Autonomic nervous system function.

Tests of autonomic function for the clinical autonomic neurophysiology laboratory should be noninvasive, sensitive, specific, reproducible, physiologically and clinically relevant, and not be excessively time-consuming. Laboratory evaluation is strongly indicated when the following are suspected: (1) generalized autonomic failure, (2) benign autonomic disorders that mimic life-threatening disorders, (3) distal small-fiber neuropathy, (4) the autonomic neuropathies, (5) sympathetically maintained pain, and (6) the postural orthostatic tachycardia syndrome. The preparation of the subject is important. Recommended tests should evaluate an adequate number of relevant autonomic systems. The Mayo laboratory utilizes two test batteries, the autonomic reflex screen and the reflex sympathetic dystrophy (RSD) screen. The former comprises the quantitative sudomotor axon reflex test (QSART), orthostatic blood pressure (BP) and heart rate (HR) responses to tilt, HR response to deep breathing, the Valsalva ratio, and beat-to-beat BP responses to the Valsalva maneuver, tilt, and deep breathing. The RSD screen comprises the recording of skin temperature, resting sweat output, and QSART distributions bilaterally.