O102 The ability of magnetoencephalography and simultaneous electroencephalography to detect focal epileptiform discharges

Objective To compare the diagnostic yield of magnetoencephalography (MEG) and electroencephalography (EEG) for detecting interictal epileptiform discharges (IEDs) in patients referred for epilepsy surgery evaluation. Method One-hundred-and-forty-one patients undergoing epilepsy surgery evaluation were prospectively analyzed. A MEG whole-head 306-channel Elekta Neuromag® system, and simultaneous high density EEG (70 electrodes, range 58–80) using a non-magnetic cap (EASYCAP) were recorded in 115 consecutive patients. Eight patients were investigated with an array of 19 EEG electrodes, according to the 10–20 system. Due to large head circumference or inability to cooperate, 18 patients were investigated without EEG. Spontaneous electric and magnetic brain activity was recorded for 1.5 h. The data were offline band-pass filtered 0.5–70 Hz. MEG-EEG was visually inspected by trained physicians (LD, SB) for well-defined IEDs using CURRY 7 Neuroimaging Suite and BESA-MEG. Results Ninety-six different clusters of IEDs were identified in 85 patients. In 73% (70/96) of the foci the IEDs were visible both in MEG and EEG. In 15% (14/96) of the foci the IEDs were visible only in EEG. In 13% (12/96) the foci were visible only in MEG. Conclusion Although the majority of IEDs are detected in raw data by both modalities, in about one third of the cases the discharges were detectable in only one modality. Discussion Simultaneous high density EEG supplements MEG recordings, and optimizes the diagnostic yield. Significance EEG should be recorded simultaneous with MEG.