TRAUMATIC BRAIN INJURY

TRAUMATIC BRAIN INJURY

Traumatic brain injury is the leading cause of disability and death in young adults in the United States. Brain injury may be caused by both closed head injury and penetrating injuries such as by bullets or foreign objects. Associated injuries, including cervical spine injury and thoracoabdominal trauma, frequently accompany acute head injury. Brain injury can be further exacerbated by systemic conditions related to trauma, including hypotension and hypoxia related to excessive bleeding, pulmonary contusion, aspiration, or adult respiratory distress syndrome.

Initial management of acute head injury patients includes immobilization of the cervical spine, establishment of a patent upper airway, and protection of the patient's lungs from aspiration of gastric contents, as well as maintaining perfusion of brain tissue by treating hypotension. The most useful diagnostic procedure, in terms of simplicity and rapidity, is CT, which should be performed as soon as possible. In this regard, CT has greatly facilitated identification of epidural or subdural hematomas. Routine CT may not be needed in patients with minor head trauma who meet the following criteria: no headache or vomiting, younger than 60 years of age, no intoxication, no deficits in short-term memory, no physical evidence of trauma above the clavicles, and no seizures.

 

It is not unusual for patients with traumatic brain injury who initially are stable and awake or in light coma to deteriorate suddenly. Delayed hematoma formation or cerebral edema is often responsible for these changes. Uncontrolled brain swelling that may not respond to conventional management may also cause sudden neurologic deterioration. Delayed secondary injury at the cellular level is an important contributor to brain swelling and subsequent irreversible brain damage.

The Glasgow Coma Scale score provides a reproducible method for assessing the seriousness of brain injury (scores of < 8 points indicate severe injury) and for following the patient's neurologic status (see Table 10A-1 ). Head injury patients with scores less than 8 are by definition in coma, and approximately 50% of these patients die or remain in vegetative states. Type of head injury and age are important determinants of outcome in the presence of low scores. For example, patients with acute subdural hematomas have a poorer prognosis than do patients with diffuse brain contusion injury. Mortality in children with severe head injury is less than in adults.

Perioperative Management

Perioperative management of patients with acute head trauma, such as those following motor vehicle accidents, must consider the risks of secondary injury due to cerebral ischemia as well as injuries affecting organ systems other than the brain. CBF is usually initially decreased and then gradually increases with time. Factors contributing to poor outcome in head injury patients are increased ICP and systolic blood pressures less than 70 mm Hg. Normal autoregulation of CBF is often impaired in patients with acute head injury, but carbon dioxide reactivity is usually preserved. Control of increased ICP with mannitol or furosemide is indicated, and in some patients craniectomy is necessary. Hyperventilation, although effective in controlling ICP, may contribute to cerebral ischemia in head injury patients, and for this reason, it is a common recommendation to avoid hyperventilation unless necessary. Barbiturate coma may be useful in some patients as a means to control intracranial hypertension when other more conservative means have failed. In adults, induced mild hypothermia in patients with acute head injury has not been shown to improve outcome. Administration of hypertonic saline and mannitol may decrease brain volume. Associated lung injuries may impair oxygenation and ventilation in these patients and necessitate mechanical ventilation. Neurogenic pulmonary edema may also contribute to acute pulmonary dysfunction. The exact mechanism for this disorder is unknown but is thought to be related to hyperactivity of the sympathetic nervous system, which results in alterations in Starling forces in the lung and pulmonary edema. Coagulopathy occurs in head injury patients and may be enhanced by hypothermia and the need for massive blood transfusions. Disseminated intravascular coagulation can occur following severe head injury. It is thought to be related to the release of brain thromboplastin into the systemic circulation. This protein is known to activate the coagulation cascade. Replacement of clotting factors may also be necessary.

Management of Anesthesia

Patients with traumatic brain injury may require anesthesia for neurosurgical interventions such as hematoma drainage, decompressive craniectomy for cerebral edema, or spinal stabilization. Anesthesia may also be required for the treatment of a variety of non-neurologic problems such as the repair of limb fractures and intra-abdominal injuries. Management of anesthesia includes efforts to optimize CPP, minimize the occurrence of cerebral ischemia, and avoid drugs and techniques that could increase ICP. CPP is maintained above 70 mm Hg if possible, and hyperventilation is not used unless it is needed as a temporizing measure to control ICP. During surgical evacuation of acute epidural or subdural hematomas, systemic blood pressure may decrease precipitously at the time of surgical decompression and require aggressive resuscitation. Patients with severe head injury may experience impaired oxygenation and ventilation that complicates the intraoperative period. Adequate fluid resuscitation and replacement are important. Hypertonic crystalloid solutions, such as 3% saline, increase the plasma osmotic pressure and thus remove water from the brain's interstitial space. Hypotonic crystalloid solutions are avoided, as they decrease plasma osmotic pressure and increase cerebral edema even in normal brains. Glucose-containing solutions should be avoided unless specifically indicated (e.g., the treatment of laboratory-diagnosed hypoglycemia), out of concern for exacerbating neuronal injury in the setting of hyperglycemia.

Induction and Maintenance of Anesthesia

In hemodynamically stable patients, the induction of anesthesia with intravenous induction drugs and nondepolarizing muscle relaxants is acceptable. Fiberoptic intubation or tracheostomy should be considered in patients when there is added concern of either the inability to safely perform tracheal intubation via direct laryngoscopy, that a neurologic deficit may be further exacerbated (i.e., cervical spine fracture), or there is already evidence of airway compromise. In moribund patients, the establishment of a safe and effective airway takes priority over concerns for anesthetic selection, as drugs may not be needed. One should also be aware of the possible presence of hidden extracranial injuries (i.e., bone fractures, pneumothorax) as they may lead to problems such as excessive blood loss and perturbations in ventilation and circulation. Maintenance of anesthesia often includes continuous infusions of intravenous drugs or low-dose volatile anesthetics, keeping in mind the goal to optimize CPP and prevent increases in ICP. Nitrous oxide should be avoided because of the risk of pneumocephalus and concern for non-neurologic injuries such as pneumothorax. Among the volatile anesthetics, low-dose sevoflurane may be unique in minimally impairing cerebral autoregulation, although low-dose isoflurane is also a good choice. If acute brain swelling develops, correctable causes such as hypercapnia, arterial hypoxemia, systemic hypertension, and venous obstruction must be considered and corrected if present. Intra-arterial monitoring of systemic blood pressure is helpful, whereas time constraints may limit the use of CVP or pulmonary artery catheter monitoring.

Postoperative Period

During the postoperative period, it is common to maintain skeletal muscle paralysis to facilitate mechanical ventilation. Continuous monitoring of ICP is also useful in many patients.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

TURP Syndrome

TURP Syndrome

Benign prostatic hyperplasia is often treated surgically by transurethral resection of the prostate (TURP). This involves resection via a cystoscope with continuous irrigation of the bladder to aid visualization while removing blood and resected material. The irrigating fluid is a nonelectrolyte fluid containing glycine, sorbitol, or mannitol, and this fluid may be absorbed rapidly via open venous sinuses in the prostate gland, causing volume overload, hyponatremia, and hypo-osmolality. This is known as TURP syndrome. This syndrome is more likely to occur if resection is prolonged (>1 hour), if the irrigating fluid is suspended more than 40 cm above the operative field, or if the pressure in the bladder is allowed to increase above 15 cm H₂O. TURP syndrome manifests principally with cardiovascular and neurologic signs and symptoms. Hypertension is common. Monitoring for development of this syndrome includes direct neurologic assessment in the patient under regional anesthesia or measurement of serum sodium concentration and osmolality in the patient under general anesthesia.

Treatment consists of terminating the surgical procedure so that no more fluid is absorbed, diuretics if needed for relief of cardiovascular symptoms, and hypertonic saline administration if severe neurologic symptoms are present or the serum sodium concentration is less than 120 mEq/L.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

Tracheal Stenosis

Tracheal Stenosis

Tracheal stenosis typically develops after prolonged endotracheal intubation. Tracheal mucosal ischemia that may progress to destruction of cartilaginous rings and subsequent circumferential constricting scar formation is minimized by the use of high-volume cuffs on tracheal tubes. Infection and hypotension may also contribute to events that culminate in tracheal stenosis.

 

Diagnosis

Tracheal stenosis becomes symptomatic when the lumen of the adult trachea is decreased to less than 5 mm. Symptoms may not develop until several weeks after tracheal extubation. Dyspnea is prominent even at rest. These patients must use accessory muscles of respiration during all phases of the breathing cycle and must breathe slowly. Peak expiratory flow rates are decreased. Stridor is usually audible. Flow-volume loops display flattened inspiratory and expiratory curves (see Fig. 9-3A ). Tomograms of the trachea demonstrate tracheal narrowing.

Management of Anesthesia

Tracheal dilation is useful in some patients, but surgical resection of the stenotic tracheal segment with primary anastomosis is often required. Translaryngeal endotracheal intubation is accomplished. After surgical exposure, the distal normal trachea is opened and a sterile cuffed tube inserted and attached to the anesthetic circuit. Maintenance of anesthesia with volatile anesthetics is useful for ensuring maximum inspired concentrations of oxygen. High-frequency ventilation is useful in selected patients. Anesthesia for tracheal resection may be facilitated by the addition of helium to the inspired gases. This decreases the density of these gases and may improve flow through the area of tracheal narrowing.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

Sitting Position and Venous Air Embolism

Sitting Position and Venous Air Embolism

Craniotomy to remove a supratentorial tumor is usually performed in the supine position with the patient's head elevated 10 to 15 degrees to facilitate cerebral venous drainage. Infratentorial tumors have more unusual patient positioning requirements and may be performed in the lateral, prone, or sitting position.

The sitting position deserves special attention since, other than for surgery on the shoulder and thyroid, it is rarely used for non-neurosurgical cases and has a variety of anesthetic implications. The sitting position is often used for exploration of the posterior cranial fossa, which may be necessary to resect intracranial tumors, clip aneurysms, decompress cranial nerves, or implant electrodes for cerebellar stimulation, as well as for surgery on the cervical spine and posterior cervical musculature. Advantages of the sitting position include excellent surgical exposure and enhanced cerebral venous and CSF drainage, thereby minimizing blood loss and reducing ICP. These advantages are offset by the decreases in systemic blood pressure and cardiac output produced by this position, and the potential hazard of venous air embolism. For these reasons, the lateral or prone position may be selected as an alternative. However, as long as no contraindication to the sitting position exists (i.e., patent foramen ovale), the outcome of patients managed in the sitting versus horizontal position is similar or superior to that of other positions. If the sitting position is used, it is mandatory to maintain a high index of suspicion for venous air embolism.

The postoperative complications that may occur after posterior fossa craniotomy include apnea due to hematoma formation, tension pneumocephalus, or cranial nerve injuries. Macroglossia is also a possibility and is presumed due to impaired venous drainage from the tongue. This is sometimes associated with excessive neck flexion and may be influenced by the use of multiple oral instruments (e.g., endotracheal tube, oral airway, esophageal stethoscope, transesophageal echocardiography probe) simultaneously.

Venous air embolism is a potential hazard whenever the operative site is above the level of the patient's heart, such that pressures in the exposed veins are subatmospheric. Although this complication is most often associated with neurosurgical procedures, venous air embolism may also occur during operations involving the neck, thorax, abdomen, and pelvis and during open heart procedures, repair of liver and vena cava lacerations, obstetric and gynecologic procedures, and total hip replacement. Patients undergoing intracranial surgery are at increased risk not only because the operative site is usually above the level of the patient's heart but also because veins in the skull may not collapse when cut, owing to their attachment to bone or dura. Indeed, the cut edge of cranial bone, including that associated with burr holes, is a common site for the entry of air into veins.

Presumably, when air enters the right atrium and ventricle, there is interference with right-sided cardiac output and blood flow into the pulmonary artery. Air that eventually enters the pulmonary artery may trigger pulmonary edema and reflex bronchoconstriction. Death is usually secondary to a vapor lock causing right-sided cardiac output to plummet, acute cor pulmonale, or arterial hypoxemia from combined cardiac and pulmonary insults.

Small quantities of air can sometimes pass through pulmonary vessels to reach the coronary and cerebral circulations; large quantities of air can travel directly to the systemic circulation through right-to-left intracardiac shunts created by a patent foramen ovale or true septal defects. This passage of air from the right to left circulation is known as paradoxical air embolism. For that reason, known foramen ovale or other cardiac defects that could possibly result in a right-to-left shunt are relative contraindications to use of the sitting position.

Fatal cerebral embolism, subsequent to entrainment of systemic venous air, has occurred even in the absence of identifiable shunt mechanisms or intracardiac defects. This may occur because of failure of contrast echocardiography to detect an existing patent foramen ovale or septal defect. There are many theoretical reasons for this failure of detection. One is that Valsalva or other provocative maneuvers are not always successful in mimicking the physiologic changes that occur during general anesthesia and true venous air embolus and, as such, may underestimate the potential for venous air to pass from the right to left circulations. Paradoxical air embolism can occur even in the absence of any detectable elevations of mean right atrial pressure compared to that of the left atrium. This occurs as a result of small differences in the timing of contraction of the various heart chambers. As a result, pressure gradients will transiently reverse, making the shunt bidirectional. An extremely brief right-to-left shunt could introduce a few air bubbles into the left-sided cardiac chambers and lead to severe consequences if those bubbles were to embolize to the brain. Also, various anesthetic drugs may diminish the ability of the pulmonary circulation to filter out air emboli and thus facilitate the passage of venous air emboli to the arterial circulation.

The use of the sitting position inherently predisposes neurosurgical patients to paradoxical air embolism, as the normal interatrial pressure gradient frequently becomes reversed in this position. When the likelihood of venous air embolism is increased, it is useful, but not mandatory, to place a right atrial catheter before beginning surgery. Death due to paradoxical air embolism results from obstruction of the coronary arteries by air, leading to myocardial ischemia and ventricular fibrillation. Neurologic damage may follow air embolism to the brain.

Early detection of venous air embolism is important for successful treatment. A Doppler sonography transducer placed over the right-sided cardiac structures is one of the most sensitive indicators of intracardiac air. Indeed, the small amount of air detected by the transducer is often clinically unimportant. In this regard, the transducer does not provide information as to the volume of air that has entered the venous circulation. Transesophageal echocardiography, by comparison, is also useful for both detecting and quantifying intracardiac air. A sudden decrease in the end-expired PaCO₂ may reflect increased alveolar dead space and/or diminished cardiac output resulting from air embolus. An increase in right atrial and pulmonary artery pressures reflects acute cor pulmonale and correlates with abrupt decreases in the end-expired CO₂. Although these changes are less sensitive indicators of the presence of air than Doppler sonography or transesophageal echocardiography, they reflect the size of the venous air embolism. Increased end-expired nitrogen concentrations identify and partially quantify venous air embolism. Changes in end-expired nitrogen concentrations often precede decreased end-expired PaCO₂ or increased pulmonary artery pressures. During controlled ventilation of the lungs, sudden attempts by patients to initiate spontaneous breaths (“gasp reflex”) may be the first indication of venous air embolism. Hypotension, tachycardia, cardiac arrhythmias, and cyanosis are late signs of venous air embolism. Certainly detection of the characteristic “millwheel” murmur, as heard through an esophageal stethoscope, is a late sign of catastrophic venous air embolism.

Upon detection of venous air, the surgeon should flood the operative site with fluid, apply occlusive material to all bone edges, and attempt to identify any other sources of air entry (e.g., perforation of a venous sinus). Aspiration of air should be attempted through the right atrial catheter. The ideal location of the right atrial catheter tip is controversial, but evidence suggests that the junction of the superior vena cava with the right atrium is preferable, as this position appears to provide the most rapid aspiration of air. Right atrial multiorifice catheters permit aspiration of larger amounts of air than do single-orifice catheters. Because of its small lumen size and slow speed of blood return, a pulmonary artery catheter is not as useful for aspirating air but may provide additional evidence that venous air embolism has occurred by virtue of increased pulmonary artery pressures. Nitrous oxide is promptly discontinued to avoid increasing the size of any venous air bubbles. Indeed, elimination of nitrous oxide from the inhaled gases after detecting a venous air embolism often results in decreased pulmonary artery pressures. At the same time oxygen is substituted for nitrous oxide, it may be helpful to apply positive end-expiratory pressure or direct jugular venous compression to increase venous pressure at the surgical site. Despite the logic of this maneuver, the prophylactic use of positive end-expiratory pressure is not of value in preventing venous air embolism.

