Abstracts

Rationale: The irritative zone and seizure onset zone can be defined during chronic subdural EEG monitoring (ECoG) in surgical candidates with refractory focal epilepsy. Interictal ECoG often shows multiple spike types, which can be focal and complex, whereas most seizure onset is focal and very stereotyped, especially those with high frequency onset. We wondered if the seizure onset zone can be predicted from the spatial characteristics of the interictal activity. Methods: 26 consecutive patients underwent a presurgical work-up with chronic ECoG monitoring using 80-120 subdural grid electrodes. Inter-electrode distance 1 cm. Post-implantation CT was matched to pre-implantation MRI and electrode positions were displayed on a cortex rendering. Different interictal ECoG spikes were marked, averaged, characterized and ranked by a combined amplitude and surface-area measure relative to background. Focal and complex interictal spikes were distinguished; the latter were defined as involving non-contiguous regions 2 or more cm apart. Their distribution was plotted on the CT-MRI. Seizure onset was defined as a sustained rhythmical change in the ECoG followed by a clinically typical event. In the 10 seconds before and first 2 second of the seizure onset a frequency analysis algorithm was used to define power changes from low to high frequency bands in each electrode. Involved electrodes, above a threshold, were also plotted on CT-MRI as seizure onset. Interictal and ictal ECoG plots were compared. Results: The mean number of spikes in irritative zone was 7. The seizure onset zone was always within the area of at least one interictal spike. Moreover, all seizure onsets were related to the interictal ECoG spikes ranked 1st, 2nd or 3rd. Of these spikes 66% were focal and 44% complex. Within the complex spike distributions, seizure onset in 64% matched with the part having the largest synchronous surface-area. Conclusions: The irritative zone encompasses the seizure onset zone. Among the different interictal spikes that make up the irritative zone, those with the largest synchronous surface-area are related to the seizure onset. The largest contiguous synchronous surface-area is also relevant within complex spikes. Synchronous surface-area is an important characteristic of interictal spikes in the corticogram that is likely related to the generation of seizures.