Autonomic Function Tests: Some Clinical Applications

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 neuropathies (such as those due to diabetes or amyloidosis) are characterized by severe generalized autonomic failure. Distal small fiber neuropathy is characterized by an absence of autonomic failure except for distal sudomotor failure. Selective autonomic failure (which only one system is affected) can be diagnosed by autonomic testing. An example is chronic idiopathic anhidrosis, where only sudomotor function is affected. Among the synucleinopathies, autonomic function tests can distinguish Parkinson's disease (PD) from multiple system atrophy (MSA). There is a gradation of autonomic failure. PD is characterized by mild autonomic failure and a length-dependent pattern of sudomotor involvement. MSA and pure autonomic failure have severe generalized autonomic failure while DLB is intermediate.