Abstracts

The localization of the epileptic focus from intracranial EEG recordings during the interictal period (i.e., without the necessity of observing any seizures) has been a focus of research in the past years. In addition to linear and nonlinear univariate measures, bivariate measures for synchronization have been shown to be capable of correctly lateralizing the focal hemisphere in the majority of cases. A more precise localization of the focal area, however, has not been achieved. Since the pathological neurons constituting the epileptic focus are assumed to drive the surrounding tissue, we used bivariate measures of directionality to investigate the driving influence of a focal area on the surrounding regions. We analyzed intracranial EEG recordings from the seizure-free intervals of 30 patients with medically intractable medial temporal lobe epilepsy undergoing invasive presurgical diagnostics. EEG signals were recorded via bilateral intrahippocampal depth electrodes, the total recording time comprised more than 83 hours. Side of focus was assessed by complete postoperative seizure control after selective amygdalo-hippocampectomy. Histopathological evaluation showed hippocampal sclerosis in all cases. A measure for directionality of phase coupling was calculated for all combinations of depth electrodes within each hemisphere using a moving window technique and then averaged over time. In 26 out of 30 cases, we found a distinct region in the ipsilateral hippocampal formation that drove the surrounding regions both in the ipsilateral and contralateral hippocampal formation. The spatial extent of this region was usually found to range between 5 and 20 mm. Even during the seizure-free interval, the focus appears to be driving its surrounding areas. Its influence can be seen not only on the ipsilateral, but also on the contralateral temporal lobe. Analysis using directionality measures may allow a functional delineation of the focal area during the presurgical evaluation as a prerequisite for super-selective resection in epilepsy surgery. (Supported by the Deutsche Forschungsgemeinschaft and the intramural research fund BONFOR of the University of Bonn.)