Abstracts

Rationale: Electric source imaging using high-density scalp encephalography (EEG) is an increasingly validated non-invasive approach for localizing the epileptogenic focus in epilepsy patients. Meanwhile, source localization of magnetoencephalography (MEG) was useful to clarify the spatial relationship of the irritative zone to structural lesions. However, these two modalities have distinct factors limiting the resolution because some spikes can be recorded only on MEG but not on EEG and vice versa. Here we recorded high-density EEG and MEG simultaneously and compared the distribution of current source and the characteristics of spikes. Methods: Our patient was a 1-year 4-month-old girl who was performed right posterior quadrantectomy at 3-month-old for the intractable epilepsy with focal cortical dysplasia in the right parietal, temporal, and occipital lobes. After surgery, she achieved seizure free, however EEG abnormalities over right anterior temporal area had continued in conventional EEG recording. The patient underwent scalp high-density EEG with 83 electrodes placed in accordance with the extended version of the international 10-10 system (Neurofax, Nihon Koden) with sampling frequency 1 kHz, and simultaneously recorded MEG (306 channels, Neuromag VectorView, Elekta) with sampling frequency of 1.2 kHz. The 83 EEG electrodes were mounted on a flexible MEG compatible electrocap (Electro-cap international). We limitedly selected the interictal spikes that appeared at the same time in both EEG and MEG. These simultaneous spikes were modeled by means of the least square minimum-norm-estimation approach using patient's own brain MRI data, and the points of maximum intensity of current source were analyzed. We also determined the peak latency of the spikes, and compared the results between EEG and MEG. Results: After 35 minuites of EEG-MEG recording, 412 spikes were detected by EEG, while 310 spikes by MEG. Simultaneous spikes in both EEG and MEG were 106 spikes, only 17% of all spikes. The current density analysis revealed that the localization of current source distributed mainly over right frontal operculum for both EEG and MEG, which were close to surgical margin. However, for the points of maximum intensity of spike source, the gap of 4.7 to 53.9 mm (average, 18.4 ± 5.7 mm) was observed between EEG and MEG. Besides, there was a lag of the peak latency of the spikes between EEG and MEG (- 4.1 to 24.6 msec), MEG peak preceded by 16.6 msec on average to EEG peak. Conclusions: There were few simultaneous spikes in high-density EEG and MEG. Even though the spikes appeared to be at the same time, the slight spatial and temporal discrepancy between EEG and MEG spike was observed. Our results indicate that these two modalities could target slightly different electromagnetic signals. Because MEG and EEG yield both complementary and confirmatory information, combined MEG-EEG recordings will further improve the noninvasive evaluation of epilepsy patients.