Abstracts

Rationale: There are accumulating evidences suggesting that high-frequency oscillations (HFOs) have a high association with epileptogenesis and removal of the brain tissue generating HFOs correlates with a better surgical outcome. HFOs are traditionally studied with intracranial electrodes such as subdural grid or depth electrodes. The aim of this study is to determine if the HFOs detected by stereo-electroencephalography (SEEG) can be also recorded on scalp EEG, as it has been reported previously and the spatial relationship of HFOs and seizure onset zone (SOZ).Methods: We retrospectively analyzed simultaneously recorded scalp and intracranial EEG from 12 adult patients with medically refractory focal epilepsy. Intracranial EEG was recorded with 1-7 depth electrodes (plus two subdural strip in one patient); scalp EEG was recorded with traditional 10-20 system. Sampling rate of 2000 Hz was used. Visual analysis of amplitude, duration, frequency and distribution of HFOs was performed with the following display settings: filters of 30-500 Hz, timescale of five sec per page, and sensitivity of 7-30 microvolts per millimeter.Results: Interical or ictal high frequency gamma activity and HFOs were observed in 8/12 (67%) patients using SEEG. However, no HFOs were observed on simultaneous recorded scalp EEG in all 8 patients. The special distribution of the HFOs is between 1 to 9 contacts on SEEG and in both mesial temporal region (in 6 patients) and temporal lobe neocortex (2 patients). The distribution of HFOs concurred with SOZ in 6/8 (75%) patients.Conclusions: Although HFOs can be readily observed on SEEG in both mesial temporal and temporal neocortex, they were not observed on scalp EEG in all 8 patients underwent chronic intracranial EEG monitoring with depth and subdural electrodes. HFOs show a high spatial correlation with SOZ and suggest it could be a reliable marker for epileptogenicity.