Abstracts

Monitoring of spontaneous seizures with video-EEG is regarded as an important aspect of the pre-surgical workup of epilepsy patients. This study investigated the differences in the informational content of ictal versus interictal EEG-MEG data, recorded simultaneously, using dipole localization and brain source montages in the non-invasive delineation of epileptogenic areas. Data were obtained from two Neuromag[trade] systems that allowed for co-registration of 306/122 MEG with up to 60 EEG channels. Seizures and interictal spikes were recorded in four patients during one hour sessions. Two patients had confirmed temporal and two extra-temporal lobe epilepsy. Brain source montages were derived using individual dipole analysis (BESA) to monitor the on-going brain activity in different brain regions independently for EEG and MEG. The region of initial seizure activity in source montages was compared to the dipole localizations during spike and seizure onset for EEG and MEG. EEG and MEG showed consistent focal unilateral onset in the right and left temporal-basal source channels in cases 1 and 2, respectively. This was confirmed by temporal-basal dipole sources that were localized consistently using either the interictal EEG or MEG. Seizure onset was more difficult to determine in the two extratemporal cases. The MRI of patient 3 showed a cortical dysplasia in the left posterior insula. During combined EEG-MEG recording (cortical lesion in left posterior insula) one seizure was observed with rhythmic 2.4 Hz onset discharges. MEG localized the 2.4 Hz activity to the border zone of the lesion. In EEG it appeared in the left parietal source montage, but dipole localization was imprecise. In the forth patient, seizure onset was seen with EEG flattening followed by mid-frontal 3.4 Hz discharges. After averaging, initial MEG and EEG dipoles localized to the left mid-frontal cortex. Interictal MEG and EEG activity was unclear in case 3 and confirmatory in case 4. Video-monitored EEGs of seizures do not always provide precise electro-clinical correlates and are frequently contaminated by muscle and movement artifact. Brain source montages applied to the scalp EEG can substantially improve the visibility of focal spike and seizure activities. But cases remain when the EEG is difficult to interpret, provides only a partial picture of epileptogenesis, and may, thereby, give potentially misleading information. Detailed work-up of co-registered EEG-MEG can provide information which is not available from either modality alone and helps in determining how many different areas of potential epileptogenicity exist in a patient.