Abstracts

Rationale: The goal of this project is to achieve a more detailed understanding of the spatial and temporal characteristics of scalp-recorded interictal epileptiform discharges. This understanding, in turn, may yield greater insight into the nature of the epileptic network for the individual subject. Methods: Four subjects with medically refractory epilepsy were studied. To improve spatial and temporal resolution of electrographic recordings, all were recorded using 256 channel, dense array EEG and sampled at 1000 Hz. Interictal discharges were identified and grouped together, based on location, to form a spike type that is defined by its unique spatial distribution. Realistic head models were constructed for each subject, using patient-specific MRIs. Source analysis was applied and each spike type was mapped to the cortical surface. Results: Multiple spike types were observed within each patient, with each spike type characterized by its distinct spatial distribution. These spikes types sometimes occur in isolation and were identified as “nodes”. We found that subjects with apparently unilateral temporal lobe spikes, for example, may exhibit nodes localized to basal-temporal, extratemporal, or even opposite basal-temporal regions. At other times nodes occur in close temporal proximity to other spike types (i.e., nodes) and thus appear to form an interictal spike network. The time differential between nodes can be calculated, and reveal patterns that demonstrate nodes either “lead” or “follow”. One subject with “left temporal spikes” was found to have nodes localized to inferior frontal, anterior basal-temporal, and posterior temporal regions, and that at different times the same nodes could either lead or follow, creating a highly variable network between them (Figure). Conclusions: Interictal epileptiform discharges are formed from a highly variable, diverse, and interconnected network of specific cortical regions. Detailed analysis of these patterns may provide a greater understanding of the overall epileptic network. Funding: Electrical Geodesics, Inc., Eugene, Oregon