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Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Boston: Butterworths; 1990.

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Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition.

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Chapter 75An Overview of the Autonomic Nervous System

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The autonomic nervous system consists of a somatic afferent pathway, a central nervous system integrating complex (brain and spinal cord), and two efferent limbs, the sympathetic and the parasympathetic nervous systems. Norepinephrine is the primary chemical neurotransmitter at sympathetic nerve endings, whereas acetylcholine is the primary chemical neurotransmitter at parasympathetic nerve endings. Acetylcholine, monamines (dopamine, etc.) and several other peptides further modulate the system centrally in the brainstem and hypothalamus.

Since the autonomic nervous system innervates all organs of the body, autonomic neuropathy is a multisystem disorder. Nevertheless, lesions tend to occur in either the afferent, central, or efferent pathways, leading to clinical findings that relate directly to the function of specific autonomic fibers.

History

Table 75.1 lists items of the history that are often abnormal in patients with autonomic dysfunction or sleep disturbances. The most common, by far, is orthostatic hypotension: dizziness or syncope on changing from the supine or sitting to the standing position. This occurs because of a marked reduction in the standing blood pressure, often unaccompanied by the usual compensatory increase in heart rate, because of the autonomic neuropathy.

Table 75.1. Symptoms of Autonomic Dysfunction and Sleep Disorders.

Table 75.1

Symptoms of Autonomic Dysfunction and Sleep Disorders.

Diabetic patients tend to develop parasympathetic autonomic neuropathy earlier than sympathetic autonomic neuropathy. Symptoms related to parasympathetic denervation of the bowel and bladder are often prominent. This contrasts with progressive autonomic failure, Shy–Drager syndrome, Riley–Day syndrome, and most drug-induced dysautonomias where the major involvement is sympathetic rather than parasympathetic.

Both sympathetic and parasympathetic autonomic fibers contribute to the arousal, erection and ejaculatory responses in sexual function. Hence, impotence is a relatively early symptom in most dysautonomias.

Physical Examination

Table 75.2 provides a list of the instruments needed for routine and specialized testing of the autonomic nervous system. A watch with a second hand is needed to record heart rates in different positions and to assure at least 7 and preferably 15 seconds" duration of forced expiration during the Valsalva maneuver. An ECG is needed to measure the heart rate responses during the Valsalva maneuver (i.e., ratio of the longest R–R interval post-Valsalva to the shortest R–R interval during the Valsalva).

Table 75.2. Instruments Used to Test the Autonomic Nervous System.

Table 75.2

Instruments Used to Test the Autonomic Nervous System.

An ECG is also needed to evaluate the parasympathetic (vagal) mediated changes in heart rate (sinus arrhythmia) during quiet deep breathing at a rate of six breaths per minute. The maximum heart rate during inspiration and the minimum heart rate during expiration are calculated for each breath and the test result is the mean of the difference for each of six breaths. Variation of 15 beats/minute or more is normal; variation of 11 to 14 beats/minute is borderline; and variation of less than 10 beats/minute is abnormal.

The reflex hammer and tuning fork are necessary for the evaluation of peripheral neuropathy, which is often present in patients who have autonomic neuropathy, especially diabetics and alcoholics.

Table 75.3 lists the signs of autonomic dysfunction that may be detected on physical examination. The hallmark of all dysautonomias is orthostatic hypotension. When orthostatic hypotension is suggested by the patient's history of dizziness or syncope upon standing, it must be documented by the physical examination. Be careful in measuring the standing blood pressure in such patients because the maximal decrease in blood pressure often occurs within the first 30 seconds, and the patient can slump to the floor with little or no measurable blood pressure until restabilized in the supine position. Have someone assist with the blood pressure and heart rate measurements in patients who are suspected of having severe orthostatic hypotension.

Table 75.3. Signs of Autonomic Dysfunction.

Table 75.3

Signs of Autonomic Dysfunction.

The pupil is an interesting example of a structure with reciprocal innervation from both sympathetic dilator and parasympathetic constrictor autonomic fibers. Adie's pupil is the classic example of parasympathetic denervation, whereas Horner's syndrome is the classic example of sympathetic denervation.

Copyright © 1990, Butterworth Publishers, a division of Reed Publishing.
Bookshelf ID: NBK399PMID: 21250240

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