Early Cortical Median Nerve Somatosensory Evoked Potentials

Subcortical and early cortical median nerve somatosensory evoked potentials (SEPs) were examined in 26 patients in hypoxic coma. The amplitude ratio between the negative slope of the scalp response and the following positive trough was determined. The latency difference between the major negative component recorded from the upper neck, N14, and the initial negative potential from the scalp, N20, was also measured. The mean of these parameters in both hemispheres was referred to as mean central conduction time (MCCT) and mean amplitude ratio (MAR). There was a significant difference in MAR in patients with a bilaterally recordable scalp response between those with a good and those with a bad outcome, but no difference in MCCT. Comparison of SEP findings with postmortem examinations suggests that a reduced MAR may yield an estimate of cortical damage in hypoxic coma.