Effects of stimulus intensity on latency and conduction time of short-latency somatosensory evoked potentials

We studied the effect of stimulus intensity on latencies of short-latency s omatosensory evoked potentials (SSEP) by measuring both onset and peak late ncies individually. The latencies of N9, N13, N20 and N9-N13 peripheral con duction time (PCT) of median nerve (MN) SSEP, and N8, N23, P37 and N8-N23 P CT of tibial nerve (TN) and sural nerve (SN) SSEP significantly shortened w ith increasing stimulus intensity by onset latency measurement. However, th ose latencies by peak latency measurement were less significantly shortened or had only a trend of latency shortening without statistical significance , in contrast to PCT, N13-N20 central conduction time (CCT) of MN-SSEP and N23-P37 CCT of TN- or SN-SSEP showed no latency changes with the increased stimulus intensity by both onset and peak latencies measurement. As peak la tencies had greater interindividual variability than onset latencies shown by larger standard deviation, shortening of onset latencies were more consi stent than that of peak latencies. We think shortening of onset latencies i ndicates the recruitment of faster conduction fiber along with increased st imulus intensity. As the degree of latency shortening was less if stimulus intensity was above 2.5 times sensory threshold, the stimulus intensity gre ater than 2.5 times the sensory threshold should be used for clinical appli cation.