[Sensory nerve conduction velocity determined by somatosensory evoked potentials in patients with hereditary motor and sensory neuropathy type I].

We evaluated sensory nerve conduction velocity (SCV) determined by somatosensory evoked potentials (SEPs) components in 2 cases of hereditary motor and sensory neuropathy whose sensory nerve action potentials (SNAPs) were not evoked. SEPs elicited by median or ulnar nerve stimulation at the wrist or elbow were recorded from 5 scalp electrodes with ear reference. In both cases, no parietal N20 or frontal P20 component was identified, but a clear parietal P27 component was noted. The onset latency of the P27 component elicited by wrist or elbow stimulation was identified as the point where the superimposed 5 waves diverged. Then, the SCV was obtained by dividing the distance between the wrist and elbow by the onset latency difference. Tibial SCV was also determined by a similar method in one patient. The SCVs of the median, ulnar and tibial nerves were markedly decreased but 10-30% faster than the motor nerve conduction velocities measured concomitantly. We showed that a multichannel scalp recording of SEPs makes it possible to determine the onset latencies of the cortical components. So, SEP testing should be applied to obtain the SCVs in patients whose SNAPs were unrecordable.