Abstracts

Patients suffering from intractable focal epilepsy may benefit from neurosurgical resection of the epileptic focus provided it can be identified via electroencephalographic recording of the seizure onset. A question of interest is whether the focus can be reliably identified from interictal recordings, i.e. without the necessity of observing seizures. Measurements of the synchronization between different EEG channels have been found to be promising for this purpose. Up to now, a number of measures for synchronization have been proposed based on different approaches. In this study a comparison between different linear and nonlinear synchronization measures was performed with respect to two aspects: 1. general performance of the measures in terms of their ability to discriminate the ipsi- from the contralateral hemisphere, 2. comparison of different measures to investigate whether the different approaches render complementary or redundant information. 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, each equipped with 10 recording contacts and implanted stereotactically along the longitudinal axis of the hippocampal formation. The total recording time comprised more than 83 hours. Several measures of synchronization were calculated for all combinations of depth electrodes within each hemisphere using a moving window technique, and then averaged over time. For each of the measures the average spatial synchronization in each hemisphere was calculated. A detection of the focal hemisphere was carried out for all measures and patients based on the assumption of a higher average synchronization in the focal hemisphere. True and false lateralizations were compared for different measures. Correct lateralization based on higher synchronization of the focal hemisphere was found to be statistically significant for all measures investigated, although actual performance differed among the investigated measures. False lateralizations for different measures ranged between 6 and 8 out of 30. A comparison of correct and false lateralizations showed a strong similarities between measures. The focal hemisphere in an epileptic brain is characterized by a pathological increase in synchronization. The different measures investigated in this study appear to be sensitive to this increased synchronization. Lateralization analysis using synchronization measures could render additional diagnostic information, although the false lateralizations found in this study could limit its clinical applicability. (Supported by the Deutsche Forschungsgemeinschaft and the intramural research fund BONFOR of the University of Bonn.)