Electrically induced quantitative sudomotor axon reflex test in human volunteers

Chemically-induced quantitative sudomotor axon reflex test (QSART) and quantitative sensory testing (QST) are established clinical tools to assess thin fiber function in humans. We investigated stimulus-response functions to transcutaneous electrical stimuli of different current intensity (3.75 to 10mA) and pulse frequency (5 to 100Hz) comparing sweat output (ml/h/m(2)) and pain intensity (numeric rating scale [NRS], 0-10). Efferent sudomotor and afferent nociceptive responses were recorded after a 30s electrical stimulation period of distal (hand and foot) and proximal (forearm and thorax) body sites with 3 repetitive measures per body site. Sweat responses increased intensity dependently and peaked (~100ml/h/m(2)) at highest currents (10mA) that had been administered. Similarly, pain ratings increased with an escalating current intensity. At a constant stimulus intensity of 7.5mA, sudomotor activity was highest (~75ml/h/m(2)) at a stimulus frequency of 20Hz without further increase at 50 or 100Hz. In contrast, pain ratings increased frequency dependently and reached NRS 7 at 100Hz. Sudomotor activity, but not pain ratings, was significantly different between the body sites (p<0.05, ANOVA) with maximum sweat responses obtained at the ventral forearm. Varying response patterns for higher stimulation frequencies between sweating (peak maximum at 20Hz) and pain (maximum at 100Hz) might indicate differential axonal properties of sympathetic efferent and nociceptive afferent fibers. Electrically induced QSART could be a useful explorative and clinical method to indirectly study characteristics of frequency-dependent axonal excitability changes of sudomotor fibers.