The epileptogenic characteristic of focal cortical dysplasias and dysembryoplastic neuroepithelial tumors explored by depth electrodes and stereoelectroencephalography is quantified using an epileptogenicity index, in a study of 36 patients with focal drug-resistant epilepsy at Universite de la Mediterranee and other centers in Marseille and Rennes, France. The epileptogenic zone is organized as a simple focal lesional site or as a complex ‘epileptogenic network’ extending beyond the lesion. Epileptogenicity index (EI) values range from 0 (none) to 1 (maximal epileptogenicity). The mean EI in lesional regions was 0.87, and 0.29 in non-lesional structures. A single focal lesion was found in 31% of patients, and more than one epileptogenic region in 25 patients (64%) (a network organization in 61% and bilateral epileptogenic zone organization in 8%). Distant structures are often involved, and in mesio-temporal epilepsy, the number of epileptogenic structures increases with epilepsy duration. None patient with bilateral organization became seizure-free, while 87% with focal organization and 57% with network organization were seizure-free. The EI is of value in the delineation of the epileptogenic zone with brain lesions and in the definition of the extent of surgical resection. [1]

COMMENT. The epileptogenicity index (EI) is a method of quantifying and defining the epileptogenicity in and around focal cortical dysplasias and neurodevelopmntal tumors during stereo-EEG with depth electrodes. Defining the extent of the epileptogenic zone is important in prognosis and in surgical resection. The November 2009 issue of Ped Neur Briefs reviews a report of a novel immunocytochemical test for epileptogenic brain tissue, independent of the histological findings [2]. With this added quantification of the epileptogenicity zone by stereo-EEG and the EI, surgical resection for refractory epilepsies should become more accurate and effective.