Abstracts

Rationale: A paradoxical observation is that memory impairment is often greater following a partial seizure than following a generalized seizure. Recent dense array EEG (dEEG) studies of generalized spike-wave (GSW) seizures in absence epilepsy have suggested that GSW seizures may be restricted to frontothalamic circuits, disrupting conscious attention but sparing the limbic circuits supporting ongoing memory consolidation. Methods: Seizures from patients with a variety of syndromes, including absence, juvenile myoclonic epilepsy, and temporal lobe epilepsy were examined with dEEG (256 channels). The head surface voltage topographies were examined with maps and waveform plots and cortical source localization was estimated with linear inverse methods registered to an MRI atlas. The pattern of discharges in each case was examined in relation to the primary circuitry of corticolimbic and corticothalamic networks characterized in the literature. Results: In five patients with absence epilepsy, GSW seizures were localized to midline frontal sources, with few signs of temporal lobe involvement. The pattern of focal spikes in frontopolar cortex suggested primary mediation by cortical BA10, the rostral thalamic reticular nucleus, and thalamocortical projections. In ten patients with juvenile myoclonic epilepsy, GSW seizures showed a similar pattern of spikes with frontopolar localization, but often involved temporal lobe discharges that preceded or accompanied the typical GSW pattern. Conclusions: The improved localization of both spike and slow wave components of GSW seizures with dEEG provides a new perspective on the specific neural circuitry that may regulate “generalized” seizures. In absence seizures, in which the GSW discharges appear restricted to frontothalamic circuitry, consciousness is abruptly impaired but the patient remains oriented (person, place, time), apparently because the discharges have not disrupted the ongoing consolidation of memory within limbic networks. In contrast, seizures that engage limbic networks disrupt ongoing memory for person, place and time, implying separable neural circuits for these critical components of conscious experience.