Objective: This study aims to identify if oscillations at frequencies higher than the traditional EEG can be recorded on the scalp EEG of patients with focal epilepsy and to analyze the association of these oscillations with interictal discharges and the seizure onset zone (SOZ).
Methods:The scalp EEG of 15 patients with focal epilepsy was studied. We analyzed the rates of gamma (40-80 Hz) and ripple (Ͼ80 Hz) oscillations, their co-occurrence with spikes, the number of channels with fast oscillations inside and outside the SOZ, and the specificity, sensitivity, and accuracy of gamma, ripples, and spikes to determine the SOZ.Results: Gamma and ripples frequently co-occurred with spikes (77.5% and 63% of cases). For all events, the proportion of channels with events was consistently higher inside than outside the SOZ: spikes (100% vs 70%), gamma (82% vs 33%), and ripples (48% vs 11%); p Ͻ 0.0001. The mean rates (events/min) were higher inside than outside the SOZ: spikes (2.64 Ϯ 1.70 vs 0.69 Ϯ 0.26, p ϭ 0.02), gamma (0.77 Ϯ 0.71 vs 0.20 Ϯ 0.25, p ϭ 0.02), and ripples (0.08 Ϯ 0.12 vs 0.04 Ϯ 0.09, p ϭ 0.04). The sensitivity to identify the SOZ was spikes 100%, gamma 82%, and ripples 48%; the specificity was spikes 30%, gamma 68%, and ripples 89%; and the accuracy was spikes 43%, gamma 70%, and ripples 81%.
Conclusion:The rates and the proportion of channels with gamma and ripple fast oscillations are higher inside the SOZ, indicating that they can be used as interictal scalp EEG markers for the SOZ. These fast oscillations are less sensitive but much more specific and accurate than spikes to delineate the SOZ. Neurology The conventional range of EEG analysis usually involves frequencies below 40 Hz. Studies over the last decade suggest, however, that high-frequency oscillations (HFOs) may have an essential role in normal and pathologic brain function. Reports using microelectrodes combined with depth EEG electrodes showed the presence of HFOs in epileptic patients.