[Somatosensory potentials evoked in patients with epilepsy recorded from the scalp and from the exposed cortex during...

Somatosensory evoked potentials (SEP) were recorded from the scalp in 47 patients and intraoperatively from the exposed cortex in 30 persons with various forms of focal epilepsy. SEP from stimulation of the median nerve were recorded from the standard derivations: F3, F4, C3, C4, T3, T4, P3, P4 and/or electrodes placed on the exposed cortex. Primary cortical negative (N20) and positive (P20) responses were analyzed. Surface recordings were normal in majority of cases (79%). The most common abnormality was the lack of cortical waves in the damaged area (mostly frontal region) or the absence of cortical responses in the whole hemisphere when central area was involved. Comparing cortical responses obtained before and during operation we found that they were similar (polarization, latency) in the majority of cases there was no response before and intraoperatively. SEP are primarily influenced by organic brain damage symptoms, but not by functional brain changes.