Abstracts

RATIONALE: The ictal onset zone (IOZ) on scalp EEG recording is difficult to detect in frontal lobe epilepsy (FLE). The aim of this study is to understand the differences between electrographical IOZ on scalp video EEG (SVEEG) and intracranial video EEG (IVEEG) in children with FLE. METHODS:_We studied 5 children with FLE ranging in age from 12 to 18 (mean 14.6). All patients were digitally recorded initially by SVEEG and later by IVEEG for epilepsy surgery. We selected seizures with the same semiology from SVEEG and IVEEG. The following data was analyzed : initial onset electrodes, propagation area using computerized voltage topographic mapping and latency prior to the clinical onset. RESULTS:_ Fourteen and 18 seizures were analyzed on SVEEG and IVEEG respectively (2-3, 3-5, 3-5, 4-2, and 2-3) in each patient. SVEEG localized the IOZ in 6 of 14 seizures in 4 patients. While IVEEG localized the IOZ in all patients. The latency from electrographical ictal onset to initial clinical presentation varied from 0 to 23 sec (mean 5.9 sec) on SVEEG and 0 to19 sec (mean 9.8sec) on IVEEG. The EEG did not change before the clinical onset in Case 3 in both methods. The activated grid electrodes involved 20 to 26 channels in 4 patients (mean 23 channels). In Case 3, SVEEG showed only movement artifact, but there was consistent spike and wave limited to 15 channels on IVEEG during the similar hypermotor seizure. Frontal lobectomy was performed in 2 and cortisectomy in 3 . TFollow-up period ranged from 2 to 45 months (mean 17). Surgical outcome was class IA in 4 patients and class II in one patient (Engel's classification). CONCLUSIONS:_IVEEG can detect the IOZ earlier than SVEEG. This suggests that cortical zone of epileptogenesis is already activated before the detection of seizure by scalp electrodes. Furthermore, the electrical field is highly affected by skull resistance in scalp EEG recording. We speculated that the cortical area which provokes ictal discharges on scalp EEG, needs more than 20 activated electrodes in FLE. Extensive subdural grid array and voltage topographic mapping are needed to delineate the precise epileptogenic zone for epilepsy surgery in FLE.