Abstracts

Rationale: Epilepsy is widely regarded as a disorder of neural synchrony. High-frequency oscillations (HFOs) have been associated with local neuronal hypersynchrony and are increasingly utilized as a biomarker for epileptogenesis. The aim of this study is to characterize the electrophysiological features of ictal HFOs (iHFOs) and its relationship with ictal onset zone during mesial temporal lobe seizures by stereo-electroencephalography (SEEG). Methods: We prospectively studied 15 adult patients with medically refractory temporal lobe epilepsy (TLE) who underwent monitoring with depth electrodes. 75 ictal EEGs recorded at 1000 Hz sampling rate (one to nine seizures recorded per patient) were studied. Visual analysis of amplitude, duration, frequency and distribution of iHFOs was performed using Nihon-Kohden Neurofax systems with the following display settings: filters of 5-300 Hz, timescale of two to five sec per page, and sensitivity of 15 microvolts per millimeter. Results: iHFOs were observed in all patients and 61/ 75 (81%) of the seizures in intracranial EEG. 90% of the HFOs were in association with conventional spikes by occurring immediately before the conventional spikes or superimposed on the ascending slope, the descending slope or the peak of the conventional spikes. However, in rare cases they occurred independent from conventional epileptiform discharges. The amplitude and duration of iHFOs increased from 40.5 ± 4.2 μV and 45.7± 3.2 msec at onset to 101.5 ±16.1 μV and 89.5±6.9 at peak, respectively (P<0.001). In 36/61 (59%) seizures, the frequency of the iHFOs increased during the seizure evolution by at least 20%. In 43/61 (70%) seizures the onset of iHFOs overlapped with the conventional EEG ictal onset zone (IOZ). iHFOs restricted to a smaller area than conventional ictal discharges in all eleven seizures with secondary generalization. Conclusions: The dynamic evolution of the amplitude, duration and frequency of iHFOs during seizures supports that iHFOs are generated by the highly synchronized neurons during seizure evolution in mesial temporal seizures. iHFOs show a high spatial correlation with IOZ and have a smaller spatial distribution than conventional epileptiform discharges during the seizure evolution suggest it could be a reliable marker for epileptogenicity.