Multifocal Source Localizations from Interictal Epileptiform Discharges (IEDs) and Seizures as Traveling Waves

Post-surgical outcomes for drug-resistant epilepsy continue to be suboptimal, particularly in the setting of multifocal epilepsy. This is due in part to the fact that the mechanisms of seizure onset and spread, particularly in multifocal disease, remain incompletely understood. Seizures may spread contiguously, as traveling waves, or via underlying structural connectivity. Distinct foci may have differing significance for seizure generation.Here, we take advantage of a recently-developed algorithm that captures time differences of traveling waves at adjacent electrodes for seizure source localization. We identify seizure source locations from 36 patients in interictal epileptiform discharges (IEDs) and during seizures. We visually counted foci of activity, and scored whether foci were resected. Next, we divided localization results into separate foci in an automated fashion, using k-means clustering. IED foci tend to be larger, but less dense, than seizure foci.Next, we asked how much of each focus should be resected to yield an Engel 1 outcome. We found that a resection extent of 50%, in seizure foci, and 75%, in IED foci, was best at stratifying patients into those with good outcome and poor outcome. On average, patients had 3 seizure foci that were at least 50% resected and on average had 1 IED focus with at least 70% resection. Finally, we asked about overlap of IED and seizure foci. We found that the degree of overlap was significantly greater in Engel 1 patients. Understanding of the extent and nature of multifocality in epilepsy may help improve post-surgical outcomes.