The autonomic nervous system regulates such important functions as blood pressure (BP), heart rate, thermoregulation, respiration, gastrointestinal, bladder, and sexual function. Autonomic dysfunction can occur as a result of many diseases that affect autonomic pathways. The clinician's role is to seek out symptoms of dysautonomia, but it is then necessary to determine if these symptoms are really due to involvement of autonomic systems. In the past, methods to evaluate autonomic function has been unavailable or too invasive. Recent advances in technology and the development/selection of autonomic function tests have resulted in the availability of quantitative, non-invasive, and reproducible tests and have made autonomic function testing accessible to the clinician. The focus of this review is to briefly describe a number of tests that are available to the clinician and how they might help in clinical situations.
The goals of autonomic function tests are summarized in clinical terms, they help the clinician diagnose the presence of dysautonomia, its distribution and severity, and since they are quantitative, whether it is getting better or worse. Goal #1 is to evaluate the severity and distribution of sudomotor, cardiovagal, and adrenergic function using non-invasive quantitative tests (described below). Goal #2 is to diagnose limited or restricted autonomic failure. When autonomic testing first began, the goal was to diagnose generalized autonomic failure alone. With increasing sophistication, we can now diagnose dysautonomia confined to a single system or area. One example is distal small fiber neuropathy (DSFN), where unmyelinated fibers to the toes and feet are affected, causing loss of sweating and pain. Another example is chronic idiopathic anhidrosis,1 where widespread sudomotor failure occurs and adrenergic and cardiovagal functions remain intact. Goal #3 is to diagnose and evaluate orthostatic intolerance. Head-up tilt (HUT) will allow the laboratory to diagnose orthostatic hypotension (OH). It now is recognized that more subtle alterations, such as the postural tachycardia syndrome (POTS), can be diagnosed on HUT by an excessive heart rate response. Goal #4 is to monitor the course of dysautonomia. The laboratory permits the clinician to quantitatively determine if the condition is getting better or worse and the rate of change. For instance, the rate of change is quite slow in Parkinson's disease and much more rapid in multiple system atrophy (MSA). Goal #5 is to monitor response to treatment either clinically or in research (Goal #6). Autonomic testing is very useful when incorporated into clinical trials. It is possible to provide a quantity of autonomic failure and determine whether treatment is making this number get better or worse. We have used this approach in evaluating if IVIG will work in treatment of autoimmune autonomic ganglionopathy.
Modern autonomic function tests can non-invasively evaluate the severity and distribution of autonomic failure. They have sufficient sensitivity to detect even subclinical dysautonomia. Standard laboratory testing evaluates cardiovagal, sudomotor and adrenergic autonomic functions. Cardiovagal function is typically evaluated by testing heart rate response to deep breathing at a defined rate and to the Valsalva maneuver. Sudomotor function can be evaluated with the quantitative sudomotor axon reflex test and the thermoregulatory sweat test. Adrenergic function is evaluated by the blood pressure and heart rate responses to the Valsalva maneuver and to head-up tilt. Tests are useful in defining the presence of autonomic failure, their natural history, and response to treatment. They can also define patterns of dysautonomia that are useful in helping the clinician diagnose certain autonomic conditions. For example, the tests are useful in the diagnosis of the autonomic neuropathies and distal small fiber neuropathy. The autonomic are characterized by severe generalized autonomic failure