6. Neurophysiological methods of autonomic nervous system testing in multiple sclerosis

Abstract

Autonomic dysfunction (AD) is commonly seen in patients with multiple sclerosis (MS). The most common manifestations of AD in MS patients include bladder or bowel dysfunction, impairment of sexual performance, pupillomotor or sweating alterations and orthostatic hypotension. Furthermore, AD is supposed to be related to fatigue. The incidence and severity of the symptoms related to AD correlated with the degree of disability. The lifetime prevalence of autonomic impairment reaches up to 80 % in MS population. Besides medical history and validated questionnaires, neurophysiological testing of cardiovascular and sudomotor functions are most commonly used for evaluation of autonomic nervous system impairment in MS patients. Of the cardiovascular tests, analysis of heart-rate variability in the time- and frequency-domain has repeatedly been published as a suitable method for evaluation of cardiovascular autonomic nervous system status in MS patients (the latter being mainly represented by spectral analysis of heart-rate variability). Autonomic challenge manoeuvres (suited for activation of sympathetic or parasympathetic nervous system) are also frequently used for this purpose and comprise Valsalva manoeuvre, deep metronomic breathing, response to active standing, sustained handgrip test, cold pressor test or cold face test. Furthermore, orthostatic challenge testing represented by head-up tilting (besides above mentioned active standing) can be useful in MS patients. Sudomotor functions in MS are most frequently tested using the sympathetic skin response (mainly because of wide availability of testing equipment), which seems to be less sensitive comparing cardiovascular tests. The availability of other testing methods, which may be used for the sweating evaluation (thermoregulatory sweat test, quantitative sudomotor axon reflex or similar tests) is lower, but they seem to have higher sensitivity in evaluation of sudomotor dysfunction in multiple sclerosis.