Paradoxical shortening of sympathetic skin response latency at distal recording sites.

Objective: The purpose of this study was to investigate and to discuss the neurophysiological mechanism of paradoxical shortening of the sympathetic skin response (SSR) latency at distal recording sites.

Methods: The latency and peak-to-peak amplitude of SSRs evoked by magnetic stimuli were analyzed. Eight active electrodes were placed on the palmar (anterior) and dorsal (posterior) sides of the hand (forearm) proximal to the distal arrangement.

Results: SSRs from two palm sites had significantly shorter latencies and larger amplitudes than the SSRs at the other six sites, including the proximal sites of the forearm.

Conclusions: This finding indicated that the SSR latency at different sites was not linearly prolonged as the distance of the recording sites from the proximal to distal areas increased. The paradoxical shortening of the latency and the large amplitude of the SSR from the palm can be explained by a recent model of the equivalent current dipole caused by the Na+ concentration gradient. The high density of sweat glands in the palm possibly produced the present findings.

Significance: We should carefully interpret the sudomotor conduction velocity derived from latency difference between two sites, especially for thermal and emotional sweating.