Extraoperative functional mapping of motor areas in epileptic patients by high-frequency cortical stimulation

The aim of this study was to investigate the usefulness of a short train of high-frequency (500 Hz) cortical stimulation to delineate the primary motor cortex (MI), supplementary motor area (SMA), primary somatosensory cortex (SI), supplementary sensory area (SSA), negative motor area (NMA), and supplementary negative motor area (SNMA) in patients with epilepsy who were undergoing functional mapping.

Seventeen patients were studied, all of whom underwent functional mapping using 50-Hz electrical stimulation. After these clinical evaluations, cortical stimulations with a short train of electrical pulses at 500 Hz were performed through subdural electrodes placed at the MI, SMA, SI, SSA, NMA, and SNMA, which had been identified by 50-Hz stimulation, and surrounding cortical areas, while surface electromyography readings were recorded.

Stimulation of the MI elicited motor evoked potentials (MEPs) in contralateral muscles. Stimulation of the SMA also induced MEPs in contralateral muscles but with longer latencies compared with the MI stimulation. Stimulation of the SMA did not elicit MEPs in ipsilateral muscles. Stimulation of the SI, SSA, NMA, and SNMA did not induce MEPs in any muscle. In one patient, MEPs were elicited without seizure induction by 500-Hz stimulation of the electrodes, whereas a 50-Hz stimulation of the same electrodes induced his habitual seizures.

Extraoperative high-frequency stimulation with MEP monitoring is a useful complementary method for cortical mapping without inducing seizure. Stimulation of SMA induces MEPs in contralateral muscles, with longer latencies compared with the stimulation of MI. This finding may be useful for the differentiation between MI and SMA, especially in the foot motor areas.