Testing the autonomic nervous system.

Noninvasive, well-validated clinical tests of autonomic function are available and are in relatively wide use. These comprise an evaluation of sudomotor, cardiovagal, and adrenergic functions. These tests are very useful and have resulted in the recognition of milder degrees of autonomic failure and the presence of orthostatic intolerance that previously were missed. An extensive normative database and commercial equipment is available. The main limitations of the tests relate to the fact that they evaluate mainly the function of target tissues so that the status of autonomic reflexes are inferred. The tests can be affected by medications. There are more invasive, more time-consuming, or less validated tests of autonomic function that can directly record from sympathetic nerve fibers (microneurography) and mesenteric bed to study cerebral vasoregulation and the status of the veins.