Abstracts

Rationale: A large proportion of pediatric patients with epilepsy is medically intractable and require surgery to remove the epileptogenic zone (EZ). High Frequency Oscillations (HFOs) (80-500 Hz) have become widely recognized as interictal biomarkers for the EZ [1]. Yet, they are not widely used during presurgical evaluation. A major challenge is the difficulty to detect and localize them noninvasively. The aim of this study is to noninvasively detect and localize interictal HFOs using simultaneous high-density scalp electroencephalography (EEG) and magnetoencephalography (MEG), and to compare their location with the irritative zone (IZ), the epileptogenic lesion and the HFOs invasively localized. Methods: We analyzed high-density scalp EEG and MEG data from children with medically refractory epilepsy. We used a semi-automatic method to detect interictal HFOs in the ripple band (80-150 Hz) on scalp EEG and MEG data. HFOs were identified by a detection algorithm and then confirmed by an EEG expert. The algorithm identifies candidate HFOs from the EEG channels based on the signal envelope in the time-domain, and disregards possible artifacts analyzing the time-frequency map [2]. During the visual review, only HFOs occurring at the same time as epileptic spikes, and visible in both EEG and MEG data, were kept in order to exclude physiological HFOs. The HFO-zone was noninvasively localized using the wavelet maximum entropy on the mean method [3]. Interictal spikes were also visually identified and the IZ was defined using equivalent current dipoles. Results: We detected and localized noninvasively HFOs in two pediatric patients (A: 15 year old girl with encephalomalacia of the right middle cerebral artery; B: 11 year old boy with left parietal/superior temporal encephalomalacia). Fig. 1 presents the HFO-zone noninvasively estimated from both scalp EEG and MEG for patient A. Both techniques localized the HFO-zone close to the lesion and to the IZ as defined with MEG. MEG and EEG presented slightly different localization results. Fig. 2 presents results from patient B, and shows that the HFO-zone noninvasively estimated by scalp EEG and MEG overlapped with the HFO-zone identified invasively with electrocorticography. Conclusions: Our results show the possibility to detect and localize noninvasively ripple HFOs using simultaneous high-density EEG and MEG. The availability of a reliable noninvasive biomarker of the EZ will reduce the requirement for long-term monitoring and intracranial recordings, improving the presurgical evaluation and postsurgical outcome of patients with epilepsy. 1. Jacobs J, et al. High-frequency oscillations (HFOs) in clinical epilepsy. Prog Neurobiol 2012;98:302?"15. 2. Burnos S, et al. Human intracranial High Frequency Oscillations (HFOs) detected by automatic time-frequency analysis. PLoS ONE 2014;9:1?"12. 3. von Ellenrieder, et al. (2016). Detection and Magnetic Source Imaging of Fast Oscillations (40?"160 Hz) Recorded with Magnetoencephalography in Focal Epilepsy Patients. Brain Topography, 218?"231. Funding: Epilepsy Foundation / American Epilepsy Society Research Grant: "Propagation of high frequency oscillations as biomarker of epilepsy" Harvard Medical School - OFD/RRRC/CTREC Shore Faculty Career Development Award:"Interictal High Frequency Oscillations as Biomarker of Epilepsy in Children"