Combined EEG and MEG Source Analysis of Interictal Epileptiform Activity in Childhood Epilepsy

Purpose: MEG is used as a non-invasive tool in an increasing number of epilepsy patients in the context of a presurgical evaluation. However, few studies applied simultaneous surface EEG in the determination of potential epileptogenic zones. A combination of both methods offers a clear benefit, since MEG only detects activities from neurons with tangential orientation and the EEG signal is dominated by radial sources. Patients: We report on 21 children and adolescents with different forms of cortical lesions (FCD 9, PMG 7, other 5) and partial epilepsy. 6 patients presented with CSWS. Methods: 122-channel whole-head MEG and 32-channel EEG were recorded simultaneously for 25 to 40 minutes. Using the BESA program, interictal spikes were identified visually and used as templates to search for similar spatio-temporal spike patterns throughout the recording. Detected similar spikes (r >0.85) were averaged, high-pass filtered (5Hz) to enhance spike onset, and subjected to multiple spatio-temporal source analysis. Source localization was visualized by superposition on T₁-weighted MRI and compared to the lesion. Results and Discussion: Intrinsic epileptogenicity in FCD was demonstrated by both EEG and MEG source analysis, since all sources modelling the spike onset were localized within the lesion visible in MRI. There were no marked differences between EEG and MEG regarding onset time and localization observed. On the contrary, EEG spike onset preceded MEG significantly in 1/3 of the spike types in PMG. This was due to radial onset activity in EEG while MEG localized propagated activity, even leading to a wrong lateralization in one case. Discrepancies in MEG and EEG were explained by the lack of deep fissures in PMG. In CSWS, combined EEG and MEG source analysis demonstrated a unilateral onset within the lesional hemisphere followed by propagation to the contralateral hemisphere. Conclusion: EEG and MEG source analysis is a valuable non-invasive tool for analyzing interictal epileptiform activity in symptomatic childhood epilepsy.