Extreme hypotension may require the support of perfusion pressure using sympathomimetic drugs. Likewise, marked decreases in cardiac output may require the infusion of β-adrenergic agonists such as dopamine or dobutamine. Bronchospasm is treated with β₂-adrenergic agonists by aerosol (metered-dose inhaler) or the intravenous route. The traditional admonition to treat venous air embolism by placing the patient in the lateral position with the right chest uppermost is rarely possible or safe during intracranial operations. It is likely that valuable time, better spent aspirating air and supporting circulation, could be lost attempting to attain this position.

After successful treatment of small or modest venous air embolism, the surgical procedure can be resumed. However, the decision to reinstitute administration of nitrous oxide must be individualized. If it is decided not to use nitrous oxide, maintenance of an adequate depth of anesthesia requires administration of larger doses of volatile or intravenous anesthetics. If nitrous oxide is added to the inhaled gases, it is possible that residual air in the circulation could again produce symptoms.

Hyperbaric therapy may be useful in the treatment of both severe venous as well as paradoxical air embolism. Transfer of patients to a hyperbaric chamber in an attempt to decrease the size of air bubbles and to improve blood flow is likely to be helpful only if the transfer can be accomplished within 8 hours.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

SPINAL CORD TUMORS

SPINAL CORD TUMORS

Spinal cord tumors can be divided into two broad categories. Intramedullary tumors are located within the spinal cord proper and account for approximately 10% of tumors affecting the spinal column, with gliomas and ependymomas accounting for the vast majority of intramedullary tumors. Extramedullary tumors can be either intradural or extradural. Neurofibromas and meningiomas account for most of the intradural tumors. In contrast, metastatic lesions, usually from lung, breast, or prostate cancer, as well as myeloma, are the most common causes of extradural lesions. Other mass lesions of the spinal cord, including abscesses and hematomas, share many of the clinical signs and symptoms seen with tumors.

 

Spinal cord tumors typically present with symptoms of cord compression. Pain is a common finding and is usually aggravated by coughing or straining. Motor symptoms and sphincter disturbances may also occur. Sometimes spinal tenderness may be present. Diagnosis is usually based on symptoms and imaging of the spinal cord; magnetic resonance imaging is the technique of choice. Treatment and prognosis depend of the nature of the lesion and may include corticosteroids, radiation therapy, chemotherapy, or surgical decompression or excision.

Management of anesthesia involves ensuring adequate spinal cord oxygenation and perfusion. This is achieved by ensuring adequate PaO₂ and by avoiding hypotension and anemia. Specifics of management will depend on the level of the lesion and the extent of neurologic impairment.

Tumors involving the cervical spinal cord may influence the approach used to secure the airway. Significant motion of the cervical spine could lead to further cord compromise via compression and decreased cord perfusion. With any form of disease that places the cervical spine in jeopardy for new injury, airway management should be similar to that discussed in the management of acute spinal cord injury. This may include in-line stabilization during direct laryngoscopy or awake fiberoptic intubation. If the approach to patient management is uncertain, it is useful, prior to administering sedatives or narcotics, to have the patient placed in position for airway management (e.g., on the operating room table) and then carefully move through the anticipated variations of head and neck movement prior to actual airway manipulation or induction of anesthesia. Exacerbation or induction of symptoms upon movement should tip the clinician toward fiberoptic laryngoscopy (with the head held in neutral position) or other options that would less likely cause movement-associated harm to the cord. For example, a light-wand or Bullard laryngoscope may facilitate intubation of the trachea without significant neck extension.

Safe resection of a tumor may require the use of intraoperative electrophysiologic monitoring of neurologic function. Techniques such as electromyography, somatosensory evoked potentials, and motor evoked potential monitoring have a variety of anesthetic implications. The preferred approach may vary from institution to institution. We refer readers to a variety of review articles discussing the intraoperative use of these monitoring modalities.

Succinylcholine should be used with caution in patients with spinal cord tumors given the risk of associated hyperkalemia. Also, neuromuscular monitoring with train-of-four should be performed on a neurologically intact extremity. Evidence of upper motor neuron impairment may lead to an up-regulation of acetylcholine receptors, thus making the extremity relatively resistant to nondepolarizing blockade. If there are any concerns regarding the possibility of altered responsiveness to neuromuscular block due to tumor-induced spinal cord dysfunction, monitoring train-of-four on the facial nerve is a reasonable option. However, one should be careful to monitor evoked muscle twitches, not direct muscle stimulation.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

PHEOCHROMOCYTOMA

PHEOCHROMOCYTOMA

Pheochromocytomas are catecholamine-secreting tumors that arise from chromaffin cells of the sympathoadrenal system. Pheochromocytomas account for less than 0.1% of all cases of hypertension in adults. Although they are an uncommon cause of hypertension, their detection is imperative since they have lethal potential and are one of the few truly curable forms of hypertension. Uncontrolled catecholamine release can result in malignant hypertension, cerebrovascular accidents, and myocardial infarctions. They present a great challenge to anesthesiologists both in the operating room and in the ICU. Before diagnostic urine screening tests became available and prior to the institution of preoperative α-adrenergic blockade (i.e., early to mid 1960s), 25% to 50% of hospital deaths in patients with a pheochromocytoma occurred during the induction of anesthesia or during surgical procedures for unrelated disorders.

The precise etiology of a pheochromocytoma is unknown. Pheochromocytomas are usually (90%) an isolated finding. Ten percent of pheochromocytomas are inherited (familial) as an autosomal dominant trait. Both sexes are equally affected, and the tumor can present at any age with the peak incidence occurring in the third to fifth decades of life. Ten percent of pheochromocytomas occur in children. Variable clinical presentations are responsible for difficulties in diagnosis. Familial pheochromocytomas usually occur as bilateral adrenal tumors or as extra-adrenal tumors that appear in the same anatomic site over successive generations. Recent advances in genetic testing allow for early identification of patients with a familial pheochromocytoma before signs and symptoms occur. Familial pheochromocytomas can also be part of the multiple endocrine neoplastic syndromes and can occur in association with several neuroectodermal dysplasias. Patients with multiple endocrine neoplastic 2a syndrome have a pheochromocytoma, medullary carcinoma of the thyroid, and hyperparathyroidism. Patients with multiple endocrine neoplastic 2b syndrome have a pheochromocytoma, medullary carcinoma of the thyroid, alimentary tract ganglioneuromatosis, thickened corneal nerves, and a marfanoid habitus. In multiple endocrine neoplastic 2a and 2b syndromes, pheochromocytomas are usually located bilaterally in the adrenal glands and are seldom malignant. Almost 100% of patients with the multiple endocrine neoplastic 2 syndromes have or will develop bilateral benign adrenal medullary pheochromocytomas. Of the neuroectodermal dysplasias, 10% to 25% of patients with von Hippel-Lindau syndrome (i.e., hemangioblastoma of the cerebellum and a retinal angioma) may have a pheochromocytoma, less than 1% of patients with von Recklinghausen’s disease (i.e., neurofibromatosis) have a pheochromocytoma, and patients with tuberous sclerosis and Sturge-Weber syndrome can have a pheochromocytoma.

Malignant spread usually proceeds via venous and lymphatic channels with a predilection for liver and bone, although spinal cord, lung, brain, and lymph nodes may also be affected. Metastatic spread from apparently benign primaries is well recognized. The incidence of malignancy is 10%, although improved diagnostic methods (i.e., ¹³¹I-metaiodobenzylguanidine [MIBG] scintigraphy) may yield a higher rate in the future. The 5-year survival rate for patients with malignancy is 44%. Following resection of benign disease, 5% to 10% of patients have a benign recurrence.

Eighty percent of pheochromocytomas are located in the adrenal medulla. The right gland is involved more often than the left. Twenty percent of pheochromocytomas are extra-adrenal in location, with the majority being located in the abdomen in association with the sympathetic ganglia. The organ of Zuckerkandl near the aortic bifurcation is the most common extra-adrenal site. Two percent of extra-adrenal pheochromocytomas occur in the neck and thorax. Failure of involution of chromaffin tissue in childhood is the best explanation for the development of extra-adrenal pheochromocytomas. Contrary to previous beliefs, most extra-adrenal pheochromocytomas follow a benign course. Adult pheochromocytomas are solid, highly vascular tumors usually 3 to 5 cm in diameter and average 100 g in weight (range, 1.0–4000 g). The average-size adult pheochromocytoma contains 100 to 800 mg of norepinephrine.

Pheochromocytomas are tumors of the SNS. The SNS remains intact and active in the presence of these tumors. The manifestations of a pheochromocytoma are the result of the hormones released by the tumor. Most pheochromocytomas secrete norepinephrine either alone or more commonly combined with a smaller amount of epinephrine in a ratio of 85:15, the inverse of the ratio secreted by the normal adrenal gland. Approximately 15% of tumors secrete predominantly epinephrine. Some dopamine-secreting pheochromocytomas have been described. Most pheochromocytomas are not under neurogenic control and secrete catecholamines autonomously.

Signs and Symptoms

Hypertension, continuous or paroxysmal, is the most frequent manifestation of the disease. Headache, sweating, pallor, and palpitations are other classic signs and symptoms. Most patients are symptomatic, and attacks range from infrequent (i.e., once a month or less) to numerous (i.e., many times per day) and may last from less than a minute to several hours. They may occur spontaneously or be precipitated by physical stimuli, psychic stimuli, or medications. Hypertension is present in more than 80% of adult patients. Paroxysmal hypertension associated with a normal blood pressure between crises occurs in 50% of patients. Thirty percent of patients will have sustained hypertension. Twenty-four–hour ambulatory blood pressure monitoring has shown that many crises are asymptomatic. Orthostatic hypotension is also a common finding and considered to be secondary to hypovolemia and impaired venous and arterial vasoconstrictor reflex responses. Hemodynamic signs depend on the predominant catecholamine secreted. With norepinephrine, α-adrenergic effects predominate, and patients usually have systolic and diastolic hypertension and a reflex bradycardia. With epinephrine, β-adrenergic effects predominate, and patients usually have systolic hypertension, diastolic hypotension, and tachycardia. Some patients remain normotensive in spite of high levels of circulating norepinephrine. The regulation of blood pressure in pheochromocytoma patients appears more complex than traditional views suggest. The extent of increases in arterial blood pressure appears to have little relation to the prevailing levels of circulating catecholamines. An imbalance between endogenous vasodilators (i.e., dopamine, serotonin, enkephalins, and vasoactive intestinal peptide) and circulating catecholamines may account for this. Despite the 10-fold higher levels of circulating catecholamines, the hemodynamics are not greatly different between patients with pheochromocytomas and patients with essential hypertension. Both groups have an increased systemic vascular resistance, usually a normal cardiac output, and a slightly decreased plasma volume. Long-term exposure to high levels of catecholamines does not appear to produce hemodynamic responses characteristic of acute administration. A desensitization of the cardiovascular system or a down-regulation of adrenergic receptors may explain this finding. The sensitivity of smooth muscle cells is decreased secondary to a decrease in the number of receptors or an alteration in receptor-effector coupling. The hypertensive crises do, however, mimic the hemodynamic responses of acute catecholamine administration. Blood vessels of pheochromocytoma patients usually require extremely high concentrations of catecholamines to constrict and produce hypertension.

A catecholamine-induced cardiomyopathy may also occur. The true incidence of clinically significant cardiomyopathies is unclear. A global reduction in myocardial pump function results from the net reduction in viable myofibrils and the down-regulation of β-receptors. The etiology appears multifactorial and includes catecholamine-induced permeability changes of the sarcolemmal membranes leading to excess calcium influx, toxicity from oxidized products of catecholamines, and damage by free radicals. In addition, high catecholamine levels affect the heart via coronary vasoconstriction through α-adrenergic pathways reducing coronary blood flow and potentially creating ischemia. Both dilated and hypertrophic cardiomyopathies, as well as left ventricular outflow tract obstruction, have been demonstrated echocardiographically. Echocardiographic findings are usually normal in patients without cardiac symptoms (dyspnea, chest pain) or other clinical evidence of cardiac involvement. Electrocardiogram abnormalities may include elevation or depression of the ST segment, flattening or inversion of T waves, prolongation of the QT interval, high or peaked P waves, left axis deviation, and arrhythmias. These changes are usually transient, diffuse, variable, and normalize with α- and/or β-blockade. The cardiomyopathy appears reversible if catecholamine stimulation is removed early before fibrosis has occurred. Distinct from a cardiomyopathy, pheochromocytoma patients may develop cardiac hypertrophy with congestive heart failure secondary to sustained hypertension.

Since pheochromocytomas are notoriously variable in their secretory activity, they have been called “great mimics,” and their presentation may be confused with thyrotoxicosis, malignant hypertension, diabetes mellitus, malignant carcinoid syndrome, or gram-negative septicemia. Although pheochromocytoma patients rarely have frank diabetes, most have an elevated blood glucose level secondary to catecholamine stimulation of glycogenolysis and an inhibition of insulin release.

 

Diagnosis

When a pheochromocytoma is clinically suspected, excess catecholamine secretion must be demonstrated. Various diagnostic tests have been suggested, but none is ideal. Regardless of which test is chosen, the clinical circumstances must be strictly controlled (i.e., for posture, exercise, emotion, medications) to yield reliable results. Concomitant medical conditions (i.e., alcoholism, hypothyroidism, hypovolemia) may yield misleading results.

The most sensitive test for high-risk patients (familial pheochromocytoma or classic symptoms) is plasma-free metanephrines. Catecholamines are metabolized to free metanephrines within tumor cells, and these metabolites are continuously released into the circulation. Plasma free normetanephrine greater than 400 pg/mL and/or metanephrine greater than 220 pg/mL is diagnostic of a pheochromocytoma. If normetanephrine is 112 to 400 pg/mL or metanephrine is 61 to 220 pg/mL, the diagnosis is equivocal. A pheochromocytoma is excluded if normetanephrine is less than 112 pg/mL and metanephrine is less than 61 pg/mL.

The determination of elevated urinary free catecholamine levels and their metabolites (i.e., metanephrine, normetanephrine, vanillylmandelic acid) is a frequently used diagnostic test. It is easy to perform and readily available; however, 24-hour collections can be inconvenient and unreliable. Measurement of vanillylmandelic acid is the oldest and least expensive test, but it is nonspecific. The determination of elevated metanephrines is the single best urine screening test. For patients with a low probability of having a pheochromocytoma, a 24-hour urine for metanephrines and catecholamines is sufficient.

Precisely executed measurement of plasma catecholamines is a favored initial test by many experts. The majority of patients have a significant elevation of norepinephrine, epinephrine, or both, although some patients with a pheochromocytoma have normal levels at rest. Plasma concentrations of total catecholamines greater than 2000 pg/mL are diagnostic of a pheochromocytoma. Values between 500 and 2000 pg/mL are equivocal, and 500 pg/mL or less rules out the diagnosis. In the majority of cases, the demonstration of increased levels of either plasma catecholamines or urinary free catecholamines and their metabolites should suffice to make the diagnosis. Results are equivocal in 5% to 10% of patients, and in these cases, the clonidine suppression test may be used. Clonidine is an α₂-agonist that acts on the central nervous system to diminish efferent sympathetic outflow. In patients with a pheochromocytoma, increased plasma catecholamines result from tumor release, bypassing normal storage and release mechanisms. Clonidine acts to lower plasma catecholamines in patients without a pheochromocytoma while having no effect on catecholamine levels in pheochromocytoma patients.

In the past, provocative testing with histamine and tyramine was used to elicit excess catecholamine release from the tumor. However, the incidence of morbidity was considered too high, and these tests have been abandoned. A glucagon stimulation test is now considered to be the safest and most specific provocative test. Glucagon acts directly on the tumor to release catecholamines. This test is limited to patients with a diastolic blood pressure of less than 100 mm Hg. A positive test yields a plasma catecholamine increase of at least three times the baseline values or more than 2000 pg/mL within 1 to 3 minutes of glucagon administration. At present, most centers diagnose a pheochromocytoma by urine testing for free catecholamines and their metabolites and/or measuring plasma catecholamines and add the clonidine suppression test and/or the glucagon stimulation test in equivocal cases. Of these tests, which is the single most reliable one remains controversial.

