Hemispheric asymmetry of callosal neuronal participation in bilaterally synchronous epileptiform discharges.

Purpose: Callosal compound action potentials (CCAPs) recorded from the corpus callosum can reveal callosal neuronal activity. In this study, hemisphere-specific activity of callosal neurons during bilateral synchrony was evaluated in relation to the postoperative EEG.

Methods: In 19 patients undergoing callosotomy for intractable epilepsy, intraoperative electrocorticograms (ECoGs) and CCAPs were simultaneously recorded prior to callosal bisection. Hemisphere-specific CCAPs during bilateral synchrony were estimated through an averaging-based method. The degree of asymmetry in hemisphere-specific CCAPs was evaluated with a laterality index (LI), and correlated with post-callosotomy outcomes.

Results: Despite preoperative frequency of bilateral synchrony, postoperative EEG outcomes were mostly categorized as either unilaterally lateralized discharge group or bilaterally asynchronous discharge group. The LI of the former group was significantly higher than that of the latter group. In addition, higher amplitudes of the hemisphere-specific CCAP coincided with the side on which epileptiform discharges persisted postoperatively in most patients of the unilaterally lateralized discharge group.

Conclusions: Electrophysiologic analysis with CCAPs revealed hemispheric asymmetry of callosal neuronal participation in bilateral synchrony. More callosal neurons fire together in the more epileptogenic hemisphere during bilateral synchrony even if preoperative EEG features are virtually symmetric. This suggests transcallosal dynamic actions on the bi-hemispheric epileptogenesis maintenance.