Abstracts

Epileptic source localization is frequently not possible in those patients with EPC due to the absence of epileptiform discharges on scalp EEG. It has been suggested that MEG may yield the critical localizing data necessary for curative surgery in these challenging cases. Combining wide-band MEG with corticomuscular coherence (CMC) analysis, we studied a 40-year-old female with EPC involving the left arm and hand, whose previous work up was extensive, but non-diagnostic. A CTF 275-channel whole-head MEG System with 0.6 KHz and 6 KHz acquisition rates was used to simultaneously acquire MEG, EEG and EMG from 4 representative muscles (left biceps, extensor digitorum communis, abductor pollicis longus and first dorsal interosseous). Muscular events were used as a marker for back-averaging MEG-EEG events. The sources of the MEG-EEG events were localized using dipole analysis and the data were co-registered into a 3D-reconstructed 3T MRI using the CTF software. Band pass filtered (0-1500Hz) data of forty-one 0.5 second epochs centered on muscle activity was used to determine CMC from the spectral cross-correlograms at each sensor. Somatosensory evoked potentials were normal in all extremities. Visual analysis of the MEG-EEG recordings revealed different types of events according to recording condition. MEG events without EEG events were identified while the patient was on full medication doses, and a combination of events after two doses were held (MEG events only, simultaneous MEG-EEG events, and EEG events only corresponding with EMG events). A pre-twitch MEG dipole was localized in the right precentral gyrus corresponding anatomically with the cortical lesion identified on gradient-echo MRI as [ldquo]hemosiderin[rdquo], while the post-twitch MEG dipole was localized more sagittally in the supplementary motor cortex. Different CMC values above those expected were found. Significant CMC in the alpha (8-13Hz) and beta (13-30Hz) bands was found in the right central and parietal regions as well as the left central regions, while in the gamma band (30-70Hz), significant CMC was found primarily in the right central and parietal regions. There was no CMC in any regions at frequencies above the gamma range. This is the first known report of non-invasively demonstrated high frequency CMC during spontaneously occurring muscle activity in EPC, suggesting that the rhythmical organization of the cortical drive to muscle extends into the high frequency (gamma) bands. This activity may be more specifically related to the spontaneous abnormal movements since CMC in the gamma band was spatially more localized and temporally more directly related to the movements. Advanced MEG methods may provide critical insights into understanding the complexity of sources in EPC, and may further clarify the physiological bases of CMC. (Supported by The NINDS protocol # 01-N-0139.)