Tumor location can be predicted by the pattern of catecholamine production ( Table 16-10 ). Specific radiographic tests can pinpoint the location. CT and MRI are the optimal noninvasive anatomic adrenal imaging studies. CT detects more than 95% of adrenal masses greater than 1.0 cm in diameter. MRI offers advantages over CT that include better differentiation of small adrenal lesions, better differentiation among different types of adrenal lesions, no intravenous contrast is needed, and no radiation exposure occurs. With certain MRI sequences, pheochromocytomas have high signal intensity and light up brightly. In contrast to CT and MRI, which provide primarily anatomic information,¹³¹I-MIBG and ¹²³I-MIBG provide functional information. MIBG is an analogue of guanethidine, similar in structure to norepinephrine, and taken up by adrenergic neurons and concentrated in catecholamine-secreting tumors. MIBG is detected by scintigraphy. This is a physiologic test that localizes based on pharmacologic activity. It is especially useful in detecting extra-adrenal pheochromocytomas, metastatic deposits, and confirming that an adrenal mass is a functional pheochromocytoma. MIBG can screen the entire body with exquisite contrast and is the initial localizing procedure of choice at many institutions. CT, MRI, and ¹³¹I-MIBG scintigraphy are complementary studies in localizing pheochromocytomas. A positron emission scan and selective venous catheterization with sampling of catecholamines from the adrenal vein and other sites are other useful tests.

Management of Anesthesia

Preoperative Management

There are no controlled, randomized, prospective clinical studies on the value of adrenergic blockade for pheochromocytoma patients in the perioperative period. However, following the introduction of α-adrenergic blockers during the preoperative period, the mortality from excision of a pheochromocytoma decreased from 40% to 60% in 1951 to 0% to 6% in 1967. Some authors attribute this result more to advances in anesthetic techniques, monitoring techniques, and the availability of fast-acting medications than to the use of α-blockers. Since most pheochromocytomas secrete predominantly norepinephrine, medical therapy has depended on α-blockade to lower blood pressure, increase intravascular volume, prevent paroxysmal hypertensive episodes, allow resensitization of adrenergic receptors, and decrease myocardial dysfunction. Although a significantly reduced intravascular volume may accompany a pheochromocytoma, the majority of patients have a normal or only slightly decreased intravascular volume. α-Blockade appears to protect myocardial performance and tissue oxygenation from adverse catecholamine effects.

Phenoxybenzamine is the most frequently prescribed α-blocker for preoperative use. It is a noncompetitive α₁-antagonist with some α₂-blocking properties. As a noncompetitive blocker, it is difficult for excess catecholamines to overcome the blockade. Its long duration of action permits oral dosing only twice daily. The usual starting regimen is 10 to 20 mg twice daily, with most patients requiring 60 to 250 mg/day. The goal of therapy is normotension, a resolution of symptoms, elimination of ST-T changes on the electrocardiogram, and elimination of arrhythmias. Overtreatment can result in severe orthostatic hypotension. The optimal duration of α-blockade therapy is undetermined and may range from 3 days to 2 weeks or longer. Because of its prolonged effect on α-receptors, it has been recommended to discontinue it 24 to 48 hours before surgery to avoid vascular unresponsiveness immediately following removal of the tumor. Some anesthesiologists administer only one half to two thirds of the morning dose preceding surgery to address similar concerns. Some surgeons request its discontinuation 48 hours preoperatively to allow them to use hypertensive episodes intraoperatively as cues to localize areas of metastasis. However, regardless of the completeness of α-blockade preoperatively, significant hypertension usually occurs with manipulation of the tumor. Unfortunately, being an α₁,₂-blocker, phenoxybenzamine may enhance catecholamine secretion through α₂-blockade, which will result in tachycardia.

Prazocin, a pure α₁-competitive blocker, can be used instead of phenoxybenzamine. It is shorter acting, causes less tachycardia, and is easier to titrate to a desired end point than phenoxybenzamine. Initial doses of 1.0 mg three times daily may be increased to 8 to 12 mg/day to obtain the desired effect. It has been criticized for its failure to prevent hypertensive episodes adequately in the preoperative period, although it has strong advocates. Other α₁-blockers include doxazosin and terazosin. Doxazosin at doses of 2 to 6 mg/day may be as effective in controlling hypertension as phenoxybenzamine and causes fewer side effects before (tachycardia) and after (hypotension) surgical removal.

If tachycardia (i.e., heart rates > 120 bpm) or other arrhythmias result following α₂-blockade from phenoxybenzamine, a β-adrenergic blocker is prescribed. A nonselective β-blocker should never be administered prior to α-blockade because blockade of vasodilatory β₂-receptors results in unopposed α-agonism, resulting in vasoconstriction and hypertensive crises. Propranolol, a nonselective β_1,2-blocker with a half-life greater than 4 hours, is most frequently used. Most patients require 80 to 120 mg/day. In some patients with epinephrine-secreting pheochromocytomas, doses up to 480 mg/day may be needed. β-Blockers must be used cautiously since a small but significant number of patients have an underlying cardiomyopathy and congestive heart failure may be precipitated. Atenolol, metoprolol, and labetalol have been used successfully, although experience is limited and complications have been reported with the latter. The degree of α- and β-blockade provided by labetalol (i.e., β effects exceed α effects) may not be appropriate for certain pheochromocytoma patients. In very rare circumstances, β-blockade has been selected before α-blockade. A patient with a solely epinephrine-secreting pheochromocytoma and coronary artery disease may benefit greatly from the β₁-selective agent esmolol. Esmolol has a fast onset and short elimination half-life and can be administered intravenously in the immediate preoperative period.

α-Methylparatyrosine (metyrosine) inhibits the rate-limiting enzyme tyrosine hydroxylase of the catecholamine synthetic pathway and may decrease catecholamine production by 50% to 80%. Usual doses range from 250 mg twice daily to 3 to 4 g/day. It is especially useful for malignant and inoperable tumors. Side effects including extrapyramidal reactions and crystalluria have limited its use. In combination with phenoxybenzamine during the preoperative period, it has been shown to facilitate intraoperative hemodynamic management.

The calcium channel blockers and the ACE inhibitors may be used to control hypertension. Calcium is a trigger for catecholamine release from the tumor and excess calcium entry into myocardial cells contributes to the catecholamine mediated cardiomyopathy. Nifedipine, diltiazem, and verapamil have all been used to control preoperative hypertension as has captopril, the ACE inhibitor. An α₁-blocker plus a calcium channel blocker (verapamil 120–240 mg every day or nifedipine 30–90 mg every day) is an effective combination for resistant cases.

Intraoperative Management

Elective surgery is recommended whenever possible. Optimal preparation with α-adrenergic blockade ± β-blockade ± α-methylparatyrosine and correction of possible hypovolemia are essential. Intraoperative goals include avoiding drugs or maneuvers that may provoke catecholamine release or potentiate catecholamine actions and maintaining cardiovascular stability, preferably with short-acting drugs. The periods of greatest danger occur secondary to hypertension and/or arrhythmias during anesthetic induction, intubation, surgical incision, abdominal exploration and particularly during tumor manipulation, and secondary to hypotension following ligation of the tumor’s venous drainage. Intraoperative monitoring should include standard monitoring devices plus an arterial catheter, a central venous pressure or pulmonary arterial catheter, and a urinary catheter. If available, transesophageal echocardiography provides additional valuable information on myocardial function. An arterial catheter enables monitoring of blood pressure on a beat-to-beat basis in addition to drawing arterial blood for necessary laboratory tests (e.g., hematocrit/hemoglobin, arterial blood gases, glucose). A central venous pressure catheter is usually sufficient for patients without cardiac symptoms or other clinical evidence of cardiac involvement. A pulmonary artery catheter may be necessary to manage the large fluid requirements, major volume shifts, and possible underlying myocardial dysfunction in patients with very active tumors. Significant fluid requirements needed to prevent hypotension after tumor removal may indicate altered pressure-volume relationships induced by sudden catecholamine withdrawal. A large positive fluid balance is usually required to keep intravascular volumes within a normal range.

Intraoperative ultrasonography can be used to localize small, functional tumors and to perform adrenal-sparing procedures or partial adrenalectomies. Adrenal-sparing procedures are particularly valuable when removing bilateral adrenal pheochromocytomas. Laparoscopy can be used for tumors less than 4 to 5 cm in size. Hypertension frequently occurs during pneumoperitoneum as well as during adrenal manipulation.

Virtually every anesthetic technique for pheochromocytoma resection has been advocated or discredited based on anecdotal reports. Both general ± regional anesthesia have been successfully administered. Medications can cause a hypertensive response via (1) direct stimulation of tumor cells, (2) stimulation of the SNS, (3) release of accumulated catecholamine stores in nerve endings, (4) interfering with neuronal uptake of catecholamines, and (5) inducing hypersensitivity of catecholamine receptors or potentiating the effect of catecholamines on arterioles. Although all anesthetic drugs have been used with some degree of success, certain drugs should theoretically be avoided to prevent possible adverse hemodynamic responses. Morphine and atracurium can cause histamine release, which may provoke release of catecholamines from the tumor. Atropine, pancuronium, and succinylcholine are examples of vagolytic or sympathomimetic drugs that may stimulate the SNS. Although halothane in high concentrations is effective in attenuating hemodynamic responses (i.e., hypertension, tachycardia) to anesthetic and surgical stimuli, it sensitizes the myocardium to catecholamines and should probably be avoided. Droperidol, chlorpromazine, metoclopramide, and ephedrine have all created significant hypertensive responses. Anesthetic drugs that appear safe include thiopental, etomidate, benzodiazepines, fentanyl, sufentanil, alfentanil, enflurane, isoflurane, nitrous oxide, vecuronium, and rocuronium. Despite these recommendations, the choice of anesthetic is not as crucial as the understanding with which the agents are used. Factors that stimulate catecholamine release such as fear, stress, pain, shivering, hypoxia, and hypercarbia must be minimized or avoided in the perioperative period.

Virtually all patients exhibit increases in systolic arterial pressure in excess of 200 mm Hg for periods of time intraoperatively irrespective of preoperative α-blockade. A number of antihypertensive drugs must be prepared and ready for immediate administration. Sodium nitroprusside, a direct vasodilator, is the agent of choice because of its potency, immediate onset of action, and short duration of action. Phentolamine, a competitive α-adrenergic blocker and a direct vasodilator, is effective, although tachyphylaxis and tachycardia are associated with its use. Nitroglycerin is effective but is required in large doses to control significant hypertensive episodes and may also cause tachycardia. Labetalol, with more β- than α-blocking properties, is preferred for predominantly epinephrine-secreting tumors. Magnesium sulfate inhibits release of catecholamines from the adrenal medulla and peripheral nerve terminals, reduces sensitivity of α-receptors to catechols, is a direct vasodilator, and is an antiarrhythmic. However, like all antihypertensive medications, it is suboptimal in controlling hypertension during tumor manipulation. Mixtures of antihypertensive drugs such as nitroprusside, esmolol, diltiazem, and phentolamine have been recommended to control refractory hypertension. Increasing the depth of anesthesia is also an option, although this approach may accentuate the hypotension accompanying tumor vein ligation.

Arrhythmias are usually ventricular in origin and managed with either lidocaine or β-blockers. Lidocaine is short acting and has minimal negative inotropic action. Although propranolol has been widely used, esmolol, a selective β₁-blocker, offers several advantages. Esmolol has a rapid onset and is short acting (i.e., elimination half-life of 9 minutes), allowing adequate control of heart rate, and may also provide protection against catecholamine-induced cardiomyopathy and ischemia and the development of postoperative hypoglycemia. Amiodarone, an antiarrhythmic agent that prolongs the duration of the action potential of atrial and ventricular muscle, has been used as an alternative to β-blockers (metoprolol) to treat supraventricular tachycardia associated with hypercatecholaminemia.

Hypotension following tumor vein ligation is usually significant and occurs secondary to a combination of factors including an immediate decrease in plasma catecholamines (i.e., half-lives of norepinephrine and epinephrine are approximately 1–2 minutes), vasodilation from residual α-blockade with phenoxybenzamine, intraoperative fluid and blood loss, and increased anesthetic depth. Hypotension with systolic pressures in the 70s is not infrequent. To prevent precipitous hypotension, volume expansion to a pulmonary capillary wedge pressure of 16 to 18 mm Hg should be attained prior to tumor vein ligation. Lactated Ringer’s solution or physiologic saline are the recommended fluids for use prior to tumor removal and a dextrose-containing solution should be added after tumor removal. A decrease in anesthetic depth will also aid in controlling hypotension. With a decrease in plasma catecholamines immediately following resection, insulin levels increase and hypoglycemia may occur. Fortunately, significant blood loss is unusual during resection of most intra-abdominal pheochromocytomas. Intraoperative blood salvage resulting in postresection hypertension secondary to catecholamine-laden blood has been reported. Vasopressors (e.g., phenylephrine, norepinephrine) and inotropes (e.g., dopamine) should be ready for administration if hypotension is slow to respond to fluid resuscitation. Adequate fluid therapy is essential and is the major factor responsible for the reduction (i.e., < 2%) in operative mortality. Vasopressors and inotropes should be viewed as a secondary treatment modality. Residual α-adrenergic blockade and down-regulation of receptors make some patients much less responsive to vasopressors. Glucocorticoid therapy should be administered if a bilateral adrenalectomy is performed or if hypoadrenalism is a possibility.

Postoperative Management

The majority of patients become normotensive following complete tumor resection. Plasma catecholamine levels do not return to normal until 7 to 10 days after surgery due to a slow release of stored catecholamines from peripheral nerves. Fifty percent of patients are hypertensive for several days following surgery, and 25% to 30% of patients remain hypertensive indefinitely. This hypertension is sustained rather than paroxysmal, lower than before surgery, and not accompanied by the classic features of hypercatecholaminemia. The differential diagnosis for persistent hypertension includes a missed pheochromocytoma, surgical complications with subsequent renal ischemia, and underlying essential hypertension.

Hypotension is the most frequent cause of death in the immediate postoperative period. Large volumes of fluid are necessary since the peripheral vasculature is unresponsive to the reduced levels of catechols. In addition to the reduction in plasma catecholamines and third-space fluid losses, the residual effects of phenoxybenzamine and α-methylparatyrosine, secondary to long half-lives, are present for up to 36 hours. Vasopressor therapy may be necessary but is a secondary consideration. Steroid supplementation is necessary for patients who had bilateral adrenalectomies or if hypoadrenalism is suspected.

Hypoglycemia may occur because of excess insulin release and inadequate lipolysis and glycogenolysis. Nonselective β-blockers (e.g., propranolol) may aggravate hypoglycemia by decreasing sympathetic tone and masking signs of hypoglycemia. Dextrose-containing solutions should be included as part of the fluid therapy, and plasma glucose levels should be monitored for 24 hours.

Patients usually remain in the ICU for at least 24 hours. Adequate pain control is essential, although somnolence and an increased sensitivity to narcotic analgesics have been observed. The need for controlled ventilation is dictated by the extent of surgery, the site of surgery, and the patient’s medical condition.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

Parkinson's Disease

Parkinson's Disease

Parkinson's disease is a neurodegenerative disorder of unknown cause. Increasing age is the single most important risk factor in the development of this disease; however, an association between manganese exposure in welders as well as a variety of genetic associations have recently been identified. There is a characteristic loss of dopaminergic fibers normally present in the basal ganglia, and, as a result, regional dopamine concentrations are depleted. Dopamine is presumed to inhibit the rate of firing of the neurons that control the extrapyramidal motor system. Depletion of dopamine results in diminished inhibition of these neurons and unopposed stimulation by acetylcholine.

 

The classic triad of major signs of Parkinson's disease consists of skeletal muscle tremor, rigidity, and akinesia. Skeletal muscle rigidity first appears in the proximal muscles of the neck. The earliest manifestations may be loss of associated arm swings when walking and absence of head rotation when turning the body. Facial immobility is characterized by infrequent blinking and by a paucity of emotional responses. Tremors are characterized as rhythmic, alternating flexion and extension of the thumbs and other digits at a rate of four or five movements per second (“pill-rolling tremor”). Tremors are most prominent in resting limbs but tend to disappear during the course of voluntary movement. Seborrhea, oily skin, diaphragmatic spasms, and oculogyric crises are frequent. Dementia and depression are often present.

Treatment of Parkinson's disease is designed to increase the concentration of dopamine in the basal ganglia or to decrease the neuronal effects of acetylcholine. Replacement therapy with the dopamine precursor levodopa combined with a decarboxylase inhibitor, which prevents peripheral conversion of levodopa to dopamine and optimizes the amount of levodopa available to enter the central nervous system, is the standard medical treatment. Indeed, levodopa is the most effective treatment for Parkinson's disease, and early treatment with this drug prolongs life. Levodopa is also associated with a number of side effects including dyskinesias (i.e., the most serious side effect, developing in 80% of patients after 1 year of treatment) and psychiatric disturbances (including agitation, hallucinations, mania, and paranoia). Increased myocardial contractility and heart rate in treated patients may reflect increased levels of circulating dopamine converted from levodopa. Orthostatic hypotension may be prominent in treated patients. Gastrointestinal side effects of levodopa therapy include nausea and vomiting, most likely reflecting stimulation of the medullary chemoreceptor trigger zone.

