Ictal and interictal high frequency oscillations in patients with focal epilepsy

ObjectiveHigh frequency oscillations (HFOs) can be recorded with depth electrodes in focal epilepsy patients. They occur during seizures and interictally and seem important in seizure genesis. We investigated whether interictal and ictal HFOs occur in the same regions and how they relate to epileptiform spikes.MethodsIn 25 patients, spikes, ripples (80-250 Hz) and fast ripples (FR: 250-500 Hz) and their co-occurrences were marked during interictal slow wave sleep (5-10 min), during 10 pre-ictal seconds and 5 s following seizure onset. We compared occurrence and spatial distribution between these periods.ResultsHFOs and spikes increased from interictal to ictal periods: the percentage of time occupied by ripples increased from 2.3% to 6.5%, FR from 0.2% to 0.8%, spikes from 1.1% to 4.8%. HFOs increased from interictal to pre-ictal periods in contrast to spikes. Spikes were in different channels in the interictal, pre-ictal and ictal periods whereas HFOs largely remained in the same channels.ConclusionsHFOs remain confined to the same, possibly epileptogenic, area, during interictal and ictal periods, while spikes are more widespread during seizures than interictally.SignificanceIctal and interictal HFOs represent the same (epileptogenic) area and are probably similar phenomena.