Abstracts

Rationale: To test the sensitivity of Magnetoencephalography (MEG) to interictal discharges arising from the irritative and the ictal onset zones (as described by intracranial EEG). Methods: 6 consecutive epileptic patients were implanted with MEG-compatible subdural/deph electrodes in order to complete prolonged presurgical epilepsy monitoring. Ictal events were reviewed to define the extent of the ictal onset zone. Then, simultaneous intracranial EEG and MEG recordings were conducted for 30 minutes to characterize interictal events. Those interictal iEEG events sharing topography with the ictal onset zone (previously defined from prolonged video iEEG) were identified. The cortical area corresponding to each interictal iEEG discharge was calculated and the presence/absence of an associated MEG discharge was tested. Results: Out of the 859 interictal iEEG discharges analyzed 311 (30%) produced an associated MEG spike. Interictal iEEG events arising from the ictal onset zone (137) produced a corresponding MEG discharge in 74% of the cases (102). MEG detected a 5% of the discharges with a source area under 8 cm2 and 70% of the discharges with a source area of 12 cm2 or higher; which included the vast majority of the interictal spikes generated from the ictal onset zone (87%). Conclusions: Interictal MEG recordings are sensitive to interictal discharges arising from the ictal onset zone. Approximately 1/3 of the spikes detected during interictal MEG recordings arise from the ictal onset zone.