Amantadine, an antiviral agent, is reported to help control the symptoms of Parkinson's disease; however, the mechanism for this effect is not fully understood. The type B monoamine oxidase inhibitor selegiline can also help control the symptoms of Parkinson's disease by inhibiting the catabolism of dopamine in the central nervous system. Selegiline has an advantage over nonspecific monoamine oxidase inhibitors since they are only weak inhibitors of type A monoamine oxidase, the isoenzyme found primarily in the gastrointestinal tract. Therefore, selegiline is not associated with tyramine-associated hypertensive crisis, which results when foods containing tyramine (i.e., cheese, wine) are consumed by those in whom type A monoamine oxidase is pharmacologically inhibited. Entry of tyramine into the systemic circulation in the setting of pharmacologically inhibited type A monoamine oxidase results in a hyperadrenergic state due to the inherent sympathomimetic activities of tyramine.

Surgical treatment of Parkinson's disease is reserved for disabling and medically refractory symptoms. Stimulation of the subthalamic nuclei via an implanted deep brain stimulator device may relieve or help to control tremor. Pallidotomy is associated with significant improvement in levodopa-induced dyskinesias, although the improvement may be short-lived. Fetal tissue transplantation for treatment of Parkinson's disease is based on the demonstration that implanted embryonic dopaminergic neurons can survive in recipients; however, the effectiveness of this treatment is currently not known.

Management of Anesthesia

Management of anesthesia in patients with Parkinson's disease is based on an understanding of the treatment of this disease and the associated potential adverse drug effects. The elimination half-time of levodopa and the dopamine it produces is brief, so interruption of therapy for more than 6 to 12 hours can result in an abrupt loss of therapeutic effects. Abrupt drug withdrawal can lead to skeletal muscle rigidity, which interferes with lung ventilation. In this regard, levodopa therapy, including the usual morning dose on the day of surgery, should be continued during the perioperative period. Oral levodopa can be administered approximately 20 minutes before inducing anesthesia and may be repeated intraoperatively and postoperatively via an oro- or nasogastric tube to minimize the likelihood of exacerbations.

The possibility of hypotension and cardiac arrhythmias must be considered during administration of anesthesia to patients treated with levodopa. Further, one must consider the ability of butyrophenones (e.g., droperidol, haloperidol) to antagonize the effects of dopamine in the basal ganglia. An acute dystonic reaction following administration of alfentanil has been speculated to reflect opioid-induced decreases in central dopaminergic transmission. Use of ketamine is questionable because of the possible provocation of exaggerated sympathetic nervous system responses. Nevertheless, ketamine has been administered safely to patients treated with levodopa. The choice of muscle relaxants does not seem to be influenced by the presence of Parkinson's disease.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

MOYAMOYA DISEASE

MOYAMOYA DISEASE

Progressive stenosis of intracranial vessels with the secondary development of an anastomotic capillary network is the hallmark of moyamoya disease. Moyamoya is the Japanese term for “puff of smoke” and refers to the angiographic finding of a cluster of small abnormal blood vessels. There seems to be a familial tendency toward the development of this disease; however, it may be seen following head trauma or in association with other disorders such as neurofibromatosis, tuberous sclerosis, and fibromuscular dysplasia. Affected arteries have a thickened intima and a thin media. Since similar pathologic findings may be found in other organs, central nervous system abnormalities may be the manifestations of a systemic disease. Intracranial aneurysms occur with increased frequency in those with moyamoya disease. Symptoms of ischemia, such as transient ischemic attacks and infarcts, are common initial findings in children, whereas hemorrhagic complications are usually the presenting symptoms in adults. The diagnosis is typically made by conventional or magnetic resonance angiography, demonstrating a cluster of small abnormal blood vessels. However, conventional MRI and CT imaging will demonstrate a tissue void or hemorrhage, respectively.

Medical treatment is usually aimed at decreasing ischemic symptoms and usually consists of a combination of vasodilators and anticoagulants. Surgical options include direct anastomosis of the superficial temporal artery to the middle cerebral artery (also known as an extracranial-intracranial bypass) as well as various indirect revascularization procedures that may be combined with an extracranial-intracranial bypass. These techniques include an encephalomyosynangiosis (laying the temporalis muscle directly on the brain surface) and encephaloduroarteriosynangiosis (suturing the superficial temporal artery to the dura mater). Despite treatment, the overall prognosis is not good; only approximately 58% of patients ever attain normal neurologic function.

Management of Anesthesia

Preoperative assessment of the patient with moyamoya disease should involve the documentation of preexisting neurologic deficits and evaluation for history of hemorrhage or the concurrent presence of an intracranial aneurysm. Anticoagulants or antiplatelet drug should be discontinued, if possible, to avoid bleeding complications intraoperatively.

The goals of induction and maintenance of anesthesia include (1) ensuring hemodynamic stability because hypotension could lead to ischemia in the distribution of the abnormal vessels and hypertension may cause hemorrhagic complications, (2) avoiding factors that lead to cerebral or peripheral vasoconstriction (e.g., hypocapnia and phenylepherine), which can compromise blood flow in the feeding or recipient vessels, and (3) facilitating a rapid emergence from anesthesia so that neurologic function can be assessed. In addition to standard monitoring, intra-arterial catheterization is essential to rapidly assess changes in blood pressure. If possible, this should be instituted prior to induction of anesthesia to help ensure a hemodynamically stable induction sequence. Central venous catheterization is not essential, but can be useful to guide fluid management and can also provide access for administering vasoactive agents or blood products. With the exception of ketamine, any intravenous induction agent can be used safely. Inhalational induction with sevoflurane is an option for children. Succinylcholine should be used with caution in patients with preexisting neurologic deficits due to the risk of hyperkalemia. As with intraoperative management of aneurysms and AVMs, the hemodynamic response to stimulating events should be blunted. A volatile anesthetic-based technique may have the theoretical advantage in that it would enhance cerebral vasodilation. Excessive hyperventilation should be avoided due to its cerebral vasoconstrictive effect. Regarding the treatment of hypotension, hypovolemia should be treated with colloid or nonhypotonic crystalloid. Dopamine and ephedrine are reasonable options for the pharmacologic treatment of hypotension as they will avoid some of the adverse effects on the cerebral vasculature that might result from the use of a pure vasoconstrictor. Anemia should be avoided to prevent ischemia in already compromised brain regions.

Postoperative complications include stroke, seizure, and hemorrhage. Any of these may present as a delay in or failure to emerge from anesthesia, or, in those who do awaken, a new neurologic deficit.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

Multiple Sclerosis

Multiple Sclerosis

Multiple sclerosis is an autoimmune disease affecting the central nervous system that seems to occur in genetically susceptible persons. Although there is a high rate of concordance among twins and an increased risk if one has a first-degree relative with the disease as well as a geographic association (e.g., highest incidence in northern Europe, southern Australia, and North America), no clear genetic, environmental, or infectious causes have been identified. There is also no clear understanding of the immunopathogenic processes that determine the sites of tissue damage in the central nervous system, the variations in natural history, or the severity of disability caused by the disease. It is twice as common in women as in men. In women with multiple sclerosis, the rate of relapse decreases during pregnancy, especially in the third trimester, and increases during the first 3 months postpartum. Exposure to viral illnesses may trigger relapses. Pathologically, multiple sclerosis is characterized by diverse combinations of inflammation, demyelination, and axonal damage in the central nervous system. The loss of myelin covering the axons is followed by formation of demyelinative plaques. Peripheral nerves are not affected by multiple sclerosis.

Clinical manifestations of multiple sclerosis reflect its multifocal involvement. Its course may be subacute, with relapses followed by remissions, or chronic and progressive. Manifestations of multiple sclerosis reflect sites of demyelination in the central nervous system and spinal cord. For example, inflammation of the optic nerves (optic neuritis) causes visual disturbances, involvement of the cerebellum leads to gait disturbances, and lesions of the spinal cord cause limb paresthesias and weakness as well as urinary incontinence and sexual impotence. Optic neuritis is characterized by diminished visual acuity and defective pupillary reaction to light. Ascending spastic paresis of the skeletal muscles is often prominent. Intramedullary disease of the cervical cord is suggested by an electrical sensation that runs down the back into the legs in response to flexion of the neck (Lhermitte's sign). Typically, symptoms develop over the course of a few days, remain stable for a few weeks, and then improve. Because remyelination in the central nervous system probably does not occur, remission of symptoms most likely results from correction of transient chemical and physiologic disturbances that have interfered with nerve conduction in the absence of complete demyelination. Further, increases in body temperature can cause exacerbation of symptoms due to further alterations in nerve conduction in regions of demyelination. There is an increased incidence of seizure disorders in patients with multiple sclerosis.

The course of multiple sclerosis is characterized by exacerbations and remissions of symptoms at unpredictable intervals over a period of several years. Residual symptoms eventually persist during remissions, leading to severe disability from visual failure, ataxia, spastic skeletal muscle weakness, and urinary incontinence. Nevertheless, the disease in some patients remains benign, with infrequent, mild episodes of demyelination, followed by prolonged, occasionally permanent remissions. The onset of multiple sclerosis after 35 years of age is typically associated with slow disease progression.

The diagnosis of multiple sclerosis can be established with different degrees of confidence (e.g., probable or definite) on the basis of clinical features alone or clinical features in combination with oligoclonal abnormalities of immunoglobulins in the CSF, prolonged latency of evoked potentials reflecting slowing of nerve conduction due to demyelination, and signal changes in white matter seen on cranial MRI.

No treatment is curative for multiple sclerosis. Instead, treatment is directed at both symptom control and methods to slow the progression of disease. Corticosteroids, the principal treatment for acute relapses of multiple sclerosis, have immunomodulatory and anti-inflammatory effects that restore the blood-brain barrier, decrease edema, and possibly improve axonal conduction. Treatment with corticosteroids shortens the duration of the relapse and accelerates recovery, but whether the overall degree of recovery or progression of the disease is altered is not known. Interferon-β is the treatment of choice for patients with relapsing-remitting multiple sclerosis. The most common side effect of interferon-β therapy is transient influenza-like symptoms for 24 to 48 hours after injection. Slight increases in serum aminotransferase concentrations, leukopenia, or anemia may be present, and co-existing depression may be exaggerated. Glatiramer acetate is a mixture of random synthetic polypeptides synthesized to mimic myelin basic protein. This drug is an alternative to interferon-β and may be most useful for patients who become resistant to interferon-β treatment owing to serum interferon-β-neutralizing activity. Mitoxantrone is an immunosuppressive agent that functions by inhibiting lymphocyte proliferation. Because of severe cardiac toxicity, its use is limited to patients with rapidly progressive disease. Azathioprine is a purine analogue that depresses both cell-mediated and humoral immunity. Treatment with this drug may decrease the rate of relapses in multiple sclerosis but has no effect on the progression of disability. Azathioprine is considered when patients do not respond to therapy with interferon-β or glatiramer acetate. Low-dose methotrexate is relatively nontoxic and inhibits both cell-mediated and humoral immunity as a result of its anti-inflammatory effects. Patients with secondary progressive multiple sclerosis may benefit most from treatment with this drug.

Management of Anesthesia

Management of anesthesia in patients with multiple sclerosis must consider the impact of surgical stress on the natural progression of the disease. For example, regardless of the anesthetic technique or drugs selected for use during the perioperative period, it is possible that symptoms of multiple sclerosis will be exacerbated postoperatively. This may be due to factors such as infection and fever. In this regard, any increase in body temperature (e.g., as little as 1°C) that follows surgery may be more likely than drugs to be responsible for exacerbations of multiple sclerosis. It is possible that increased body temperature results in complete block of conduction in demyelinated nerves. Of note, the unpredictable cycle of clinical exacerbations and remissions could lead to erroneous conclusions that there are cause-and-effect relationship between disease severity and drugs or events present during the perioperative period.

The changing and unpredictable neurologic presentation in patients with multiple sclerosis during the perioperative period must be appreciated when selecting regional anesthetic techniques. Indeed, spinal anesthesia has been implicated in postoperative exacerbations of multiple sclerosis, whereas exacerbations of the disease after epidural anesthesia or peripheral nerve blocks have not been described. The mechanism by which spinal anesthesia might differ from epidural anesthesia is unknown but might reflect local anesthetic neurotoxicity. Specifically, it is speculated that the demyelination associated with multiple sclerosis renders the spinal cord more susceptible to the neurotoxic effects of local anesthetics. Epidural anesthesia may be less of a risk than spinal anesthesia because the concentration of local anesthetics in the white matter of the spinal cord is lower than after spinal anesthesia. Nevertheless, both epidural anesthesia and spinal anesthesia have been used in parturients with multiple sclerosis.

General anesthesia is the most often used technique in patients with multiple sclerosis. There are no unique interactions between multiple sclerosis and the drugs used to provide general anesthesia, and there is no evidence to support use of one inhaled or injected anesthetic drug over another. When selecting muscle relaxants, one should consider the possibility of exaggerated release of muscle potassium, causing hyperkalemia, following administration of succinylcholine to these patients. Prolonged responses to the paralyzing effects of nondepolarizing muscle relaxants would be consistent with co-existing skeletal muscle weakness (myasthenia-like) and decreased skeletal muscle mass. Conversely, resistance to the effects of nondepolarizing muscle relaxants has been observed, perhaps reflecting proliferation of extrajunctional cholinergic receptors characteristic of upper motor neuron lesions.

Corticosteroid supplementation during the perioperative period may be indicated in patients being treated long-term with these drugs. Efforts must be made to recognize and prevent even modest increases in body temperature (more than 1°C), as this change might exacerbate symptoms. Periodic neurologic evaluation during the postoperative period may be useful for detecting exacerbations.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

Lymphangioleiomyomatosis

Lymphangioleiomyomatosis

Lymphangioleiomyomatosis is the proliferation of smooth muscle in airways, lymphatics, and blood vessels that occurs in females of reproductive age. Pulmonary function tests show restrictive and obstructive lung disease with decreases in diffusing capacity. Lymphangioleiomyomatosis presents clinically as progressive dyspnea, hemoptysis, recurrent pneumothorax, and pleural effusions. Nearly all lymphangioleiomyomatosis cells express progesterone receptors. Progesterone or tamoxifen can be used for treatment, but there is progressive deterioration in pulmonary function, and most patients die within 10 years of the onset of symptoms.

Management of Anesthesia

Preoperative

Patients usually present with dyspnea and nonproductive cough. Cor pulmonale may be present. Coarse breath sounds with crepitations are heard on auscultation. A chest radiograph may show a ground glass or nodular pattern. Arterial blood gases reveal hypoxemia with normocarbia. Pulmonary function tests show restrictive ventilatory defects and CO diffusing capacity is decreased. A vital capacity of less than 15 mL/kg indicates severe pulmonary dysfunction. Infection should be treated, secretions cleared, and smoking stopped preoperatively.

Intraoperative

Patients with restrictive lung disease tolerate apneic periods very poorly due to their small FRC and low oxygen stores. General anesthesia, the supine position, and controlled ventilation all contribute to further decreases in FRC. Alterations in FRC and the risk of hypoxia continue into the postoperative period. Uptake of inhaled anesthetics is faster in these patients because of the small FRC. Peak airway pressures should be kept as low as possible to minimize the risk of barotrauma.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

INTRACRANIAL TUMORS

INTRACRANIAL TUMORS

Intracranial tumors may be classified as primary (those arising from the brain and its coverings) or metastatic. Tumors can originate from virtually any cell type within the central nervous system. Supratentorial tumors are more common in adults and often present with headache, seizures, or new neurologic deficits, whereas infratentorial tumors are more common in children and often present with obstructive hydrocephalus and ataxia. Treatment and prognosis depend both on the tumor type and location. Treatment may consist of surgical resection or debulking, chemotherapy, or radiation. Gamma knife irradiation differs from traditional radiation therapy in that multiple radiation sources are used, and by addressing the tumor from multiple angles, radiation to the tumor can be maximized while radiation dose to any single area of surrounding brain can be diminished. This same approach can be accomplished with the use of radiation produced by a linear accelerator.

Tumor Types

Astrocytoma

Astrocytes are the most prevalent neuroglial cell in the central nervous system and give rise to many types of infra- and supratentorial tumors. Well-differentiated (low-grade) gliomas are the least aggressive class of astrocytic-derived tumors. They often present in young adults with new-onset seizures. Upon imaging, they generally show minimal enhancement with contrast. Surgical or radiation treatment of low-grade gliomas usually results in symptom-free long-term survival.

