Functional localization of sensorimotor cortex by somatosensory evoked potentials produced by femoral nerve stimulation.

Cortical somatosensory evoked potentials (SSEPs) can be used to localize the central sulcus during a craniotomy. In particular, contralateral median nerve stimulation producing SSEPs can disclose the location of the central sulcus around the sensorimotor hand representation area. However, the median nerve cannot be stimulated in patients who undergo craniotomy at locations other than the hand representation area. The present study attempts to localize the central sulcus in the lateral surface of the brain near the interhemispheric fissure by stimulating the contralateral femoral nerve to produce SSEPs. Somatosensory evoked potentials were recorded between the superior lip of the interhemispheric fissure and 1.5 to 2 cm laterally in the cortex. Only seven of the 12 patients studied showed a phase reversal of the initial component across the central sulcus. The polarity was negative in the postcentral gyrus and positive in the precentral gyrus. The other five patients did not show a phase reversal of the initial component across the central sulcus. The amplitude was highest in the postcentral gyrus and the polarity was positive. Based on these results, the authors hypothesize that stimulating the contralateral femoral nerve to produce SSEPs and then analyzing the distribution of the SSEPs may provide a method for functional localization of the sensorimotor cortex around the interhemispheric fissure during craniotomy.