Detection of Interictal Epileptiform Activity during Spontaneous Sleep: An MEG-EEG Comparison

Introduction: Simultaneous interictal EEG and MEG sleep recordings were compared as to their sensitivity in the visual detection of focal epileptiform activity. Because MEG is indifferent to radial activity, i.e., sleep changes, a higher ratio of epileptic discharges (ED) unique to MEG compared to EEG was presumed in the case of overlapping sleep changes. Patients: We studied 14 children and adolescents aged 3.5–17 years with localization-related epilepsy. Methods: The 122-channel whole-head MEGs and 33-channel EEGs were recorded simultaneously for 20–40min, most of them in segments of 5–20min. Segments of artefacts or bilateral polyspikes (>2 ED/200 ms) were excluded from the evaluation. The EEG and MEG data were separated and a blinded independent review of the 28 data segments was performed by 4 raters for the presence oEf D. EEG and MEG data were matched and ED identified by 3 reviewers were classified in three categories: 1. EEG=MEG (3 raters each), 2. EEG >MEG (difference of 3 raters) and 3. MEG > EEG (dto). The presence of simultaneous sleep changes (spindles, vertex waves, K-complexes, slow wave sleep) was visually determined for every single ED segment (±500 ms). Results: Out of 4704 detected ED, 1387 spikes fulfilled the above criteria; 689 spikes unique to MEG (54% within EEG sleep changes) and 136 spikes unique to EEG (21% sleep changes). 562 spikes were identified by 3 raters in both EEG and MEG (42% sleep changes). The presence of vertex waves alone led to a significantly higher number of spikes unique to MEG. Conclusion: A higher number of spikes was detected by MEG compared to EEG. Beside various factors such as focus localization and source orientation, overlapping sleep changes contribute to the differences observed.