Abstracts

Rationale: High Frequency Oscillations (HFOs) are emerging as a reliable biomarker of epileptogenic tissue. As HFOs are produced by small brain regions, and since the EEG is greatly attenuated before reaching the scalp, HFOs are mostly recorded with intracranial electrodes. However, recent studies showed that HFOs can be also recorded from the scalp. Using simultaneous scalp and intracranial recordings, we aimed to analyze the characteristics of the scalp EEG at the time of intracranial HFOs. In this way, features that could make some HFOs visible on the scalp might be identified.Methods: Simultaneous scalp and subdural EEGs from 6 patients with focal cortical dysplasia were processed (low-pass filtered at 344Hz and sampled at 1024Hz). Intracranial HFOs were automatically detected in 10min of slow wave sleep. An expert reviewer validated all detections and divided them in co-occurring or not with spikes. Three scalp channels close to (on top of) the subdural contacts showing HFOs and one remote channel were selected. Segments contaminated with EMG were excluded. Time-frequency (TF) characteristics were obtained using a Morlet wavelet. Baseline was considered as segments of EEG free of HFOs in all subdural channels. In each scalp channel and neighboring subdural channel, averaged TF maps were obtained by temporally aligning the signal with respect to the maximum power of each subdural HFO. TF maps relative to baseline were used to normalize across channels. To obtain a global TF map for each scalp channel, the relative TF maps obtained from HFOs in each subdural channel were averaged. In this way, the characteristics of the scalp EEG locked to the intracranial HFOs were obtained.Results: A total of 32025 HFOs (16238 HFOs alone and 15787 co-occurring with spikes) were detected and validated in 87 channels. In four patients, the averaged TF map of the scalp EEG for HFOs alone showed a similar pattern to the subdural averaged TF map of HFOs alone in at least one neighboring channel. This was not the case for remote scalp channels, where the TF maps did not show a clear pattern. Examples from two patients are shown in Figures 1 and 2. The patterns were similar but not as clear for HFOs co-occurring with spikes as for HFOs alone.Conclusions: When averaging the scalp EEG that corresponds to a large number of intracranial HFOs, similar TF characteristics are observed on the scalp in two thirds of the patients. This suggests that some of the intracranial HFO events can be observed simultaneously on scalp and on subdural contacts because they have common characteristics. The possibility of observing individual HFOs on the scalp is uncertain however, as it depends on the signal to noise ratio and the spatial extent of the intracranial event. Finding correlates on the scalp of intracranial HFOs opens the possibility of detecting HFOs noninvasively (on scalp EEG). Scalp HFOs could be valuable to evaluate large patient populations, to predict surgical outcome, and to plan electrode implantation. Supported by NSERC PGSD, DAAD short term scholarship and CIHR MOP-102710.