The autonomic nervous system (ANS) (Fig. 189-1. Schematic Representation of the Autonomic Nervous System) innervates the entire neuraxis and permeates all organ systems. It regulates bp, heart rate, sleep, glandular, pupillary, and bladder and bowel function. It maintains organ homeostasis and operates automatically, its full importance becomes recognized only when ANS function is compromised, resulting in dysautonomia.
Key features of the ANS are summarized in Table 189-1 Functional Consequences of Normal ANS Activation. Responses to sympathetic or parasympathetic activation are frequently antagonistic; partial activation of both systems allows for simultaneous integration of multiple body functions.
Consider disorders of autonomic function in the differential diagnosis of pts with unexplained orthostatic hypotension (OH), sleep dysfunction, impotence, bladder dysfunction (urinary frequency, hesitancy, or incontinence), diarrhea, constipation, upper gastrointestinal symptoms (bloating, nausea, vomiting of old food), impaired lacrimation, or altered sweating (hyperhidrosis or hypohidrosis).
OH is often the most disabling feature of autonomic dysfunction. Syncope results when the drop in bp impairs cerebral perfusion (Chap. 52 Syncope). Other manifestations of impaired baroreflexes are supine hypertension, a fixed heart rate regardless of posture, postprandial hypotension, and a high nocturnal bp. Many pts with OH have a preceding diagnosis of hypertension. Most causes of OH are not neurologic in origin; these must be distinguished from neurogenic causes.
| APPROACH TO THE PATIENT | ||
Autonomic Nervous System DisordersThe first step in the evaluation of OH is to exclude treatable causes. History should include a review of medications that may cause OH (e.g., diuretics, antihypertensives, antidepressants, ethanol, opioids, insulin, dopamine agonists, and barbiturates); precipitation of OH by medications may also be the first sign of an underlying autonomic disorder. History may reveal an underlying cause for symptoms (e.g., diabetes, Parkinson's disease) or identify causative mechanisms (e.g., cardiac pump failure, reduced intravascular volume). Any relationship of symptoms to meals (splanchnic pooling), standing on awakening in the morning (intravascular volume depletion), ambient warming (vasodilatation), or exercise (muscle arteriolar vasodilatation) should be sought. Physical examination includes measurement of supine and standing pulse and bp. OH is defined as a sustained drop in systolic (≥20 mmHg) or diastolic (≥10 mmHg) bp within 2-3 min of standing. In nonneurogenic causes of OH (such as hypovolemia), the bp drop is accompanied by a compensatory increase in heart rate of >15 beats/min. A clue to neurogenic OH is aggravation or precipitation of OH by autonomic stressors (such as a meal, hot tub/hot bath, and exercise). Neurologic evaluation should include a mental status examination (to exclude neurodegenerative disorders such as Lewy body dementia), cranial nerve examination (impaired downgaze in progressive supranuclear palsy, abnormal pupils with Horner's or Adie's syndrome), motor tone (Parkinsonian syndromes), motor strength and sensory examination (polyneuropathies). In pts without a clear initial diagnosis, follow-up evaluations every few months or whenever symptoms worsen may reveal the underlying cause. Autonomic testing: Autonomic function tests are helpful when history and physical examination findings are inconclusive, to detect subclinical involvement, or to follow the course of an autonomic disorder. Heart rate variation with deep breathing is a measure of vagal function. The Valsalva maneuver measures changes in heart rate and bp while a constant expiratory pressure of 40 mmHg is maintained for 15 s. The Valsalva ratio is the maximum heart rate during the maneuver divided by the minimum heart rate following the maneuver; the ratio reflects cardiovagal function. Tilt-table beat-to-beat bp measurements in the supine, 70° tilt, and tilt-back positions can be used to evaluate orthostatic failure in bp control in pts with unexplained syncope. Most pts with syncope do not have autonomic failure; the tilt-table test can be used to diagnose vasovagal syncope with high sensitivity, specificity, and reproducibility. Other tests of autonomic function include the quantitative sudomotor axon reflex test (QSART) and the thermoregulatory sweat test (TST). The QSART provides quantitative measure of regional autonomic function mediated by acetylcholine (ACh)-induced sweating. The TST provides a qualitative measure of sweating in response to a standardized elevation of body temperature. |
Disorders of the Autonomic Nervous System 
Autonomic disorders may occur with many disorders of the central and/or peripheral nervous systems (Table 189-2 Classification of Clinical Autonomic Disorders). Diseases of the CNS may cause ANS dysfunction at many levels, including hypothalamus, brainstem, or spinal cord.
