Quantification and localization of EEG interictal spike activity in patients with surgically removed epileptogenic foci.

Objective: To demonstrate if interictal spike activity was localized within the resected area in surgically treated epilepsy patients; and if there is correspondence between the degree of localization and improvement after surgery.

Methods: We analyzed long-term EEGs from 34 patients. Interictal spikes were grouped in clusters and averaged according to morphology and topography. The relative contribution of each cluster to the total number of spikes in each patient was estimated. Dipole and distributed EEG source localization of each cluster was made. The percentage of spike activity localized within the site of resection (SR) during the onset was estimated. The relationship between the percentage of activity within SR and the surgery outcome was assessed.

Results: Fourteen patients had 90-100% of spikes within SR, 9 had 50-89%. Most patients with more than 50% of activity localized within SR were seizure free, while the 5 patients who had all activity localized outside SR were not seizure free.

Conclusions: Localization of clusters containing the largest quantity of interictal spikes during onset generally corresponded to the likely location of the epileptogenic cortex.

Significance: Computer-assisted EEG source localization with our methodology can be a useful adjunct for the evaluation of candidates for epilepsy surgery.