The autonomic nervous system (ANS) regulates physiologic processes, such as blood pressure, heart rate, body temperature, digestion, metabolism, fluid and electrolyte balance, sweating, urination, defecation, sexual response, and other processes. Regulation occurs without conscious control, i.e., autonomously. The ANS has two major divisions: the sympathetic and parasympathetic systems. Many organs are controlled primarily by either the sympathetic or parasympathetic system, although they may receive input from both; occasionally, functions are reciprocal (e.g., sympathetic input increases heart rate; parasympathetic decreases it).
The sympathetic nervous system is catabolic and activates fight-or-flight responses. Thus, sympathetic output increases heart rate and contractility, bronchodilation, hepatic glycogenolysis and glucose release, BMR (basal metabolism rate), and muscular strength; it also causes sweaty palms. Less immediately-life-preserving functions (e.g., digestion, renal filtration) are decreased.
The parasympathetic nervous system is anabolic; it conserves and restores. Gastrointestinal secretions and motility (including evacuation) are stimulated, heart rate is slowed, and blood pressure decreases.
Disorders of the ANS can affect any system of the body; they can originate in the peripheral or central nervous system and may be primary or secondary to other disorders. Symptoms suggesting autonomic dysfunction include orthostatic hypotension, heat intolerance, nausea, constipation, urinary retention or incontinence, nocturia, impotence, and dry mucous membranes. If a patient has symptoms suggesting autonomic dysfunction, cardiovagal, adrenergic, and sudomotor tests are usually done to help determine severity and distribution of the dysfunction.
Cardiovagal innervation testing evaluates heart rate response to deep breathing and to the Valsalva maneuver, via electrocardiogram rhythm strip. If the ANS is intact, heart rate varies with these maneuvers; the ratio of longest to shortest R-R interval (Valsalva ratio) should be 1.4 or greater.
Vasomotor adrenergic innervation testing evaluates response of beat-to-beat blood pressure to the head-up tilt and Valsalva maneuver. The head-up tilt shifts blood to dependent parts, causing reflex responses. The Valsalva maneuver increases intrathoracic pressure and reduces venous return, causing blood pressure changes and reflex vasoconstriction. In both tests, the pattern of responses is an index of adrenergic function.
The quantitative sudomotor axon reflex test (QSART) evaluates integrity of postganglionic neurons using iontophoresis; electrodes filled with acetylcholine are placed on the legs and wrist to stimulate sweat glands, and the volume of sweat is then measured. The test can detect decreased, absent, or persistent (after stimulus discontinuation) sweat production. The silastic sweat imprint differs from QSART in that the recording is an imprint of the sweat droplets appearing as indentations on silastic material.
The thermoregulatory sweat test (TST) evaluates both preganglionic and postganglionic pathways. After a dye is applied to the skin, patients enter a closed compartment that is heated to cause sweating. Sweating causes the dye to change color, so that areas of anhidrosis and hypohidrosis are apparent and can be calculated as a percentage of body surface area.
Sympathetic skin response (SSR) provides an index of sweat production by measuring change in skin resistance following random electrical stimulation over the palms and soles.
Quantitative direct and indirect axon reflex testing (QDIRT) is defined by Illigens and Gibbons (2009) as a novel new technique to evaluate the postganglionic sympathetic cholinergic sudomotor function by measuring the direct and axon-reflex mediated sweat response in a dynamic fashion. Sweat glands are stimulated by acetylcholine iontophoresis and sweat is displayed via an activator dye followed by digital photographs over time.
Pupillometry measures the responses of the pupil to light. Evaluating pupil size and response to light is an established tool in the clinical setting. Traditionally, this is measured using a penlight and a pupil gauge, which may be subjective. The NeurOptics® NPi™-100 Pupillometer is a hand-held, portable, infrared device used to measure pupil size and the pupillary light reflex. (11)
A variety of monitoring and testing equipment can be used for ANS testing; one example is the ANX 3.0™. In 1994, the Ansar Group, Inc. received U.S. Food and drug Administration (FDA) 510(K) clearance for their new autonomic nervous system monitoring technology; in 2004 Ansar introduced the ANX 3.0, which is their latest generation non-invasive, real-time, digital monitor of autonomic nervous system functioning. The ANX 3.0 monitors both branches of the ANS simultaneously.