Multiple system atrophy (MSA) is a progressive neurodegenerative disorder comprising autonomic failure (OH and/or a neurogenic bladder) combined with either parkinsonism (MSA-p) or cerebellar signs (MSA-c), often with progressive cognitive dysfunction. Dysautonomia is also common in advanced Parkinson's disease and in dementia with Lewy bodies.
Spinal cord injury may produce autonomic hyperreflexia affecting bowel, bladder, sexual, temperature-regulation, or cardiovascular functions. With spinal cord lesions above T6, increased autonomic discharges (autonomic dysreflexia) can be elicited by irritation of bladder, skin, or muscles. Bladder distention from palpation, catheter insertion, catheter obstruction, or urinary infection is a common and correctable trigger of autonomic dysreflexia. Dangerous increases or decreases in body temperature can result from an inability to experience the sensory accompaniments of heat or cold exposure below the level of injury.
Peripheral neuropathies affecting small myelinated and unmyelinated fibers of the sympathetic and parasympathetic nerves are the most common cause of chronic autonomic insufficiency (Chap. 196 Peripheral Neuropathies, Including Guillain-Barré Syndrome). Autonomic involvement in diabetes mellitus typically begins ∼10 years after onset of diabetes and slowly progresses. Diabetic enteric neuropathy produces gastroparesis, nausea and vomiting, malnutrition, achlorhydria, and bowel incontinence. Impotence, urinary incontinence, pupillary abnormalities, and OH may occur as well. Prolongation of the QT interval increases risk of sudden death. Autonomic neuropathy occurs in sporadic and familial forms of amyloidosis. Pts typically present with distal, painful polyneuropathy. Alcoholic polyneuropathy produces symptoms of autonomic failure when the neuropathy is severe, and autonomic involvement contributes to the high mortality rates associated with alcoholism. Attacks of acute intermittent porphyria (AIP) are associated with tachycardia, sweating, urinary retention, and hypertension; other prominent symptoms include anxiety, abdominal pain, nausea, and vomiting. BP fluctuation and cardiac arrhythmias can be severe in Guillain-Barré syndrome. Autoimmune autonomic ganglionopathy presents as the subacute development of autonomic failure with OH, enteric neuropathy (gastroparesis, ileus, constipation/diarrhea), flaccid bladder, loss of sweating, sicca complex, and a tonic pupil. Onset may follow a viral infection; serum antibodies to the α3 subunit of the ganglionic ACh receptor (α3AChR) are diagnostic, and some pts appear to respond to immunotherapy. Rare pts develop dysautonomia as a paraneoplastic disorder (Chap. 79 Neurologic Paraneoplastic Syndromes). There are five known hereditary sensory and autonomic neuropathies (HSAN I-V).
Botulism is associated with blurred vision, dry mouth, nausea, unreactive or sluggishly reactive pupils, urinary retention, and constipation. Postural orthostatic tachycardia syndrome (POTS) presents with symptoms of orthostatic intolerance (not OH), including shortness of breath, light-headedness, and exercise intolerance accompanied by an increase in heart rate but no drop in bp. Primary hyperhidrosis affects 0.6-1.0% of the population; the usual symptoms are excessive sweating of the palms and soles. Onset is in adolescence, and symptoms tend to improve with age. Although not dangerous, this condition is socially embarrassing; treatment with sympathectomy or local injection of botulinum toxin is often effective.
Complex Regional Pain Syndromes
Complex regional pain syndrome (CRPS) type I is a regional pain syndrome that usually develops after tissue trauma. Allodynia (the perception of a nonpainful stimulus as painful), hyperpathia (an exaggerated pain response to a painful stimulus), and spontaneous pain occur. Symptoms are unrelated to the severity of the initial trauma and are not confined to the distribution of a single peripheral nerve. CRPS type II is a regional pain syndrome that develops after injury to a peripheral nerve, usually a major nerve trunk. Spontaneous pain initially develops within the territory of the affected nerve but eventually may spread outside the nerve distribution.
- Early mobilization with physical therapy or a brief course of glucocorticoids may be helpful for CRPS type I or II. Chronic glucocorticoid treatment is not recommended.
- Current treatment paradigms are multidisciplinary with a focus on early mobilization, physical therapy, pain management, pt education, and psychological support.
| TREATMENT | ||
Autonomic Nervous System Disorders
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