Autonomic testing can distinguish primary from secondary autonomic disorders, true autonomic neuropathy from conditions that mimic it, and psychogenic from organic conditions. It can also help to differentiate progressive diseases and serve as a means of monitoring disease progression and response to treatment.
Referrals to Mayo Clinic for autonomic evaluation are increasingly coming from both within and outside the field of neurology, as physicians develop a wider appreciation for the role of autonomic testing in differential diagnosis and management.
Routine autonomic tests of sudomotor, cardiovagal, and adrenergic function are noninvasive. Because test results are quantifiable, responses from the 3 systems can be compared to determine selective and/or relative autonomic dysfunction.
For example, a patient might have moderate involvement of the cardiovagal system and severe involvement of adrenergic function without impacting sudomotor function. Or, both the cardiovagal and adrenergic systems may be normal in the face of isolated anhidrosis, suggesting the possibility of chronic idiopathic anhidrosis. Chronic idiopathic anhidrosis is a more benign disorder that does not progress to widespread autonomic failure and for which symptomatic and sometimes specific treatment is available.
Only by testing all 3 systems can such diagnostically informative patterns emerge.
The quantitative sudomotor axon reflex test (QSART) is used to evaluate postganglionic sudomotor function. The thermoregulatory sweat test (TST) is used to evaluate both preganglionic and postganglionic function over the entire anterior body surface. When evaluated together in the same patient, TST and QSART can differentiate preganglionic from postganglionic lesions.
The distribution of abnormal sweat responses measured by QSART and TST is of diagnostic importance for a number of conditions such as peripheral neuropathy.
For example, small-fiber neuropathy with the symptom of burning feet can be associated with idiopathic disease and also with diabetes. In such cases, the most distal sites may have abnormal QSART and TST responses with more proximal sites becoming involved as the disease progresses.
Using a 10-point composite autonomic severity score that they developed, Mayo physicians found that sudomotor testing is highly sensitive in identifying clinical distal small-fiber neuropathy in patients who have normal or unrelated abnormalities on electromyographic testing.
The 2 main tests of cardiovagal function in the autonomic test sequence are heart rate response to deep breathing and the Valsalva ratio, which involves several calculations and up to 4 maneuvers. The measure used for Valsalva ratio is beat-to-beat blood pressure. Once an invasive technique, measurement of beat-to-beat blood pressure can now be done with a recording device placed on the patient's finger.
Beat-to-beat blood pressure under various laboratory conditions is a proven method of testing adrenergic function for many conditions, including orthostatic hypotension (OH). OH is well recognized as a potential consequence of Parkinson's disease and diabetes and is increasingly recognized as a common disorder among the elderly.
Symptoms of OH, such as fatigue and impaired concentration, can be subtle and difficult to diagnose. Even when mild, symptoms of OH can be debilitating and markedly affect activities of daily living. Severe and sustained OH can induce syncope, with resultant falls and injury. In younger patients, symptoms may include palpitations, anxiety, and nausea and may be indicative of autonomic neuropathy.
Adrenergic testing helps distinguish OH syncope from psychogenic disorders and from other conditions that induce loss of consciousness, such as seizures and transient ischemic attacks. Autonomic tests also can determine severity of OH, an important factor when considering behavioral, pharmacologic, and nonpharmacologic treatments.
The Mayo Clinic autonomic testing laboratory highlights the complexity of the autonomic system. Although physicians cannot dissect all aspects of autonomic function, autonomic testing helps detect deficits that may not have been diagnosed, and determine if the problems are benign or represent a true autonomic failure and, if so, quantify its severity and distribution.