Tests of parasympathetic cardiovagal function include the analysis of heart rate response to standing (the 30:15 ratio), heart rate variation with deep breathing, and the Valsalva ratio. Breathing tests measure the heart rate and blood pressure response to breathing exercises such as the Valsalva maneuver.

Tests of sympathetic adrenergic vascular function include blood pressure analysis while standing, the Valsalva maneuver, sustained handgrip, and cold water immersion. The tilt-table test monitors the blood pressure and heart rate response to changes in posture and position, simulating what occurs when the patient stands up after lying down. The patient lies flat on a table, which is then tilted to raise the upper part of the body. Normally, the body compensates for the drop in blood pressure that occurs when a person stands up by narrowing the blood vessels and increasing the heart rate. This response may be slowed or abnormal if the person has autonomic neuropathy. A simpler way to test for postural changes in blood pressure involves standing for a minute, then squatting for a minute, and then standing again. Blood pressure and heart rate are monitored throughout this test. Breathing tests measure how the heart rate and blood pressure respond to breathing exercises such as the Valsalva maneuver.

Tests of sympathetic cholinergic sudomotor function include the sympathetic skin response (SSR), quantitative sudomotor axon reflex test, sweat box testing, and quantification of sweat imprints. The quantitative sudomotor axon reflex test (QSART) evaluates the nerves that regulate the sweat glands’ response to stimulation. A small electrical current is passed through capsules, placed on the patient’s forearm, foot, and leg, while a computer analyzes how the nerves and sweat glands react. The patient may feel warmth or a tingling sensation during the test.

During this test, the patient is coated with a powder that changes color with sweating. The patient enters a chamber with slowly increasing temperature, which will eventually make the patient perspire. Digital photos document the results. The sweat pattern may help confirm a diagnosis of autonomic neuropathy or other causes of decreased or increased sweating.

Pupil function is tested pharmacologically and with pupillographic techniques.

The SSR is a potential generated in skin sweat glands. This response originates from activation of the reflex arch by different kinds of stimuli. The response typically produces biphasic or triphasic slow wave activity with relatively stable latency and variable amplitude. In healthy subjects younger than 60 years of age the response is always present in all extremities. The SSR is most frequently used in diagnosing the functional impairment of non-myelinated postganglionic sudomotor sympathetic fibers in peripheral neuropathies, but it is a poor man’s test, with inadequate sensitivity and specificity for autonomic dysfunction, and there is no close correlation between the presence or absence of the SSR and the severity of autonomic dysfunction. The reader is recommended to read the article by Claus et al. to gain further understanding of the methodology and interpretation of autonomic studies (http://​www.​clinph-journal.​com/​pb/​assets/​raw/​Health%20​Advance/​journals/​clinph/​Chapter7.​1.​pdf).