Intraoperative localization of sensorimotor cortex by cortical somatosensory evoked potentials

Intraoperative cortical somatosensory potentials were evoked to localize the sensorimotor cortex under general anesthesia by contralateral median nerve stimulation in 12 patients. Eight patients underwent an excision of the epileptogenic focus for intractable complex partial seizures and the other 4 patients underwent tumor surgery following successful localization of the sensorimotor cortex. The relationships between the somatosensory evoked potentials and the anatomy of the sensorimotor cortex allowed rapid and accurate localization of the sensorimotor hand area: (1) the P2 recorded from the postcentral gyrus had a positivity of 49.0 microV on the average with a mean latency of 26.9 msec; (2) the N1 and P2 consistently showed phase-reversal across the Rolandic fissure; and (3) maximum SEP amplitudes were recorded from the hand representation area of the sensorimotor cortex. The neurophysiological and neurosurgical results demonstrate the feasibility of recording cortical SEP's under general anesthesia and the improvement of safety in surgical excisions near the sensorimotor cortex.