Bilateral sympathetic skin response following nociceptive stimulation: study in healthy individuals

Bilateral sympathetic skin response (SSR) was evaluated in 25 normal subjects aged (29 +/- 5 years). The stimulation (an electrical pulse train randomly applied to the sural nerve) was equal to 0.5-1.0-1.5 times the nociceptive flexion reflex (RIII) thresholds. This method allowed us to quantify the pain threshold, since a close relationship between the RIII threshold and subjective pain threshold has been described. Decrease in latency, increase in amplitude and duration were observed when increasing intensity of stimulation was used. The right-side stimulation produced significantly shorter latencies and higher amplitudes than the left one. The asymmetry index showed a trend of larger responses on the left hand and shorter latencies on the right hand in males. Females displayed less left-right asymmetry than males. A positive correlation was recognized between RIII threshold and amplitude and duration of SSR. Test-retest evaluation in 12 individuals revealed good reproducibility of SSR in terms of latency and duration, while amplitudes showed large variability. The accuracy of SSR response concerning latency and duration was better (range 0.88-0.92) when 1.5 RIII threshold was used, showing that painful stimulation should be used to obtain reliable SSR in clinical practice.