The autonomic nervous system (ANS) (Fig. 189-1) 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.

FIGURE 189-1

Schematic representation of the autonomic nervous system. (From Moskowitz MA: 9—Diseases of the autonomic nervous system. Clin Endocrinol Metab 6:745, 1977.)

Key features of the ANS are summarized in Table 189-1. 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.