Chapter 2 . 4 Somatosensory evoked potentials

Somatosensory evoked potentials (SEPs) are the electrical potentials generated in sensory pathways at peripheral, spinal, subcortical and cortical levels of the nervous system. SEPs can be elicited from almost any nerve, although the median and posterior tibial nerves are usually chosen in clinical practice. Recently up-dated reviews on SEP recording, normal waveforms, normative data and clinical applications can be found in Jones (1993) and MauguieÁre (1999). Recommendations from an IFCN committee published in 1994 (Nuwer et al. 1994) have been updated in this chapter. The short latency responses occur within the ®rst 60 ms after an electrical stimulus. Later middle latency and long latency SEPs show a wider range of normal variability making their clinical use more dif®cult. SEPs are usually evoked by bipolar transcutaneous electrical stimulation applied on the skin over the trajectory of peripheral nerves. Electrical stimuli used in clinical practice (see below) produce a twitch in the muscles innervated by the stimulated nerve when it contains a contingent of motor ®bers. At this intensity the rapidly conducting large myelinated ®bers, including ®bers subserving touch and joint sensation but also muscle afferents, are activated because of their higher resistance. Muscle afferents have been shown to elicit cortical SEPs 79 Recommendations for the Practice of Clinical Neurophysiology: Guidelines of the International Federation of Clinical Physiology (EEG Suppl. 52) Editors: G. Deuschl and A. Eisen q 1999 International Federation of Clinical Neurophysiology. All rights reserved. Published by Elsevier Science B.V.