Pilocytic astrocytomas usually affect children and young adults. They often arise in the cerebellum (cerebellar astrocytoma), cerebral hemispheres, hypothalamus, or optic pathways (optic glioma). The tumor usually appears as a contrast-enhancing, well-demarcated lesion with minimal to no surrounding edema. Because of its benign pathologic characteristics, prognosis following surgical resection is generally very good; however, the location of the lesion, such as within the brainstem, may preclude resection.

Anaplastic astrocytomas are poorly differentiated, usually appear as a contrast-enhancing lesion on imaging due to disruption of the blood-brain barrier, and usually evolve into glioblastoma multiforme. Treatment usually involves resection, radiation, or chemotherapy. Prognosis is intermediate between low-grade gliomas and glioblastoma multiforme.

Gliobastoma multiforme (grade IV glioma) accounts for 30% of all primary brain tumors in adults. Imaging usually reveals a ring-enhancing lesion due to central necrosis as well as surrounding edema. Treatment typically involves debulking combined with radiation and chemotherapy. Due to microscopic infiltration of normal brain by tumor cells, resection alone is usually inadequate. Instead, treatment usually consists of surgical debulking combined with chemotherapy and radiation and is aimed at palliation, not cure. Despite treatment, life expectancy is usually on the order of weeks.

Oligodendroglioma

Arising from myelin-producing cells within the central nervous system, oligodendrogliomas account for only 6% of primary intracranial tumors. Classically, seizures usually predate the appearance of tumor on imaging, often by many years. Calcifications within the tumor are common and are visualized on CT imaging. The tumor usually consists of a mixture of both oligodendrocytic and astrocytic cells. Treatment and prognosis depend on the pathologic features. Initial treatment involves resection since, early in the course, the tumor typically consists of primarily oligodendrocytic cells, which are radioresistant. Because of the presence of astrocytic cells, these tumors commonly behave more like anaplastic astrocytomas or glioblastoma multiforme later in their course.

Ependymoma

Arising from cells lining the ventricles and central canal of the spinal cord, ependymomas commonly present in childhood and young adulthood. Their most common location is the floor of the fourth ventricle. Symptoms include obstructive hydrocephalus, headache, nausea, vomiting, and ataxia. Treatment consists of resection and radiation. Tumor infiltration into surrounding tissues may preclude complete resection. Prognosis usually depends on the extent of resection.

Primitive Neuroectodermal Tumor

Primitive neuroectodermal tumor represents a diverse class of tumors including retinoblastoma, medulloblastoma, pineoblastoma, and neuroblastoma, all believed to arise from primitive neuroectodermal cells. Medulloblastoma is the most common pediatric primary malignant brain tumor and may disseminate via the CSF to encompass the spinal cord. Presentation of medulloblastoma is similar to ependymoma. Treatment usually involves a combination of resection and radiation given its high radiosensitivity. Prognosis is very good in children, if there is disappearance with treatment of both tumor on MRI and tumor cells within the CSF.

Meningioma

Meningiomas are usually extra-axial (arising outside of the brain proper), slow-growing, well-circumscribed, benign tumors arising from arachnoid cap cells, not the dura mater. Because of their slow-growing nature, they can be very large at the time of diagnosis. They can occur anywhere arachnoid cap cells exist, but are most common near the sagittal sinus, falx cerebri, and cerebral convexity. Tumors are usually apparent on plain radiographs and CT due to the presence of calcifications. On MRI and conventional angiography, these tumors often receive their blood supply from the external carotid artery. Surgical resection is the mainstay of treatment. Prognosis is usually excellent; however, some tumors may be recurrent and require additional resection. Malignant meningiomas are rare.

Pituitary Tumors

Pituitary adenomas usually arise from cells of the anterior pituitary gland. They may occur along with tumors of the parathyroids and pancreatic islet cells as part of multiple endocrine neoplasia type 1. Tumors are usually divided into functional (i.e., hormone secreting) and nonfunctional. The former usually present as a result of an endocrinologic disturbance related to the hormone secreted by the tumor. Functional tumors are usually smaller (<1 cm in diameter) at the time of diagnosis; hence, they are often called microadenomas. Macroadenomas are usually nonfunctional, present with symptoms related to their mass (i.e., headache or visual changes due to compression of the optic chiasm), and are larger at the time of diagnosis, usually greater than 1 cm in diameter. Panhypopituitarism may be caused by either tumor type due to compression of the normal functioning pituitary gland. Pituitary tumors may also present as apoplexy, which is the abrupt onset of headache, visual changes, ophthalmoplegia, and altered mental status secondary to hemorrhage, necrosis, or infarction within the tumor. Finally, tumors can invade the cavernous sinus or internal carotid artery or compress various cranial nerves, causing an array of symptoms. Treatment well may depend on tumor type. Prolactinomas are often initially treated medically with bromocriptine. Surgical resection via the transsphenoidal or open craniotomy approach is often curative for most pituitary tumors.

Acoustic Neuroma

Usually the result of a benign schwannoma involving the vestibular component of cranial nerve VIII within the internal auditory canal, an acoustic neuroma typically occurs as a single mass. However, bilateral tumors may occur as part of neurofibromatosis type 2. Common presenting symptoms include hearing loss, tinnitus, and disequilibrium. Larger tumors, which grow out of the internal auditory canal and into the cerebellopontine angle, may cause symptoms related to compression of cranial nerves, most commonly the facial nerve (cranial nerve VII) as well as the brainstem. Treatment usually consists of surgical resection with or without radiation therapy. Surgery usually involves intraoperative cranial nerve monitoring with electromyography or brainstem auditory evoked potentials. Prognosis is usually very good; however, recurrence of tumor is not uncommon.

Central Nervous System Lymphoma

This is a rare tumor that can arise as a primary brain tumor, also known as a microglioma, or via metastatic spread from a systemic lymphoma. Primary central nervous system lymphoma can occur anywhere within the brain but is most common in supratentorial locations, especially in deep gray matter or the corpus callosum. Primary central nervous system lymphoma is thought to be associated with a variety of systemic disorders including systemic lupus erythematosus, Sjögren's syndrome, rheumatoid arthritis, immunosuppressed states, and infection with Epstein-Barr virus. Symptoms depend on the location of the tumor. Diagnosis is made by imaging as well as biopsy. During biopsy, it may be reasonable to wait to administer corticosteroids, such as dexamethasone, until after pathologic findings are obtained since these tumors may be sensitive to steroids. As such, steroid-associated tumor lysis prior to performing a biopsy may result in failure to obtain an adequate sample to make the diagnosis. The mainstay of treatment is chemotherapy (including intraventricularly delivered drugs) and whole-brain radiation. Prognosis is poor despite treatment.

Metastatic Tumor

Metastatic brain tumors originate most often from primary sites in the lungs or breasts. Malignant melanoma, hypernephroma, and carcinoma of the colon are also likely to spread to the brain. Metastatic brain tumor is the likely diagnosis when more than one intracranial lesion is present.

Management of Anesthesia

Anesthetic management of patients undergoing tumor resection can be challenging since it may involve patients of any age group as well as a variety of intraoperative patient positions. Further, some procedures may be conducted with electrophysiologic monitoring, which may have implications for anesthetic choice and the use of muscle relaxants. Some procedures may even be conducted in awake patients to facilitate resection of a mass located near an eloquent region of brain, such as the motor cortex. Major goals during anesthesia include (1) maintenance of adequate perfusion and oxygenation of normal brain, (2) optimizing operative conditions to facilitate resection, (3) ensuring a rapid emergence from anesthesia at the conclusion of the procedure to facilitate neurologic assessment, and, when appropriate, (4) accommodating intraoperative electrophysiologic monitoring.

Preoperative Management

Preoperative evaluation of a patient with an intracranial tumor is directed toward identifying the presence or absence of increased ICP. Symptoms of increased ICP include nausea and vomiting, altered levels of consciousness, mydriasis and decreased reactivity of pupils to light, papilledema, bradycardia, systemic hypertension, and breathing disturbances. Evidence of midline shifts (>0.5 cm) on CT or MRI suggests the presence of increased ICP.

Patients with intracranial pathology may be extremely sensitive to the central nervous system depressant effects of opioids and sedatives. Drug-induced hypoventilation can lead to accumulation of arterial carbon dioxide and further increases in ICP. Likewise, drug-induced sedation can mask alterations in the levels of consciousness that accompany intracranial hypertension. Conversely, preoperative sedation can unmask subtle neurologic deficits that may not usually be apparent. This is thought to be a result of increased sensitivity of injured neurons to the depressant effects of various anesthetic and sedative agents. Considering all the potential adverse effects of preoperative medication, it is an inescapable conclusion that pharmacologic premedication should be used sparingly, if at all, in patients with intracranial tumors. Preoperative depressant drugs are particularly best avoided in patients with diminished levels of consciousness. In alert adult patients with intracranial tumors, benzodiazepines in small doses can provide anxiety relief without meaningfully affecting ventilation. Decisions to administer anticholinergic drugs or H₂-receptor antagonists are not influenced by the presence or absence of increased ICP.

Induction of Anesthesia

Anesthesia induction is achieved with drugs (e.g., thiopental, etomidate, propofol) that produce a rapid, reliable onset of unconsciousness without increasing ICP. This is often followed by a nondepolarizing muscle relaxant to facilitate tracheal intubation. Administration of succinylcholine may be associated with modest, transient increases in ICP. Mechanical hyperventilation of the patient's lungs is initiated with the goal of decreasing the PaCO₂ to near 35 mm Hg. Adequate depth of anesthesia and profound skeletal muscle paralysis should be achieved prior to laryngoscopy, as noxious stimulation or patient movement can abruptly increase CBF, CBV, and ICP.

Direct laryngoscopy for tracheal intubation is accomplished during profound skeletal muscle paralysis as confirmed by the absence of electrically evoked neuromuscular transmission. Additional doses of intravenous induction drugs, lidocaine 1.5 mg/kg IV, or potent short-acting opioids may blunt the responses laryngoscopy or other forms of intraoperative stimulation (e.g., placement of pinions, skin incision).

Abrupt, sustained increases in systemic blood pressure, particularly in areas of impaired cerebrovascular tone, may be accompanied by undesirable increases in CBF, CBV, and ICP, and followed by cerebral edema. Sustained hypotension must also be avoided, as brain ischemia can occur in the presence of decreased CPP. Skeletal muscle responses during tracheal intubation typically reflect inadequate anesthesia or incomplete skeletal muscle paralysis, both of which may confound management of ICP and brain volume. New-onset seizures or repeat episodes of seizures are another possible origin of unexpected movement. Following tracheal intubation, the patient's lungs are ventilated at a rate and tidal volume that maintain the PaCO₂ near 35 mm Hg. Positive end-expiratory pressure has a highly variable effect on ICP, resulting in increases, decreases, or no change in ICP. Hence, it should be used with caution, with attention paid to the ICP, MAP, and CPP effects of the intervention.

Maintenance of Anesthesia

The maintenance of anesthesia in patients undergoing surgical resection of supratentorial brain tumors is often achieved by combining drugs of various classes, including nitrous oxide, volatile anesthetics, opioids, barbiturates, and propofol. Although modest cerebrovascular differences can be demonstrated among different combinations of drugs, there is no evidence that any particular combination is significantly different from another in terms of effects on ICP and short-term patient outcome.

The use of nitrous oxide is controversial if there is any potential for venous air embolism (e.g., operations performed with patients in the sitting position). Despite theoretical concerns, the incidence of venous air embolism in sitting patients is not influenced by nitrous oxide use. Once a venous air embolism has been detected, nitrous oxide use must be discontinued out of concern that the embolus volume will expand, exacerbating the physiologic consequences. Both nitrous oxide and the potent volatile anesthetics have the potential to increase CBV and ICP as a result of direct cerebral vasodilation. However, low concentrations of volatile anesthetics (0.6–1.0 MAC) may be useful for preventing or treating increases in systemic blood pressure related to noxious surgical stimulation. Additionally, volatile anesthetic–associated increases in anesthetic depth and diminution of the physiologic responses to noxious stimuli will help preserve CBV and ICP. Administration of peripheral vasodilating drugs, such as nitroprusside or nitroglycerin, may increase CBV and ICP despite accompanying decreases in systemic blood pressure. This, in turn, can dramatically reduce CPP, which is dependent on both MAP and ICP. For this reason, vasodilating drugs are best used after craniotomy and opening of the dura.

Spontaneous movement by patients undergoing surgical resection of brain tumors must be prevented. Such movement could result in dangerous increases in ICP, herniation of the brain, or bleeding at the operative site, making surgical exposure difficult. Therefore, in addition to adequate depths of anesthesia, skeletal muscle paralysis is typically maintained during intracranial surgery.

Fluid Therapy

Relatively iso-osmolar solutions (e.g., 0.9% saline, lactated Ringer's solution) do not adversely affect brain water or edema formation, provided there is an intact blood-brain barrier and they are used in modest doses. In contrast, free water in hypo-osmolar solutions (e.g, 0.45% sodium chloride) is rapidly distributed throughout body water, including brain water, and may adversely affect ICP management. Hyper-osmolar solutions, such as 3% sodium chloride, initially tend to decrease brain water by increasing the osmolarity of plasma. Regardless of the crystalloid solutions selected, any solution administered in large amounts can increase CBV and ICP in patients with brain tumors. Therefore, the rate of fluid infusion should be titrated to maintain euvolemia with measures taken to avoid hypervolemia. Intravascular fluid volume depletion due to blood loss during surgery should be corrected with packed red blood cells or colloid solutions supplemented with balanced salt solutions. Glucose-containing solutions should be used with caution since hyperglycemia, in the setting of central nervous system ischemia, will exacerbate neuronal injury and worsen outcome.

Monitoring

The insertion of a peripheral arterial catheter is useful for continuous monitoring of systemic blood pressure and repetitive blood sampling. Capnography can facilitate ventilation and PaCO₂ management as well as detecting venous air embolism (see “Sitting Position and Venous Air Embolism”). Continuous ICP monitoring, although not routine, is of obvious value. Nasopharyngeal or esophageal temperature is monitored to prevent hyperthermia or uncontrolled hypothermia. A bladder catheter has utility for managing perioperative fluid volume. It is necessary if drug-induced diuresis is planned; in patients who have diabetes insipidus, syndrome of inappropriate antidiuretic hormone, or other aberrations of salt or water physiology; or if a lengthy surgical procedure is anticipated and bladder distention is a concern.

Intravenous access with large-bore catheters should be obtained, given the likelihood of bleeding and the need for transfusion or rapid administration of fluids. Central venous catheterization can be useful as reliable means of large-bore intravenous access, as well as a monitor of fluid status. Central venous cannulation also has utility during cases performed in the sitting position as a means to aspirate intracardiac air following venous air embolism. Transesophageal echocardiography can also be useful in sitting position cases to identify intravenous air and help assess cardiac function. Pulmonary artery catheterization should be considered in patients with cardiac disease.

A peripheral nerve stimulator is helpful for monitoring the persistence of drug-induced skeletal muscle weakness or paralysis. If paresis or paralysis of an extremity is associated with the brain tumor, it is important to appreciate resistance (decreased sensitivity) to nondepolarizing muscle relaxants in the paretic extremity, compared with the normal extremity ( Fig. 10A-4 ). Therefore, monitoring skeletal muscle paralysis on the paretic limb may be misleading. For example, the evoked response may be erroneously interpreted as inadequate skeletal muscle paralysis. Likewise, at the conclusion of surgery, the same response could be assumed to reflect recovery from the muscle relaxant when substantial neuromuscular block persists. In these instances, the altered muscle response to relaxants may reflect the proliferation of acetylcholine-responsive cholinergic receptors that can occur after denervation.

Monitoring the electrocardiographic activity is necessary to detect responses related to intracranial tumors or from surgery. Electrocardiographic changes can reflect increased ICP or, more importantly, surgical retraction or manipulation of the brainstem or cranial nerves. Indeed, the cardiovascular centers, respiratory control areas, and nuclei of the lower cranial nerves lie in close proximity in the brainstem. Manipulation of the brainstem may produce systemic hypertension and bradycardia or hypotension and tachycardia. Cardiac arrhythmias range from acute sinus arrhythmias to ventricular premature beats or ventricular tachycardia.

