Abstracts

Rationale: The background electroencephalogram (EEG) activity in patients with focal epilepsy often shows focal or localized slow activities. Intermittent delta activity in the EEG of patients with focal epileptogenic brain lesions has been reported to be a marker of an epileptogenic focus. The delta activity coming from cortical regions is thought to be close to the brain lesion or epileptogenic focus. This study investigated the concordance between the current source distribution (CSD) of interictal spikes and that of the background delta frequency band (DFB) of the scalp EEG. Methods: We collected scalp EEGs of 13 focal epileptic patients that contained unifocal interictal spikes and unilateral delta to theta slow waves based on visual inspection. The patients included 4 females and 9 males, with a mean age of 45.5 ± 18.0 years (mean ± SD). The locations of the spikes included the frontal, temporal, parietal, and central areas. We recorded the EEGs for at least 30 min using a 32-channel digital EEG machine, and 21 or 25 electrodes placed on the scalp according to the international 10-20 system with anterior temporal or subtemporal electrodes. The sampling rate was 400 Hz. We applied a distributed source model using LORETA® to determine the CSD of the peak points of the interictal spikes and the DFB of the background activity. To obtain the CSD of the interictal spikes, we averaged 20 typical spikes in each patient. For the current source analysis of the background DFB, we did a cross-spectral analysis using five 5-sec artifact-free segments in each patient. In order to define the CSD responsible for the spikes and the background DFB objectively, we used the percentile of the current density as a threshold of significance, using a current density over the 95th percentile as the threshold of significance. We compared the CSD of the averaged spikes with that of the background DFB in each patient. Results: The CSDs of the averaged spikes were located unilaterally and concordant with the location of the spikes in all cases. The CSDs of the DFB were ipsilateral in ten patients, and bilateral with ipsilateral predominance to the background slow activity in three patients. In the cases with a CSD ipsilateral to the DFB, 8 of 10 patients had concordance of the CSD localization between the spikes and the DFB. Concordance of the CSD lateralization between them was observed in all ten patients. In the cases with bilateral CSD of the DFB, 2 of 3 patients had concordance of the CSD localization between the spikes and the DFB. Concordance of the CSD lateralization between them was observed in all three patients. Conclusions: The CSD localization and lateralization appear to be concordant between the interictal spikes and the DFB of background activity in epileptic patients. Therefore, the CSD of the DFB in EEGs with visually observable slow activities may predict those of epileptiform discharges.