Hypofunction or Aberration in Function of the Sympathetic Nervous System (Dysautonomia)

Disorders of the sympathetic nervous system include Shy-Drager syndrome, Riley-Day syndrome, Lesch-Nyhan syndrome, Gill's familial dysautonomia, diabetic dysautonomia, and the dysautonomia of spinal cord transection.

Although individuals can function well without an adrenal medulla, a deficient peripheral sympathetic nervous system occurring late in life poses major problems for many facets of life^{[213] [214] [215] [216] [217] [218] [219] [220] [221] [222] [223]} ; nevertheless, perioperative sympathectomy or its equivalent has been recommended by some.^{[224] [225] [226] [227] [228] [229] [230] [231] [232] [233] [234] [235] [236] [237] [238]} A primary function of the sympathetic nervous system appears to be regulation of BP and intravascular fluid volume during changing of body position. Common features of all the syndromes of hypofunctioning of the sympathetic nervous system are orthostatic hypotension and decreased beat-to-beat variability in heart rate. These conditions can be caused by deficient intravascular volume, deficient baroreceptor function (as also occurs in carotid artery disease ^[239] ), abnormalities in CNS function (as in Wernicke or Shy-Drager syndrome), deficient neuronal stores of norepinephrine (as in idiopathic orthostatic hypotension^[214] and diabetes^[19] ), or deficient release of norepinephrine (as in traumatic spinal cord injury^[213] ). These patients may have an increased number of available adrenergic receptors (a compensatory response) and an exaggerated response to sympathomimetic drugs.^[240] In addition to other abnormalities, such as retention of urine or feces and deficient heat exchange, hypofunctioning of the sympathetic nervous system is often accompanied by renal amyloidosis. Thus, electrolyte and intravascular fluid volume status should be evaluated preoperatively. Because many of these patients have cardiac abnormalities, intravascular fluid volume might be assessed preoperatively with a Swan-Ganz catheter or transesophageal echocardiography rather than measurement of CVP (also see Chapter 32 and Chapter 33 ).

Inasmuch as functioning of the sympathetic nervous system is not predictable in these patients, we generally use slow, gentle induction of anesthesia and treat sympathetic excess or deficiency by infusing, with careful titration, drugs that directly constrict (phenylephrine) or dilate (nitroprusside) blood vessels or that stimulate (isoproterenol) or depress (esmolol) the heart rate. We prefer these drugs to agonists or antagonists, which may indirectly release catecholamines. A 20% perioperative mortality rate for 2600 patients with spinal cord transection has been reported,^[221] thus indicating that such patients are difficult to manage and deserve particularly close attention.

After reviewing 300 patients with spinal cord injuries, Kendrick and coworkers^[241] concluded that autonomic hyperreflexia syndrome does not develop if the lesion is below spinal dermatome T7. If the lesion is above that level (splanchnic outflow), 60% to 70% of patients experience extreme vascular instability. The trigger to this instability, or a mass reflex involving noradrenergic and motor hypertonus,^[215] can be a cutaneous, proprioceptive, or visceral stimulus (a full bladder is a common initiator). The sensation enters the spinal cord and causes a spinal reflex, which in normal persons is inhibited from above. Sudden increases in BP are sensed in the pressure receptors of the aorta and carotid sinus. The resulting vagal hyperactivity produces bradycardia, ventricular ectopia, or various degrees of heart block. Reflex vasodilation may occur above the level of the lesion and result in flushing of the head and neck.

Depending on the length of time since spinal cord transection, other abnormalities may occur. Acutely (i.e., <3 weeks from the time of spinal injury), retention of urine and feces is common and, by elevating the diaphragm, may impair respiration. Disimpaction of the intestine alleviates this respiratory problem. Hyperesthesia is present above the lesion; reflexes and flaccid paralysis are present below the lesion. The intermediate period (3 days to 6 months) is marked by a hyperkalemic response to depolarizing drugs.^[220] The chronic phase is characterized by a return of muscle tone, a positive Babinski sign, and frequently, the occurrence of hyperreflexia syndromes (e.g., mass reflex, see earlier).

Thus, in addition to meticulous attention to perioperative intravascular volume and electrolyte status, the anesthesiologist should know—by history taking, physical examination, and laboratory data—the status of the patient's myocardial conduction (as revealed by ECG), the status of renal functioning (by noting the ratio of

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