Abstracts

Rationale: The interictal-ictal transition is important for localization of the epileptogenic zone. One pattern of interictal-ictal transition is the transitional sharp wave (TShW), a high voltage sharp wave immediately followed by voltage attenuation and classical ictal rhythmic activity. We observed that this pattern occurs with seizures of neocortical origin. We analyzed the localization of the epileptogenic zone in a series of consecutive recordings with the TShW ictal onset pattern.Methods: We identified all scalp ictal recordings with a TShW at ictal onset in the Vanderbilt Epilepsy Monitoring Unit over a period of 12 months. We recorded the field distribution of the TShW and the subsequent rhythmic ictal discharges, as well as the final localization of the epileptogenic zone based on the overall presurgical evaluation and postoperative outcome in operated patients. For a control group, we identified and similarly analyzed all ictal discharges in patients with temporal lobe epilepsy and hippocampal sclerosis, over the same period of time.Results: A total of 39 ictal discharges in 10 patients started with a TShW. The final localization was confirmed with subdural grid recordings in four patients and structural and functional brain imaging in the remaining six patients. The localization was neocortical temporal in five patients (25 ictal discharges), neocortical frontal in four patients (11 ictal discharges), and occipital in one patient (three ictal discharges). The center of TShW field was concordant with the final localization in all of the reviewed ictal recordings, while the center of the subsequent ictal discharge field was concordant in only two patients (18 ictal discharges). The subsequent rhythmic discharge was non-localizing in another five patients (twelve ictal discharges) and misleading in the remaining three patients (nine ictal discharges). None of the 79 ictal discharges in the 22 patients with hippocampal onset seizures started with a TShW.Conclusions: The TShW was a marker of neocortical onset in our study. The TShW field provides more reliable electrographic localization of the epileptogenic zone than the subsequent rhythmic ictal activity.