Abstracts

To determine the cerebral EEG substrates of scalp EEG seizure patterns, and thus to assess the value of scalp ictal recording in the lateralization and localization of seizure onset zones in patients with temporal lobe epilepsy. We recorded simultaneously 26 channels of scalp EEG with sub-temporal supplementary electrodes and 46-98 channels of intracranial EEG in presurgical candidates with temporal lobe epilepsy. Subdural electrodes covered the anterior two-thirds of the temporal lobe in all patients. We studied the influence of intracranial EEG source area and synchrony at seizure onset on corresponding scalp EEG correlates. Eighty-six simultaneous intracranial- and scalp-recorded seizures from 23 patients were evaluated. Thirty-four intracranial ictal discharges (40%) from 9 patients (39%) had sufficient source area (namely [gt] 10 cm²) and synchrony at seizure onset to produce a nearly simultaneous focal scalp EEG ictal pattern. Forty-one intracranial ictal discharges (48%) from 10 patients (43%) gradually achieved the necessary source area and synchrony over several seconds to generate a scalp EEG ictal pattern. These scalp rhythms were lateralized, but less localizable. Eleven intracranial ictal discharges (13%) from 4 patients (17%) recruited the necessary source area, but lacked sufficient synchrony to generate clearly localized or lateralized scalp EEG ictal patterns. Sufficient source area and synchrony are mandatory cerebral EEG substrates for generating scalp-recordable ictal EEG patterns. When the necessary source area and synchrony are achieved at or near seizure onset, the scalp EEG pattern can localize the cerebral seizure onset zone. However, if achieving necessary source area and synchrony requires several seconds, during which time more distant propagation may occur, the resultant scalp ictal pattern may only lateralize, but not localize, the seizure onset zone. (Supported by Brain Research Fundation.)