Somatosensory evoked potentials after head injury: The significance of the double peak

Abstract

The calculation of central somatosensory conduction time by subtracting the peak latency of the cervical potential from that of the initial cortical response in patients in posttraumatic coma provides both a reliable indication of outcome plus an assessment of brain damage. The interpretation of such recordings is confounded in a minority of patients by the presence in the cortical SEP of a biphasic cortical negativity, giving the impression that the initial cortical response (N20) has bifurcated into two subcomponents. Depending upon which subcomponent is assumed to be N20, an individual recording may ambiguously appear to have a central conduction time either within normal limits or very abnormally prolonged. The aetiology and significance of this double peak phenomenon was studied in 30 patients with severe head trauma. It is concluded that the first of the subcomponents invariably represents the genuine N20 potential irrespective of its relative amplitude. The second subcomponent is assumed to be N25, a minor cortical potential which can be recorded in only a limited number of normal subjects. The double peak effect appears to be created by a distortion in the early cortical waveform probably due to the characteristic reduction in its amplitude following head injury.