Postoperative Management

Ideally, the effects of anesthetics and muscle relaxants are dissipated or pharmacologically reversed at the conclusion of intracranial surgery. This facilitates monitoring the neurologic status and recognizing any adverse effects of the surgery. It is important to limit reaction to the tracheal tube as patients are awakening. Intraoperative use of narcotics and the optimal timing of extubation are of value. Lidocaine, 0.5 to 1.5 mg/kg IV, may also attenuate the physiologic response to the tracheal tube. However, it must be appreciated that this local anesthetic has general anesthetic properties and can produce central nervous system depression and reduce the activity of protective upper airway reflexes. If consciousness was depressed preoperatively or new-onset neurologic deficits are anticipated as a result of the surgical course, it may be best to delay tracheal extubation until return of airway reflexes are confirmed and spontaneous ventilation is sufficient to prevent CO₂ retention. Hypothermia must be considered a possible cause of slow postoperative awakening. Other causes of delayed emergence from anesthesia include residual neuromuscular block, residual effects of drugs with sedative effects (i.e., narcotics, benzodiazepines, volatile anesthetics), or a primary central nervous system event such as ischemia, hematoma, and tension pneumocephalus.

Following anesthesia, preexisting neurologic deficits may be exacerbated by the sedative effects of anesthetic agents making a subtle preoperative deficit appear more severe. This differential awakening is thought to be due to increased sensitivity of injured neurons to the depressant effects of anesthetic agents. Often, these deficits will disappear and neurologic function will return to its baseline state with time. Any persistent new deficit that does not quickly resolve should be further investigated.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

Hyperthyroidism

Hyperthyroidism

Signs and Symptoms

Hyperthyroidism refers to hyperfunctioning of the thyroid gland with excessive secretion of active thyroid hormones. The majority of cases (i.e., 99%) of hyperthyroidism result from one of three pathologic processes: Graves’ disease, toxic multinodular goiter, or a toxic adenoma. Regardless of the etiology, the signs and symptoms of hyperthyroidism are those of a hypermetabolic state. The patient is anxious, restless, and hyperkinetic and may be emotionally unstable. The skin is warm and moist, the face is flushed, the hair is fine, and the nails are soft and fragile. The eyes exhibit a wide-eyed stare with retraction of the upper eyelids. The patient may demonstrate increased sweating and complain of heat intolerance. Wasting, weakness, and fatigue of the proximal limb muscles is common. The patient usually complains of extreme fatigue but an inability to sleep. Increased bone turnover and osteoporosis may occur. A fine tremor of the hands and hyperactive tendon reflexes are common. Weight loss despite an increased appetite occurs secondary to increased calorigenesis. Bowel movements are frequent and diarrhea is not uncommon. The cardiovascular system is most threatened with hypermetabolism of peripheral tissues, increased cardiac work with tachycardia, arrhythmias (commonly atrial) and palpitations, a hyperdynamic circulation, increased myocardial contractility and cardiac output, and cardiomegaly. The etiology of cardiac responses is due to the direct effects of T₃ on the myocardium and the peripheral vasculature (see Table 16-8 ). Although cardiac failure rarely occurs, a thyrotoxic cardiomyopathy has been described with a lymphocytic and eosinophilic infiltration of the myocardium with fibrotic and fatty changes. Elderly patients with unexplained cardiac failure or rhythm disturbances, especially atrial in origin, should be evaluated for thyrotoxicosis.

Patients with subclinical hyperthyroidism are usually detected on routine laboratory screening. Most patients present with few if any signs or symptoms, although some may present with an elevated heart rate. Thyroid function tests reveal a normal T₃ and T₄ but a reduced TSH. Whether these patients should be treated is controversial. Benefits of treatment are not clearly established. If the TSH is between 0.1 and 0.5 mU/L, many clinicians will not treat. When the TSH drops below 0.1 mU/L, most patients receive treatment. Patients with subclinical hyperthyroidism are potentially at future risk of cardiac (atrial fibrillation) and central nervous system (emboli, cerebrovascular accident) complications.

Graves’ disease or toxic diffuse goiter occurs in 0.4% of the United States population and is the leading cause of hyperthyroidism. The disease typically occurs in females (female-to-male ratio is 7:1) between the ages of 20 and 40 years. Although the etiology is unknown, Graves’ disease appears to be a systemic autoimmune disease with thyroid-stimulating antibodies (long-acting thyroid stimulator, thyroid-stimulating immunoglobulins), binding to TSH receptors in the thyroid, activating adenyl cyclase and stimulating thyroid growth, vascularity, and hypersecretion of T₄ and T₃. The manifestations of the disease vary in intensity with the severity of the thyrotoxicosis, the age of the patient, the duration of the illness, and the involvement of other organ systems, especially cardiovascular. The disease is characterized by a classic triad of hyperthyroidism, exophthalmos, and dermopathy. The thyroid is usually diffusely enlarged, becoming two to three times normal size. Some glands secrete T₄ and T₃ at five to 15 times the normal amount. An ophthalmopathy occurs in 30% of cases and may include upper lid retraction, a wide-eyed stare, muscle weakness, proptosis, and an increase in intraocular pressure. Proptosis and muscle weakness are secondary to immunologic changes of extraocular muscles and retro-orbital tissues (i.e., edema, inflammation). When severe, the condition is termed malignant exophthalmos. Steroids, bilateral tarsorrhaphies, external radiation, or surgical decompression may be necessary in these cases. Fortunately, most cases are mild and follow a benign course and remit spontaneously. The dermopathy is characterized by edematous skin (pretibial myxedema) over the dorsum of the legs and feet and occurs in only 10% to 15% of cases.

Diagnosis

The diagnosis of Graves’ disease is confirmed by elevated F T₄, T₃, F T₄ index, and R T₃U and an elevated radioactive iodine uptake. The TSH level is often low, and thyroid-stimulating antibodies are increased. In severe hyperthyroidism, the basal metabolic rate is markedly increased (30%–60%).

Toxic multinodular goiters usually arise from long-standing simple goiters and therefore occur mostly in patients older than 50 years of age. They may produce the most extreme thyroid enlargements, with some weighing more than 2000 g. They may cause dysphagia from esophageal compression, and a choking sensation and possibly inspiratory stridor from tracheal compression, especially with extension of the mass into the thoracic inlet behind the sternum. In severe cases, superior vena cava obstruction syndrome may also occur. However, hypermetabolism is usually less severe than with Graves’ disease. There is no associated opthalmopathy or dermopathy. The diagnosis is confirmed by a thyroid scan demonstrating “hot” patchy foci throughout the gland or one or two “hot” nodules. Radioactive iodine uptake and serum T₄ and T₃ may only be slightly elevated. The goiter must be differentiated from a neoplasm, and a computed tomography (CT) scan and biopsy may be necessary.

A solitary toxic nodule (toxic adenoma) usually occurs in patients 30 to 40 years of age and may cause hyperthyroidism if the lesion exceeds 3 cm in diameter. The same diagnostic tests used for multinodular goiters are used for toxic adenomas.

An unusual presentation of thyrotoxicosis that may occur in association with Graves’ disease, multinodular goiter, or toxic adenoma is T₃ toxicosis. In these patients, the serum T₄ and F T₄ are normal or low, while T₃ is increased. These patients may be more likely to have a long-term remission after withdrawal of antithyroid drug therapy than patients with the usual form of thyrotoxicosis in whom production of both T₄ and T₃ is increased.

Treatment

The first line of treatment for hyperthyroidism is the antithyroid drug propylthiouracil (PTU) or methimazole (Tapazole). Methimazole is currently more popular because of its faster response time and its ability to be administered as a single daily dose. These agents interfere with the synthesis of thyroid hormones by inhibiting organification and coupling. PTU has the added advantage of inhibiting the peripheral conversion of T₄ to T₃. PTU is prescribed for adults as 200 to 300 mg orally every 8 to 12 hours and methimazole as 10 to 20 mg orally every 12 hours. A euthyroid state can almost always be achieved in 6 to 8 weeks with either drug if a sufficient dose is given. The delay in effect is secondary to the large store of hormones existing in the gland prior to initiating therapy. Once euthyroidism is achieved, the dose is reduced and continued for 6 to 12 months and in some cases for 24 months. After euthyroidism has been achieved, a natural remission of the disease will often occur, but only less than 40% of patients remain well indefinitely after discontinuation of medication. Side effects occur in 3% to 12% of patients, with agranulocytosis being the most serious.

Iodide in high concentration inhibits release of hormones from the hyperfunctioning gland. Its effects occur immediately but last for only several weeks. Therefore, iodide is usually reserved for preparing hyperthyroid patients for surgery, managing patients with actual or impending thyroid storm, or treating patients with severe thyrocardiac disease. There is no need to delay surgery in an otherwise well-controlled thyrotoxic patient to initiate iodide therapy. High concentrations of iodide decrease all phases of thyroid synthesis and release and result in reduced gland size and possibly a decrease in vascularity. Iodide is administered orally as an SSKI (saturated solution of potassium iodide) solution, three drops orally every 8 hours for 10 to 14 days. Antithyroid drug therapy should precede the initiation of iodide because iodide alone will increase thyroid hormone stores and exacerbate the thyrotoxic state. Although parenteral NaI has been withdrawn from clinical use, oral iodide is equally efficacious. The radiographic contrast dye ipodate or iopanoic acid (0.5–3.0 g every day) contains iodide and demonstrates beneficial effects similar to those of inorganic iodide. In addition, ipodate inhibits the peripheral conversion of T₄ to T₃ and may also antagonize thyroid hormone binding to receptors. It is especially useful in the preoperative preparation of the thyrotoxic patient, reducing T₃ levels by 50% to 75% in 6 to 12 hours. Lithium carbonate 300 mg orally every 6 hours may be given in place of potassium iodide or ipodate if the patient is allergic to iodide.

β-Adrenergic antagonists do not affect the underlying thyroid pathology but may relieve signs and symptoms of increased adrenergic activity such as anxiety, sweating, heat intolerance, tremors, and tachycardia. Propranolol 40 to 80 mg orally every 6 to 8 hours, esmolol, metoprolol, and atenolol are effective. Propranolol has added features of impairing the peripheral conversion of T₄ to T₃ and reducing metabolic rate. For emergency use, intravenous propranolol in 0.2- to 1.0-mg boluses followed by an infusion or an intravenous esmolol 0.5 mg/kg bolus followed by an infusion is titrated to restore a normal heart rate.

Ablative therapy with radioactive ¹³¹I or surgery is recommended for patients with Graves’ disease in whom antithyroid drugs were ineffective or toxic or in whom a relapse occurred after 1 to 2 years of drug treatment and for patients with toxic multinodular goiter or a toxic adenoma. Also, patients who fail to follow medical regimens or fail to return for periodic examinations are candidates.

Radioactive ¹³¹I therapy is the treatment of choice for hyperthyroidism in many large series because it is simple, effective, and economical. Standard doses deliver approximately 8500 rad to the thyroid and destroy the follicular cells. The remission rate is 80% to 98%. A major disadvantage of therapy is that 40% to 70% of treated patients become hypothyroid within 10 years. Patients are usually made euthyroid by antithyroid drugs prior to radioactive iodine therapy to avoid possible thyrotoxicosis from a radiation-induced thyroiditis. Radioactive iodine therapy has replaced subtotal thyroidectomy as the standard form of therapy for patients with Graves’ disease requiring ablative therapy.

Surgery (i.e., subtotal thyroidectomy or possibly total thyroidectomy) results in prompt control of disease and a lower incidence of hypothyroidism (10%–30%) than radioactive iodine. Subtotal thyroidectomy corrects thyrotoxicosis in more than 95% of patients with a mortality rate of less than 0.1%. Complications from surgery are a major disadvantage of this form of therapy and include, in addition to hypothyroidism, hemorrhage with tracheal compression, unilateral or bilateral damage to the recurrent laryngeal nerve(s), damage to the motor branch of the superior laryngeal nerve, and damage to or inadvertent removal of the parathyroid glands.

Preparation of the hyperthyroid patient for surgery is extremely important. For elective surgery, all patients should be made euthyroid with a course of an antithyroid drug (PTU or methimazole) for 6 to 8 weeks preoperatively. A low TSH value should not be a contraindication to surgery. TSH values remain suppressed from prolonged hyperthyroidism in patients who have normalized T₃ and T₄ values. In addition, potassium iodide (SSKI solution) should be given for 7 to 14 days prior to surgery to reduce the gland’s vascularity and hormonal release. β-Adrenergic blockers may be added in the preoperative period to control heart rate. Optimal patient response should dictate the timing of surgery.

For emergency surgery, an antithyroid drug (PTU or methimazole) should be administered even though it has a limited effect if taken for less than 2 weeks. No intravenous preparation is available, so the drug must be taken orally, via a nasogastric tube, or rectally. The antithyroid drug should precede iodide by 2 to 3 hours. Sodium iopanoate 500 mg orally twice daily and an intravenous β-blocker, preferably propranol, are essential for effective management. Glucocorticoids (dexamethasone 2 mg IV every 6 hours) should be administered to decrease hormone release and reduce the peripheral conversion of T₄ to T₃. This combination of medications is effective for any thyrotoxic patient presenting for thyroid or nonthyroid surgery. Euthyroidism can be achieved surprisingly quickly in approximately 5 to 7 days.

The treatment of hyperthyroidism during pregnancy includes low doses of antithyroid drugs. However, these drugs do cross the placenta and can cause fetal hypothyroidism. If the mother remains euthyroid on small doses of an antithyroid drug, the occurrence of fetal hypothyroidism is rare. Radioactive iodine treatment is contraindicated during pregnancy, and oral iodide therapy causes fetal goiter and hypothyroidism and is therefore contraindicated. The long-term use of propranolol is controversial since intrauterine growth retardation has been attributed to its use. Fortunately, pregnancy appears to attenuate the severity of hyperthyroidism and doses of antithyroid drugs can be kept low (i.e., PTU < 200 mg/day). If doses greater than 300 mg/day of PTU are needed during the first trimester, a subtotal thyroidectomy should be performed in the second trimester. Thyroid storm occurring in pregnancy is managed the same as in the nonpregnant patient.

Management of Anesthesia

In managing hyperthyroid patients for surgery, euthyroidism should definitely be established preoperatively. In elective cases, this may mean waiting a substantial time (6–8 weeks) for antithyroid drugs to become effective. In emergency cases, the use of an intravenous β-blocker, ipodate, cortisol, or dexamethasone and PTU is usually necessary. The anesthesiologist should be prepared to manage thyroid storm, especially in patients with uncontrolled or poorly controlled disease who present for emergency surgery. Premedication may include the use of a barbiturate, benzodiazepine, and/or a narcotic. Anticholinergic drugs (i.e., atropine) should be avoided since they may precipitate tachycardia and alter heat-regulating mechanisms. Intraoperatively, the need for invasive monitoring is determined on an individual basis and depends on the type of surgery to be performed and the medical condition of the patient. Controlled studies in hyperthyroid animals demonstrate no clinically significant increase in anesthetic requirements (i.e., minimum alveolar concentration, [MAC]). Establishing adequate anesthetic depth is extremely important to avoid exaggerated sympathetic nervous system (SNS) responses. Drugs that stimulate the SNS should be avoided (i.e., ketamine, pancuronium, atropine, ephedrine, epinephrine). No controlled patient studies have demonstrated one preferred anesthetic technique or anesthetic agent(s). For induction, thiopental, secondary to its thiourylene nucleus, decreases the peripheral conversion of T₄ to T₃ and may have a slight advantage over other induction agents. Succinylcholine and the nondepolarizing muscle relaxants with limited hemodynamic effects (e.g., vecuronium, rocuronium) have been used safely for intubation. Eye protection (eyedrops, lubricant, eye pads) is important, especially for patients with proptosis. For maintenance of anesthesia, any of the potent inhalation agents may be used. A concern in hyperthyroid patients is organ toxicity secondary to an increase in drug metabolism. Although animal studies demonstrate an increase in hepatotoxicity in hyperthyroid rats following exposure to isoflurane, no alterations in liver function have been demonstrated postoperatively in hyperthyroid patients rendered euthyroid preoperatively and administered this agent for surgery. Nitrous oxide and opioids are safe and effective in hyperthyroid patients. Muscle relaxants should be chosen based on their interaction with the SNS and their hemodynamic effects. Also, hyperthyroid patients may have co-existing muscle disease (e.g., myasthenia gravis) with reduced requirements for the nondepolarizing muscle relaxants necessitating careful titration to effect. Reversal of muscle relaxants should include glycopyrrolate instead of atropine in combination with an acetylcholinesterase inhibitor. For the treatment of intraoperative hypotension, a direct-acting vasopressor (phenylephrine) is preferred. Ephedrine, epinephrine, norepinephrine, and dopamine are avoided or administered in extremely low doses to prevent exaggerated hemodynamic responses. Regional anesthesia can be safely performed and in fact may be a preferred technique. Epinephrine containing local anesthetic solutions should be avoided. Fluids and phenylephrine are used to treat hypotension secondary to SNS blockade.

