Abstracts

Rationale: Paroxysmal sharp transients whether physiologic or epileptic differ in waveform when registered in MEG and EEG. MEG transients are shorter in duration, and smaller in area of involvement. This is due to the fact that MEG records the tangential component of the spikes from cortical sulci, while EEG records both the radial component from the crest of the cortical gyri and the distorted tangential component from the sulci after volume conduction. It has been suggested that the earlier peak of the MEG spike may be due to the epileptiform activity starting in the sulci and propagating to the gyri. In this study we analyzed 744 spikes in regard to their MEG and EEG peaks. Methods: 744 spikes were analyzed from five consecutive patients with co-registered MEG and EEG data. For each patient, 10-20 minutes of data was reviewed by two independent reviewers. Only spikes chosen by both reviewers were included in the study. Inclusion Criteria for both MEG and EEG spikes included morphology that stands out from the background activity, sharp peak, and duration of 20-200 msec. Additional inclusion criteria for MEG spikes included dipole stability of 20 msec or more. Spikes that were concomitant with EKG or arouse from an artifactual background were excluded. Spikes were divided to 3 subgroups: MEG only spikes (M-spikes), EEG only spikes (E-spikes) and MEG+EEG spikes (M-E-spikes). The M-E-spike subgroup was further subdivided depending on the timing of MEG and EEG spike peaks. If the MEG and EEG spike peaks occurred within 5 msec of each other, they were considered concomitant, if MEG spike preceded the EEG spike by > 5 msec, a MEG earlier spike was identified and if the EEG spike preceded the MEG spike by > 5 msec, an EEG earlier spike was noted. Results: A total of 744 spikes were analyzed. 14% (106 spikes) were E-spikes, 60% (442 spikes) were M-spikes and 26% (196 spikes) were M-E-spikes. Of the 196 M-E-spikes, 71% were concomitant, 23% were MEG earlier spikes, 6% were EEG earlier spikes. Conclusions: The majority of M-E spikes peak concomitantly in the MEG and EEG, or earlier in MEG. In the minority of M-E spikes, the peak is earlier in the EEG than the MEG. This may point to possible activation of cortical gyri prior to sulci in minority of epileptiform discharges.