Abstracts

Rationale: Rationale: High frequency oscillations (HFOs) are often recorded from implanted intracranial electrodes and occur at the time of interictal spikes. The spatial distribution of these HFOs shows strong correlation with the epileptogenic zone. Magnetoencephalography (MEG) is being increasingly used in the pre surgical evaluation of intractable epilepsy. Clinical MEG analysis routinely utilizes the equivalent current dipole model (ECD). As it is affected by the signal to noise ratio, the ECD model will not be suitable for analysis of low amplitude HFOs of interictal spikes. We utilized an event related beamformer (ERB) (Cheyne et al., 2007) to localize interictal spike-locked oscillations (40-120 Hz) and compared the localization accuracy of the ERB to the epileptogenic onset zone identified by intracranial EEG. Methods: Methods: MEG recordings were performed utilizing whole head 151-channel axial gradiometer system (VSM MedTech). A Minimum of 15 two-minutes data sets were collected with a sampling rate of 625 Hz/1250 Hz. MEG data were inspected at the 3-70 Hz band to mark earliest peaks of MEG interictal spikes. We subsequently applied a band bass filter of 40-120 Hz. We excluded spikes in which gamma and beta oscillations were seen only in the context of generalized discharges or if no 40-120 Hz oscillations were seen in the earliest phase of the spike. We localized the sources of individual interictal spikes utilizing an event-related beamformer. We compared the beamformer source localization to the ictal onset zone identified in intracranial EEGs. Results: Results: We identified spike-locked oscillations (40-120Hz) in 19 patients with extratemporal intractable pediatric epilepsy. Eighteen patients underwent surgical resection. Surgical resections involved the rolandic cortex in 11 patients, Frontal lobe in four patients, Occipital resection in two patients and hemispherectomy in one patient. One patient declined surgery. Beamformer localization was concordant with the intracranial monitoring results in 18 patients aand was discordant in one patient with tuberous sclerosis. Conclusions: Beamformer source localization of spike-locked 40-120 Hz oscillations shows strong correlations with the ictal onset zone as identified by intracranial EEGs in a subset of cases of pediatric extratemporal lobe epilepsy.