Investigation of bilateral synchronous spike-wave discharge by EEG topography

To investigate the generator mechanism of the generalized discharge in childhood epilepsy, we studied its voltage field distribution using linked ear reference (L+R), common averaged reference (CA), and source derivation methods (SD). Two cases of epilepsy with clinical lapse of consciousness, and diffuse EEG spike-wave burst were divided into primary bilateral synchrony (PBS) and secondary bilateral synchrony (SBS), using coherence-phase analysis. Spike topograph images were shown by three different references. The difference in spike voltage between CA and SD was also investigated (CA-SD). By all three different references, PBS spike topography showed a broad symmetrical voltage gradient. In SBS, a left frontal focal negativity was shown with a steep voltage gradient in the topographic images of spikes by L+R, CA and SD. This asymmetry disappeared gradually at the end of the spike-wave burst. Both patients showed almost identical broad symmetrical voltage gradients on CA-SD topogram. EEG topography using various references suggested that the potential fields produced by deep generators were identical for PBS and SBS, although those generated by surface generators were different. These findings support the hypothesis that SBS may be projected from the subcortical system, and activated by cortical firing.