Intracranially recorded interictal spikes: Relation to seizure onset area and effect of medication and time of day

•In patients with temporal or extratemporal epilepsy interictal spikes can be observed in the intracranial EEG over multiple areas of the hemisphere ipsilateral to the seizure onset area.•The single brain area with the highest number of interictal spikes, or interictal spike rate, is not a reliable indicator of the seizure onset area.•The spatial distribution of spike rates over the multiple areas of the ipsilateral hemisphere is strongly related to the seizure onset area and may hold value for its localization.

ObjectiveThe relationship between seizures and interictal spikes remains undetermined. We analyzed intracranial EEG (icEEG) recordings to examine the relationship between the seizure onset area and interictal spikes.Methods80 unselected patients were placed into 5 temporal, 4 extratemporal, and one unlocalized groups based on the location of the seizure onset area. We studied 4-h icEEG epochs, removed from seizures, from day-time and night-time during both on- and off-medication periods. Spikes were detected automatically from electrode contacts sampling the hemisphere ipsilateral to the seizure onset area.ResultsThere was a widespread occurrence of spikes over the hemisphere ipsilateral to the seizure onset area. The spatial distributions of spike rates for the different patient groups were different (p < 0.0001, chi-square test). The area with the highest spike rate coincided with the seizure onset area only in half of the patients.ConclusionThe spatial distribution of spike rates is strongly associated with the location of the seizure onset area, suggesting the presence of a distributed spike generation network, which is related to the seizure onset area.SignificanceThe spatial distribution of spike rates, but not the area with the highest spike rate, may hold value for the localization of the seizure onset area.