Posterior tibial and sural nerve somatosensory evoked potentials in dystrophia myotonica.

Somatosensory evoked potentials (SEPs) were recorded in a group of 18 patients with dystrophia myotonica and in 28 control subjects after stimulating the right and left posterior tibial (SEP-PT) and sural (SEP-S) nerves at the ankles. Recording electrodes were placed in the popliteal fossae, overlying the L3 spinal vertebrae, and at the appropriate scalp sites. In all control subjects and dystrophia myotonica subjects SEP-PT latencies were shorter than equivalent SEP-S latencies, probably reflecting conduction along group I muscle afferents and along slower conducting cutaneous afferents, respectively. Intergroup comparisons revealed prolonged absolute and interpeak latencies in the dystrophia myotonica group, showing both peripheral and central somatosensory pathway involvement. Individual abnormal latencies which exceeded the control group mean plus 3 standard deviations were found in 66% of the dystrophia myotonica group, mainly due to prolonged peripheral conduction times. Results pointed to the concomitant involvement of both the posterior tibial and sural nerve somatosensory pathways in dystrophia myotonica.