Abstracts

Rationale: Interictal spikes play an important role in the diagnosis of epilepsy and delineation of the irritative zone helps to estimate the epileptogenic zone for epilepsy surgery in focal epilepsy. Magnetoencephalography (MEG) is used as a noninvasive method of source localization, for which the invasive (chronic) electrocorticogram (ECoG) can be seen as a gold standard. We wondered whether, and to what extent interictal MEG sources are reflected in the ECoG. Methods: From 1998 to 2008, 31 patients underwent simultaneous MEG/EEG, high-resolution MRI and chronic ECoG as a part of their presurgical evaluation. A 151-channel whole-head MEG was recorded during 60-75 minutes with intermittent head position measurement. Epileptic interictal transients were marked in MEG by two independent observers. Only consensus interictal spikes (kappa ≥ 0.40) were clustered, averaged, and then modelled using the MUSIC algorithm and co-registered to 3D MRI using CURRY software. Subdural ECoG was continuously recorded with grids and strips (interelectrode distance 1 cm) for approximately one week. Electrode positions in CT were matched to the pre-implantation MRI. In a representative 60 minutes sample of awake interictal ECoG, 2 seconds of background activity was selected and different interictal spikes were marked. Different spike-types were averaged and characterized by a combined amplitude and surface-area measure relative to background. They were ranked according to this measure and plotted on the MRI surface rendering. Each MEG cluster of spikes was then associated with one ECoG spike-type by defining the maximum overlap of the area of the MUSIC result and the interictal ECoG area. Results: In all the analyzed patients, at least a MEG MUSIC result coincided with one top ranking interictal ECoG spike-type. However, not all interictal ECoG spike-types had an association with the MEG clusters of spikes. Conclusions: Interictal MEG clusters of spikes have corresponding ECoG spike types, localized to the same area, and therefore contribute to the noninvasive localization of the irritative zone. However, not all interictal spikes of chronic ECoG can be observed in MEG.