Basic Tests of Autonomic Function

SUMMARY: Over the past 3 decades, tests of autonomic function have become increasingly standardized across most laboratories, particularly with commercially available equipment similar to other neurophysiologic tests. Most neurologically based laboratories perform four or five tests of autonomic function. Two of these, the sudomotor axon reflex response and the thermoregulatory sweat test (which some laboratories do not perform because it requires extensive equipment), examine sudomotor autonomic function. The remaining three, the cardiovascular response to a tilt table test, the cardiovascular response to the Valsalva maneuver, and the cardiac response to deep breathing examine cardiovascular autonomic function. Tests of sweating typically localize the lesion in the neuraxis, differentiating between central nervous system pathways, the spinal cord, or pre- or postganglionic roots or nerves. Tests of cardiovascular function delineate specific autonomic subsystem involvement, whether vagal parasympathetic as reflected in the deep breathing response and specific phases of the Valsalva maneuver or sympathetic adrenergic as reflected in the tilt table test and the other phases of the Valsalva. This review details the basic performance, analysis, and interpretation of these and a few other tests, with illustrative patient cases.