Abstracts

RATIONALE: Interictal epileptogenic events generate electromagnetic signals that can be recorded by both MEG and EEG. The magnetic and electric signals derive from a common physiology, but actual identification of an event as epileptiform is dependent on waveform and signal-to-noise ratio. In many cases, visual inspection of concurrent MEG and EEG data will lead to identification of common time-points for epileptogenic events, but it is not uncommon for EEG events to be without obvious MEG correlates, and vice-versa. METHODS: The objective of this study was to examine the concordance and discordance of transients as identified independently by MEG and EEG. Data were obtained from 3 patients with frequent interictal discharges, MRI indicative of mesial temporal sclerosis, semiology and ictal EEG data indicative of mesial temporal onset. MEG data were collected using a 306-channel, whole-head biomagnetometer unit. Twenty-one channel EEG data were simultaneously collected. RESULTS: Of a total of 221 transients, 145 were anterior-basal, identified across the methods. Sixty-nine events were commonly identified in MEG and EEG. There were 58 seen on EEG without clear MEG correlates, while 18 MEG transients were without EEG correlates. In addition, 5 events revealed posterior temporal locations (4 EEG only, 1 MEG only). Two EEG transients were contralateral to mesial temporal sclerosis. Sixty-nine transients were localized outside basal temporal regions (7 EEG-MEG concordant, 17 EEG only, 45 MEG only). CONCLUSIONS: Only about 30% of the transients were concordant for basal temporal localization, but overall, MEG and EEG were complementary. Extratemporal transients were seen more often in MEG than in EEG and tended to be discordant. At present, the full significance of extratemporal interictal events (especially those seen in MEG only) is uncertain.