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. 
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 . 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.