Abstracts

Patients suffering from intractable focal epilepsy may benefit from neurosurgical resection of the epileptic focus if this focus can be identified via electroencephalographic recording of the seizure onset. In cases where the seizure origin can not be identified unequivocally from surface EEG recordings, implantation of intracranial electrodes can be indicated to achieve a better spatial resolution. Exact knowledge of the location of the implanted electrode contacts is crucial for evaluating these recordings. Electrode positions are usually verified by MRI scans, which are sometimes difficult to interpret due to artifacts caused by the electrode material. In this study we evaluated spatial synchronization maps calculated from intracranial EEG recordings to detect structural and functional boundaries and thereby supply additional information about electrode positions. We analyzed intracranial EEG recordings from the seizure-free intervals of 19 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 40 hours. As a measure for phase synchronization, the mean phase coherence R was calculated for all channel combinations within each hemisphere using a moving window technique resulting in a time series of spatial synchronization maps for each hemisphere. After evaluating the temporal stability of the maps, an automated cluster analysis was performed to track boundaries characterized by a spatial gap in synchronization along the hippocampal formation. Findings were then compared to post-implantation MRI scans. Spatial synchronization maps from both hemispheres proved stable over time. Synchronization gaps obtained from the automated cluster analysis corresponded well to anatomical boundaries of the hippocampal formation as evidenced by post-implantation MRI scans. Spatial synchronization maps obtained from intracranial EEG recordings can provide valuable information about electrode placement with respect to anatomical and functional boundaries. Due to the temporal stability of these maps, even short recordings provide a robust estimation of these boundaries. (Supported by The intramural research fund BONFOR of the University of Bonn and the Deutsche Forschungsgemeinschaft.)