Removal of the thyrotoxic gland does not mean immediate resolution of thyrotoxicosis. The T_1/2 of T₄ is 7 to 8 days; therefore, β-blocker therapy may need to be continued in the postoperative period. Antithyroid drug therapy can be discontinued.

Thyroid storm and malignant hyperthermia can present with similar intraoperative and postoperative signs and symptoms (i.e., hyperpyrexia, tachycardia, hypermetabolism). Differentiation between the two may be extremely difficult. The preoperative detection of thyrotoxicosis (tremors, diaphoresis, fatigue, tachypnea, tachycardia, fever, an enlarged thyroid) is very important. Although thyrotoxicosis is an uncommon adult endocrine disorder, it is very rare in children. Regardless, thyrotoxicosis should be considered in the differential diagnosis of malignant hyperthermia in any age group.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

Huntington's Disease

Huntington's Disease

Huntington's disease is a premature degenerative disease of the central nervous system characterized by marked atrophy of the caudate nucleus and, to a lesser degree, the putamen and globus pallidus. Biochemical abnormalities include deficiencies in the basal ganglia of acetylcholine (and its synthesizing enzyme choline acetyltransferase) and γ-aminobutyric acid. Selective loss of γ-aminobutyric acid may decrease inhibition of the dopamine nigrostriatal system. This disease is transmitted as an autosomal dominant trait, but its delayed appearance until 35 to 40 years of age interferes with effective genetic counseling. Identification of the genetic defect may be useful for disease risk prediction in those who have inherited the defective gene, as applied to both prenatal and postnatal (including adult) testing.

Manifestations of Huntington's disease consist of progressive dementia combined with choreoathetosis. Chorea is usually considered the first sign of Huntington's disease; hence, the former designation of this disease as Huntington's chorea. Behavioral changes (e.g, depression, aggressive outbursts, mood swings) may precede the onset of involuntary movement by several years. Involvement of the pharyngeal muscles makes these patients susceptible to pulmonary aspiration. The disease progresses over several years, and accompanying mental depression makes suicide a frequent cause of death. The duration of Huntington's disease, from clinical onset to death, averages 17 years.

Treatment of Huntington's disease is symptomatic and is directed at decreasing the choreiform movements. Haloperidol and other butyrophenones may be administered to control the chorea and emotional lability associated with the disease. The most useful therapy for controlling involuntary movements is with drugs that interfere with the neurotransmitter effects of dopamine either via antagonism (i.e., haloperidol, fluphenazine) or via depletion of dopamine stores (i.e., reserpine, tetrabenazine).

Experience with the management of anesthesia in patients with Huntington's chorea is too limited to recommend specific anesthetic drugs or techniques. Preoperative sedation using butyrophenones such as droperidol or haloperidol may be helpful in controlling choreiform movements. The increased likelihood of pulmonary aspiration must be considered if pharyngeal muscles are involved. Nitrous oxide and volatile anesthetic use is acceptable. Thiopental, succinylcholine, and mivacurium have been administered without adverse effects, but decreased plasma cholinesterase activity, with prolonged responses to succinylcholine, has been observed. Likewise, it has been suggested that these patients may be sensitive to the effects of nondepolarizing muscle relaxants.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

DEGENERATIVE DISEASES OF THE BRAIN

DEGENERATIVE DISEASES OF THE BRAIN

Degenerative diseases of the central nervous system usually involve neuronal malfunction or loss within specific anatomic regions and represent a diffuse group of disease states.

Alzheimer's Disease

Alzheimer's disease is a chronic neurodegenerative disorder. It is the most common cause of dementia in patients older than 65 years of age, and the fourth most common cause of death from disease in patients older than 65. Diffuse amyloid-rich senile plaques and neurofibrillary tangles are the hallmark pathologic findings. There are also changes in synapses and the activity of multiple major neurotransmitters, especially involving acetylcholine and central nervous system nicotinic receptors. Two types of Alzheimer's disease have been described: early onset and late onset. Early-onset Alzheimer's disease usually presents before age 60 and is thought to be due to missense mutations on up to three genes leading to an autosomal dominant mode of transmission. Late-onset Alzheimer's disease usually develops after age 60, and genetic transmission appears to play a relatively minor role in the risk of developing this disorder. With both forms of the disease, patients typically develop progressive cognitive impairment that can consist of problems with memory as well as apraxia, aphasia, and agnosia. Definitive diagnosis is usually made on postmortem examination, usually making premortem diagnosis of Alzheimer's disease one of exclusion. There is currently no cure for Alzheimer's disease, and treatment usually focuses on control of symptoms. Pharmacologic options include cholinesterase inhibitors, such as tacrine, donepezil, rivastigmine, and galantamine. Pharmacologic therapy should be combined with nonpharmacologic therapy including caregiver education and family support. Despite treatment, the prognosis for patients with Alzheimer's disease is poor.

Patients with Alzheimer's disease may present for a variety of surgical interventions that are common in the elderly population. Patients are often confused and sometimes uncooperative, making monitored anesthesia care or regional anesthesia challenging. However, there is probably no one single anesthesia technique or agent that is superior in this group of patients. Shorter acting sedative/hypnotic drugs, anesthetic agents, and narcotics are preferred since they may allow a more rapid return to baseline mental status. Finally, one should be aware of potential drug interactions, especially prolongation of the effect of succinylcholine and relative resistance to nondepolarizing muscle relaxants due to the use of cholinesterase inhibitors.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

Hallervorden-Spatz Disease

Hallervorden-Spatz Disease

Hallervorden-Spatz disease is a rare autosomal recessive disorder of the basal ganglia. It follows a slowly progressive course from its onset during late childhood to death in approximately 10 years. No specific laboratory tests are diagnostic for this condition, and no effective treatment is known. Dementia and dystonia with torticollis, as well as scoliosis, are commonly present. Dystonic posturing is likely to disappear with the induction of anesthesia, although skeletal muscle contractures and bony changes may accompany the chronic forms of the disease, leading to immobility of the temporomandibular joint and cervical spine, even in the presence of deep general anesthesia or drug-induced skeletal muscle paralysis.

Management of anesthesia must consider the possibility of being unable to position these patients optimally for tracheal intubation following the induction of anesthesia. Noxious stimulation, as produced by attempted awake tracheal intubation, can intensify dystonia. For these reasons, induction of anesthesia may be achieved by inhalation and maintenance of spontaneous ventilation. Administration of succinylcholine is questionable, as skeletal muscle wasting and diffuse axonal changes in the brain, which may involve the upper motor neurons, could accentuate the release of potassium; however, succinylcholine has been reported to have been used safely. Offsetting this centrally mediated propensity for muscle wasting may be that chronic muscle hyperactivity produces muscular and cardiovascular effects similar to that of a trained athlete. Any required skeletal muscle relaxation is probably best provided by increased concentrations of volatile anesthetics or administration of nondepolarizing neuromuscular blocking drugs. Emergence from anesthesia is predictably accompanied by return of dystonic posturing.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

Cystic Fibrosis

Cystic Fibrosis

Cystic fibrosis is the most common life-shortening autosomal recessive disorder. It affects an estimated 30,000 persons in the United States.

Pathophysiology

The cause of cystic fibrosis is a mutation in a single gene on chromosome 7 that encodes the cystic fibrosis transmembrane conductance regulator. The result of this mutation is defective chloride ion transport in epithelial cells in the lungs, pancreas, liver, gastrointestinal tract, and reproductive organs. Decreased chloride transport is accompanied by decreased transport of sodium and water, resulting in dehydrated, viscous secretions that are associated with luminal obstruction as well as destruction and scarring of various exocrine glands. Pancreatic insufficiency, meconium ileus at birth, diabetes mellitus, obstructive hepatobiliary tract disease, and azoospermia are often present but the primary cause of morbidity and mortality in patients with cystic fibrosis is chronic pulmonary infection.

Diagnosis

The presence of a sweat chloride concentration higher than 80 mEq/L plus the characteristic clinical manifestations (cough, chronic purulent sputum production, exertional dyspnea) or a family history of the disease confirm the diagnosis of cystic fibrosis. Chronic pansinusitis is almost universal. The presence of normal sinuses on radiographic examination is strong evidence that cystic fibrosis is not present. Malabsorption with a response to pancreatic enzyme treatment is evidence of the exocrine insufficiency associated with cystic fibrosis. Obstructive azoospermia confirmed by testicular biopsy is also strong evidence of cystic fibrosis. Bronchoalveolar lavage typically shows a high percentage of neutrophils, a sign of airway inflammation. COPD is present in virtually all adult patients with cystic fibrosis and follows a relentless course.

Treatment

Treatment of cystic fibrosis is similar to that for bronchiectasis and is directed toward alleviation of symptoms (mobilization and clearance of lower airway secretions and treatment of pulmonary infection) and correction of organ dysfunction (pancreatic enzyme replacement).

Clearance of Airway Secretions

The abnormal viscoelastic properties of the sputum in patients with cystic fibrosis lead to sputum retention, resulting in airway obstruction. The principal nonpharmacologic approach to enhancing clearance of pulmonary secretions is chest physiotherapy with postural drainage. High-frequency chest compression with an inflatable vest and airway oscillation with a flutter valve may provide alternative methods to physiotherapy that are less time-consuming and do not require trained personnel.

Bronchodilator Therapy

Bronchial reactivity to histamine and other provocative stimuli is greater in patients with cystic fibrosis than in normal subjects. Bronchodilator therapy is considered if patients have an increase of 10% or more in FEV₁ in response to an inhaled bronchodilator.

Reduction in Viscoelasticity of Sputum

The abnormal viscosity of airway secretions is primarily due to the presence of neutrophils and their degradation products. DNA released from neutrophils forms long fibrils that contribute to the viscosity of the sputum. Recombinant human deoxyribonuclease I can cleave this DNA and increase the clearance of sputum in these patients.

Antibiotic Therapy

Patients with cystic fibrosis have periodic exacerbations of pulmonary infection that are identified primarily based on an increase in symptoms and in sputum production. Antibiotic therapy is based on identification and susceptibility testing of bacteria isolated from the sputum. In patients in whom cultures yield no pathogens, bronchoscopy to remove lower airway secretions may be indicated. Many patients with cystic fibrosis are given long-term maintenance antibiotic therapy in the hope of suppressing chronic infection and the development of bronchiectasis.

Management of Anesthesia

Management of anesthesia in patients with cystic fibrosis invokes the same principles as outlined for patients with COPD and bronchiectasis. Elective surgical procedures should be delayed until optimal pulmonary function can be ensured by controlling bronchial infection and facilitating removal of airway secretions. Vitamin K treatment may be necessary if hepatic function is poor or if absorption of fat-soluble vitamins from the gastrointestinal tract is impaired. Maintenance of anesthesia with volatile anesthetics permits the use of high inspired concentrations of oxygen, decreases airway resistance by decreasing bronchial smooth muscle tone, and decreases the responsiveness of hyperreactive airways. Humidification of inspired gases, hydration, and avoidance of anticholinergic drugs is important for maintaining secretions in a less viscous state. Frequent tracheal suctioning may be necessary.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

Bronchiectasis

Bronchiectasis

Bronchiectasis is a chronic suppurative disease of the airways that, if sufficiently widespread, may cause expiratory airflow obstruction similar to that seen with COPD. Despite the availability of antibiotics, bronchiectasis is an important cause of chronic productive cough with purulent sputum and accounts for a significant number of patients who develop massive hemoptysis.

Pathophysiology

Bronchiectasis is characterized by a localized, irreversible dilation of a bronchus caused by destructive inflammatory processes involving the bronchial wall. Bacterial or mycobacterial infections are presumed to be responsible for most cases of bronchiectasis. The most important consequence of bronchiectatic destruction of airways is an increased susceptibility to recurrent or persistent bacterial infection, reflecting impaired mucociliary activity and pooling of mucus in dilated airways. Once bacterial superinfection is established, it is nearly impossible to eradicate and daily expectoration of purulent sputum persists.

Diagnosis

The history of a chronic cough productive of purulent sputum is highly suggestive of bronchiectasis. Digital clubbing occurs in most patients with significant bronchiectasis and is a valuable diagnostic clue, especially since this change is not characteristic of COPD. Pulmonary function changes vary considerably and range from no change to alterations characteristic of COPD or restrictive lung disease. Computed tomography provides excellent images of bronchiectatic airways and can be used to confirm the presence and extent of the disease.

Treatment

Bronchiectasis is treated by administration of antibiotics and postural drainage. Periodic sputum culture guides antibiotic selection. Pseudomonas is the most common organism cultured. Hemoptysis can be controlled with appropriate antibiotic therapy. However, massive hemoptysis (>200 mL over a 24-hour period) may require surgical resection of the involved lung or selective bronchial arterial embolization. Postural drainage is useful to assist in expectoration of secretions that pool distal to the diseased airways. Chest physiotherapy with chest percussion and vibration is another aid for bronchopulmonary drainage. Surgical resection has played a declining role in the management of bronchiectasis during the modern antibiotic era and is only considered in the rare instance when severe symptoms persist or recurrent complications occur.

Management of Anesthesia

Prior to elective surgery, the pulmonary status of patients with bronchiectasis is optimized by antibiotic therapy and postural drainage. Airway management might include use of a double-lumen endobronchial tube to prevent spillage of purulent sputum into normal areas of the lungs. Instrumentation of the nares should be avoided because of the high incidence of chronic sinusitis in these patients.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

Arteriovenous Fistula

Arteriovenous Fistula  

Arteriovenous (AV) fistulas are direct communications between an artery and a vein without an intervening nidus of vessels. They commonly occur between meningeal vessels within the dura mater or between the carotid artery and venous sinuses within the cavernous sinus. Some AV fistulas are thought to spontaneously occur. Many others are associated with a previous traumatic injury or, in the case of carotid-cavernous fistulas, are associated with previous (presumably silent) rupture of an intracavernous carotid artery aneurysm. Dural AV fistulas commonly present with pulsatile tinnitus or headache, and of note, an occipital bruit can be appreciated in 24% of cases given that the occipital artery is a common arterial feeder. Treatment options include angiographically guided embolization or surgical ligation, bearing in mind the risk of rapid blood loss associated with surgical treatment.

Patients with carotid-cavernous AV fistulas often present with orbital or retro-orbital pain, arterialization of the conjunctiva, or visual changes. Diagnosis is made by magnetic resonance or conventional angiography. Embolization is usually an effective option for treatment.

Management of Anesthesia

Surgical resection of low-flow vascular malformations (i.e., venous angiomas and cavernous angiomas) is generally not associated with the degree of both intraoperative and postoperative complications as associated with the resection of high-flow vascular lesions (i.e., AVMs and AV fistulas). Further, since AVMs are often associated with multiple feeding and draining vessels, unlike AV fistulas which involve a single feeding and a single draining vessel, surgical resection of AVMs often pose great clinical challenges during resection and postoperative care.

Preoperatively, the patient with an intracranial vascular malformation should be evaluated for evidence of cerebral ischemia or increased ICP. The nature of the malformations, such as size, location, mechanism of venous drainage, presence of associated aneurysms, and a history of treatment, should be elicited since these factors may help one to anticipate perioperative complications such as the risk of intraoperative bleeding as well as postoperative complications. Adjunct medications, including antiepileptics (if the patient has a concurrent seizure disorder), should be administered. Patients exposed to preoperative angiography may experience fluid and electrolyte abnormalities secondary to the administration of hypertonic contrast material.

In addition to standard monitors, an arterial catheter placed prior to the induction of anesthesia may be helpful in that it will allow rapid evaluation of systemic blood pressure. Blood pressure control is critical, given that hypotension may result in ischemia in hypoperfused areas and hypertension may increase the risk of rupture of an associated aneurysm, worsen intraoperative bleeding, or worsen intracranial hypertension. For embolization or surgical resection of a vascular malformation in an eloquent region of brain, monitored anesthesia care is an attractive option. For cases requiring general anesthesia, a smooth, hemodynamically stable induction of general anesthesia is paramount. Thiopental, propofol, or etomidate are all effective and safe induction agents. Muscle relaxation should be accomplished with a nondepolarizing neuromuscular blocking agent such as succinylcholine may induce further increases in ICP as well as cause hyperkalemia if motor deficits are present. Techniques to blunt the hemodynamic responses to stimulating events such as laryngoscopy, pinion placement, and incision, should be used. These may include the administration of lidocaine, short-acting β-adrenergic antagonists (i.e., esmolol), and nitroprusside or deepening the anesthetic state with either higher concentrations of volatile anesthetics, small doses of induction agents, short-acting opioids, or intravenous lidocaine. Given the risk of severe and rapid intraoperative hemorrhage, especially with AVMs and AV fistulas, adequate intravenous access is essential. Further, central venous access may be useful in these cases to monitor volume status or for rapid administration of large volumes of fluids or blood products. A pulmonary artery catheter or transesophageal echocardiography can be useful in patients with cardiac disease.

