Comparison of somatosensory and somatomotor evoked responses to median nerve and digital nerve stimulation

Abstract

Scalp recorded cerebral and myogenic evoked responses to stimulation of the median nerve at the wrist and digital nerve at the index finger were summated and compared in eleven normal adults. Myogenic responses were differentiated from cerebral responses on the basis that the former occurred only with applied local muscle tension or were radically augmented by it, whereas the latter were relatively unaffected by muscle tensing maneuvers. Myogenic and cerebral responses had similar latencies, but their distributions were different. Myogenic potentials for both stimulus sites were widespread, bilateral and their maximal amplitudes and phase reversals occurred at electrodes overlying the contracting muscles. An almost linear relationship existed between the amount of applied muscle tension and the amplitude of the myogenic response. Early cerebral responses for both stimulus sites had a clear contralateral predominance, whereas the later potentials were more generalized. The cerebral response to median nerve stimulation was greater in amplitude, shorter in latency, better defined and similar in configuration to the digital nerve evoked response. Therefore, in terms of amplitude, definition of the response and potential myogenic contamination, digital nerve stimulation offers no advantage for evoking the somatosensory response. In subjects with prominent myogenic responses, recordings surrounding the site of maximal cerebral response showed no area where a cerebral response could be recorded free of potential myogenic contamination. This suggests that the contaminating effect of myogenic responses cannot be eliminated by the judicious choice of recording locations. Since myogenic responses are widely distributed and often much greater in amplitude and similar in latency to the cerebral response, they may potentially cause very serious problems in the interpretation of the scalp response. However, they do not appear to pose an insurmountable problem in normal cooperative subjects if care is taken to ensure relaxation of the scalp musculature.