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

OBJECTIVE: The 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. METHODS: 80 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. RESULTS: There 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. CONCLUSION: The 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. SIGNIFICANCE: The 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.