Abstracts

MEG and EEG correlation of epileptic interictal spikes has been reported to help understanding of electromagnetic neurophysiology. The radial orientation or deep location of dipoles has been considered to yield EEG spikes without MEG correlates (E+M- spikes). This study systematically collects, analyzes and characterizes E+M- spikes, for understanding the different expressions between EEG and MEG from epileptic neuronal activities., We studied five patients (age ranging 6-38 years) who had E+M- spikes and presented temporal lobe epilepsy in four patients and extra-temporal epilepsy in one. We used either whole head 151 (four patients) or 64 (one patient) channel gradiometers (VSM MedTech Ltd., Port Coquitlam, BC, Canada), with simultaneous EEG recording using international 10-20 electrodes. Patients were under spontaneous or drug induced sleep. We selected E+M- spikes from whole data sets. We averaged MEG data by the peak of EEG spikes in 3 patients in whom more than ten spikes were recorded in a single dataset (2 minutes). We used a single dipole model for averaged MEG waveforms. We also analyzed electrical source of individual E+M- spikes by BESA5.1 (MEGIS, Grafelfing,Germany) to estimate EEG dipoles and compare them with dipoles of averaged MEG spikes., We found E+M- spikes (ranging4-30, mean 21) in temporal region in four patients and Rolandic region in one. Averaged MEG waveforms became apparent in three patients (two temporal, one Rolandic). Single dipole analysis of averaged MEG spikes showed tangential dipoles of 30-100 nAm. In all five patients, EEG source analysis of individual E+M- spikes revealed predominantly radial dipoles., Dipoles of averaging MEG using EEG peaks in E+M- spikes revealed small tangential components associated with predominantly radial dipoles of EEG. Both EEG and MEG dipole analyses achieve valuable additional information of radial components of neuronal source, although the locations of EEG dipoles were generally considered not so accurate as MEG dipoles. This study suggests that the orientation of dipoles is the main factor of E+M- spikes. EEG dipole analysis is useful to understand dipole orientation including accompanied radial component which can [apos]t be detected by MEG analysis.,