With large or high-flow vascular malformations, frequent communication with the surgeon is of paramount importance because impressions of the lesions and the surgical and anesthetic requirements for safe resection may change during the operation. This is due, in part, to somewhat less than definitive imaging assessment preoperatively or changing surgical requirements during various stages of resection of a large, complex lesion. Hemodynamic stability, optimal surgical conditions, and rapid emergence at the end of surgery are appropriate goals when selecting maintenance techniques. Both intravenous and volatile-based techniques are appropriate and agents should be selected on a case-by-case basis.

Hypotonic and glucose-containing solutions should be avoided, given that the former can exacerbate cerebral edema and the latter can worsen the outcome from neurologic ischemia. Mild hyperventilation (PaCO₂ of 30–35 mm Hg) will help facilitate surgical exposure. Lumbar CSF drainage may also help to decrease intracranial volume and improve exposure. Cerebral edema can be a significant problem during AVM treatment. Because AVMs represent a high-flow, low-resistance vascular lesion, as arterial feeders are ligated during resection or embolization, blood flow is thus directed toward normal brain tissue, resulting in possible cerebral edema. Mechanisms to be considered for the treatment of cerebral edema include moderate hyperventilation as a temporizing measure, diuretics such as mannitol and furosemide, and blood pressure reductions. In extreme cases, high-dose barbiturate or propofol anesthesia, or temporary craniectomy, with postoperative ventilatory support, may be useful.

Most patients, however, should respond quite well to surgical resection, and emergence from anesthesia should be smooth and rapid. Agents such as β-adrenergic antagonists as well as lidocaine or nitroprusside can be used to control short-term hypertensive events. Prompt neurologic assessment should follow emergence.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

Acute Intrinsic Restrictive Lung Disease

Acute Intrinsic Restrictive Lung Disease

Pulmonary edema is due to leakage of intravascular fluid into the interstitium of the lungs and into the alveoli. Acute pulmonary edema can be caused by increased capillary pressure (hydrostatic or cardiogenic pulmonary edema) or by increased capillary permeability. Pulmonary edema typically manifests as bilateral symmetrical opacities on chest radiography. A perihilar distribution (“butterfly pattern”) of the lung opacity is common. However, this pattern of lung opacity is more commonly seen with increased capillary pressure than with increased capillary permeability. The presence of air bronchograms on chest radiograph suggests permeability pulmonary edema. Cardiogenic pulmonary edema is characterized by extreme dyspnea, tachypnea, and signs of sympathetic nervous system activation (hypertension, tachycardia, diaphoresis) that may be more pronounced than in patients with capillary permeability pulmonary edema. Pulmonary edema caused by increased capillary permeability is characterized by a high concentration of protein and secretory products in the edema fluid. Diffuse alveolar damage is typically present with the increased permeability pulmonary edema associated with acute respiratory distress syndrome (ARDS).

Aspiration Pneumonitis

Aspirated acidic gastric fluid is rapidly distributed throughout the lung and produces destruction of surfactant-producing cells and damage to the pulmonary capillary endothelium. As a result, there is atelectasis and leakage of intravascular fluid into the lungs producing capillary permeability pulmonary edema. The clinical picture is similar to that of ARDS. Arterial hypoxemia is typically present. In addition, there may be tachypnea, bronchospasm, and acute pulmonary hypertension. Chest radiographs may not demonstrate evidence of aspiration pneumonitis for 6 to 12 hours after the event. Evidence of aspiration, when it does appear, is most likely to be in the right lower lobe if the patient aspirated while in the supine position.

Measurement of gastric fluid pH is useful as it reflects the pH of the aspirated fluid. Measurement of tracheal aspirate pH is of no value because the aspirated gastric fluid is rapidly diluted by airway secretions. The aspirated gastric fluid is also rapidly distributed to peripheral lung regions so lung lavage is not useful unless there has been aspiration of particulate material.

Aspiration pneumonitis is best treated by delivery of supplemental oxygen and PEEP. Bronchodilation may be needed to relieve bronchospasm. There is no evidence that prophylactic antibiotics decrease the incidence of pulmonary infection or alter outcome. Corticosteroid treatment of aspiration pneumonitis is controversial. Despite the absence of confirmatory evidence that corticosteroids are beneficial, some will treat aspiration pneumonitis with very large doses of methylprednisolone or dexamethasone.

Neurogenic Pulmonary Edema

Neurogenic pulmonary edema develops in a small proportion of patients experiencing acute brain injury. Typically, this form of pulmonary edema occurs minutes to hours after central nervous system injury and may manifest during the perioperative period. There is a massive outpouring of sympathetic impulses from the injured central nervous system, resulting in generalized vasoconstriction and a shift of blood volume into the pulmonary circulation. Presumably, increased pulmonary capillary pressure leads to transudation of fluid into the interstitium and alveoli. Pulmonary hypertension and hypervolemia can also injure blood vessels in the lungs.

The association of pulmonary edema with a recent central nervous system injury should suggest the diagnosis of neurogenic pulmonary edema. The principal entity in the differential diagnosis is aspiration pneumonitis. Unlike neurogenic pulmonary edema, chemical pneumonitis resulting from aspiration frequently persists longer and is often complicated by secondary bacterial infection.

Treatment of neurogenic pulmonary edema is directed at the cause of the central nervous system injury, at decreasing intracranial pressure, and at support of oxygenation and ventilation. Diuretics should not be used unless there is hypervolemia because development of hypovolemic hypotension could aggravate the central nervous system injury.

Drug-Induced Pulmonary Edema

Acute noncardiogenic pulmonary edema can occur after administration of a number of drugs, especially opioids (heroin) and cocaine. High permeability pulmonary edema is suggested by high protein concentrations in the pulmonary edema fluid. Cocaine can also cause pulmonary vasoconstriction, acute myocardial ischemia, and myocardial infarction. There is no evidence that naloxone speeds resolution of opioid-induced pulmonary edema. Treatment of patients who develop drug-induced pulmonary edema is supportive and may include tracheal intubation for airway protection and mechanical ventilation.

High-Altitude Pulmonary Edema

High-altitude pulmonary edema may occur at heights ranging from 2500 to 5000 meters and is influenced by the rate of ascent to that altitude. The onset of symptoms is often gradual but typically occurs within 48 to 72 hours at high altitude. Fulminant pulmonary edema may be preceded by the less severe symptoms of acute mountain sickness. The etiology of this high permeability pulmonary edema is presumed to be hypoxic pulmonary vasoconstriction, which increases pulmonary vascular pressures. Treatment includes administration of oxygen and prompt descent from the high altitude. Inhalation of nitric oxide may improve oxygenation.

Reexpansion of Collapsed Lung

Rapid expansion of a collapsed lung may lead to pulmonary edema in that lung. The risk of reexpansion pulmonary edema after relieving a pneumothorax or pleural effusion is related to the amount of air or liquid that was present in the pleural space (>1 L increases the risk), the duration of collapse (> 24 hours), and the rapidity of reexpansion. High protein concentrations in the edema fluid suggest that enhanced capillary membrane permeability is important in the development of this form of pulmonary edema. Treatment of reexpansion pulmonary edema is supportive.

Negative-Pressure Pulmonary Edema

Negative-pressure pulmonary edema may follow relief of acute upper airway obstruction (postobstructive pulmonary edema) caused by postextubation laryngospasm, epiglottitis, tumors, obesity, hiccups, or obstructive sleep apnea in spontaneously breathing patients. The time at onset of pulmonary edema after relief of airway obstruction ranges from a few minutes to as long as 2 to 3 hours. Tachypnea, cough, and failure to maintain oxygen saturation above 95% are common presenting signs and may be confused with pulmonary aspiration or pulmonary embolism. It is possible that many cases of postoperative oxygen desaturation are due to unrecognized negative-pressure pulmonary edema.

The pathogenesis of negative-pressure pulmonary edema is related to the development of high negative intrapleural pressure by vigorous inspiratory efforts against an obstructed upper airway. High negative intrapleural pressure decreases the interstitial hydrostatic pressure, increases venous return, and increases left ventricular afterload. In addition, such negative pressure leads to intense sympathetic nervous system activation, hypertension, and central displacement of blood volume. Together these factors produce acute pulmonary edema by increasing the transcapillary pressure gradient.

Maintenance of a patent upper airway and administration of supplemental oxygen are sufficient treatment because this form of pulmonary edema is typically transient and self-limited. Mechanical ventilation may occasionally be needed for a brief period of time. Hemodynamic monitoring reveals normal right and left ventricular function. Central venous pressure and pulmonary artery occlusion pressure are normal. Radiographic evidence of pulmonary edema resolves within 12 to 24 hours.

Management of Anesthesia

Preoperative

Elective surgery should be delayed in patients with acute restrictive pulmonary disease, and every effort must be made to optimize cardiorespiratory function. Large pleural effusions may need to be drained. Persistent hypoxemia may require mechanical ventilation and PEEP. Hemodynamic monitoring may be useful in both the assessment and treatment of pulmonary edema.

Intraoperative

These patients are critically ill. Intraoperative management should be a continuation of critical care management and include a plan for intraoperative ventilator management. The best way to ventilate patients with acute respiratory failure and restrictive lung disease has not been determined by clinical trials. However, because the pathophysiology is similar to that of acute lung injury and because there is the risk of hemodynamic compromise and barotrauma with the use of large tidal volumes and high airway pressures, it is reasonable to ventilate with low tidal volumes (e.g., 6 mL/kg) with a compensatory increase in ventilatory rate (14–18 breaths per minute) attempting to keep the end-inspiratory plateau pressure less than 30 cm H₂O. Typical anesthesia ventilators may not be adequate for patients with severe ARDS and more sophisticated intensive care unit ventilators may be needed in occasional patients. Patients with restrictive lung disease typically breathe rapidly and shallowly, so tachypnea is likely during the weaning process and should not be used as the sole reason for delaying extubation if gas exchange and other assessments are satisfactory.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com

 

ACUTE CERVICAL SPINAL CORD INJURY

ACUTE CERVICAL SPINAL CORD INJURY

Cervical spine radiographs are obtained in a large fraction of patients who present with various forms of trauma for fear of missing occult cervical spine injuries. Nevertheless, the probability of cervical spine injury is minimal in patients who meet the following five criteria: (1) no midline cervical spine tenderness, (2) no focal neurologic deficits, (3) normal sensorium, (4) no intoxication, and (5) no painful distracting injury. Patients who meet these criteria do not require routine imaging studies to rule out occult cervical spine injury.

An estimated two thirds of trauma patients have multiple injuries that can interfere with cervical spine evaluation. Evaluation usually includes computed tomography or magnetic resonance imaging. Nevertheless, routine imaging may not be practical in some, considering the risk of transporting unstable patients. For this reason, standard radiographic views of the patient's cervical spine, often taken with a portable x-ray machine, are frequently relied on to evaluate the presence of cervical spine injury and associated instability. Regardless of the form of cervical spine imaging employed, the entire cervical spine including the body of the first thoracic vertebra must be seen and evaluated. Alignment of the vertebrae (lateral view), fractures (all views), and evaluation of disc and soft-tissue spaces are analyzed on the radiographic examination. The sensitivity of plain radiographs is less than 100%, and therefore the likelihood of cervical spine injury must be interpreted in conjunction with other clinical symptoms and risk factors. If there is any doubt, it is prudent to treat all acute cervical spinal injuries as potentially unstable.

Treatment of a cervical fracture dislocation entails immediate immobilization to limit neck flexion and extension. In addition, soft neck collars have almost no effect on limiting neck flexion, and neck extension is only modestly limited. Hard neck collars limit neck flexion and extension by only approximately 25%. Immobilization and traction as provided by halo-thoracic devices are most effective in preventing cervical spine movement. Manual in-line stabilization (the assistant's hands are placed on each side of the patient's face with the fingertips resting on the mastoid process with downward pressure against a firm table surface to hold the head immobile in a neutral position) is recommended to help minimize cervical spine flexion and extension during direct laryngoscopy for tracheal intubation.

Cervical spine movement during direct laryngoscopy is likely to be concentrated at the occipitoatlantoaxial area, suggesting an increased risk of spinal cord injury at this level in vulnerable patients, even with the use of manual in-line stabilization.

In addition to mechanical deformation of the spinal cord produced by movement of the neck in the presence of cervical spine injury, there is perhaps an even greater risk of compromise of the blood supply to the spinal cord produced by neck motion that elongates the cord, with resultant narrowing of the longitudinal blood vessels. In fact, maintenance of perfusion pressure may be more important than positioning for preventing spinal cord injury in the presence of cervical spine injury.

Management of Anesthesia

Patients with acute spinal cord transections often require special precautions during airway management. The key principle when performing direct laryngoscopy is to minimize neck movements during the procedure. However, fear of possible spinal cord compression (from an unstable cervical spine injury) must not prevent necessary airway intervention. Extensive clinical experience seems to support the use of direct laryngoscopy for orotracheal intubation provided that (1) maneuvers are taken to stabilize the head during the procedure (avoiding hyperextension of the patient's neck) and (2) evaluation of the patient's airway did not suggest the likelihood of any associated technical difficulty.

Topical anesthesia and awake fiberoptic laryngoscopy are an alternative to direct laryngoscopy if patients are cooperative and airway trauma—with ensuing blood, secretions, and anatomic deformities—does not preclude visualization with the fiberscope. Of note, coughing during both topicalization of the airway and fiberoptic intubation may result in cervical spine movement. It is reasonable to have an assistant maintain manual in-line stabilization of the cervical spine during both interventions. Another alternative is rapid-sequence induction of anesthesia with intravenous anesthetics and a muscle relaxant. When the cervical spine is unstable or there is a high index of suspicion for the presence of cervical spine injury, it is important to proceed carefully, as neck hyperextension could further damage the spinal cord. Nevertheless, there is no evidence of increased neurologic morbidity after elective or emergency orotracheal intubation of anesthetized or awake patients who have an unstable cervical spine if appropriate and safe steps are taken to minimize neck movement. Awake tracheostomy is reserved for the most challenging airway conditions, in which neck injury, combined with facial fractures or other severe anomalies of airway anatomy, make safely securing the airway by nonsurgical means difficult or unsafe. All factors considered, airway management in the presence of cervical spine injury should be dictated by common sense, not dogmatic approaches. Certainly, clinical experience supports the safety of a variety of techniques just described.

The absence of compensatory sympathetic nervous system responses makes patients with cervical or high thoracic spinal cord injury particularly vulnerable to dramatic decreases in systemic blood pressure following acute changes in body posture, blood loss, or positive airway pressure. To minimize these effects, liberal intravenous infusion of crystalloid solutions may be necessary to replete intravascular volume, which has been abruptly compromised by vasodilation. Likewise, acute blood loss should be replaced promptly. Electrocardiogram abnormalities are common during the acute phase of spinal cord injury, especially with cervical cord injuries. Breathing is best managed by mechanical ventilation, as abdominal and intercostal muscle weakness or paralysis, exacerbated by general anesthesia, increases the chances of respiratory failure with ensuing hypoxia and hypercapnia. Body temperature should be monitored and manipulated as patients tend to become poikilothermic below the spinal cord transection. Anesthetic maintenance is targeted at ensuring physiologic stability and facilitating tolerance of the tracheal tube. Volatile or injected anesthetics are satisfactory for this purpose. Nitrous oxide should be used with caution, given concerns for co-existing trauma and air entrainment in closed spaces (e.g., as occurs with basilar skull fracture or rib fracture, which could potentially contribute to pneumocephalus or pneumothorax, respectively). Arterial hypoxemia is common following spinal cord injury, emphasizing the need for continuous pulse oximetry and delivery of supplemental oxygen.

Muscle relaxant use should be determined by the operative site and the level of spinal cord transection. If muscle relaxants are necessary, the sympathomimetic effects of pancuronium makes this drug an attractive choice; however, other nondepolarizing muscle relaxants can be used safely. Succinylcholine is unlikely to provoke excessive release of potassium during the first few hours after spinal cord transection. Even in these instances, the benefits of succinylcholine, which include rapid onset of action and short duration of relaxation, should be weighed against potential side effects. Use of a nondepolarizing relaxant, with mask ventilation while employing cricoid pressure, is another alternative to airway management during anesthesia induction and prior to laryngoscopy. Benefits of the latter approach are that once the endotracheal tube is placed, the longer duration of the nondepolarizing relaxant has utility during patient positioning.

Copyright © 2008 Elsevier Inc. All rights reserved. - www.mdconsult.com