Abstracts

Rationale: We reported recently that the high frequency oscillations (HFO: ≥70 Hz) at seizure onset had a more focal distribution than the conventional frequency activity (CFA: 1-70 Hz) in a patient with nonlesional frontal lobe epilepsy who underwent successful subpial transections (Modur and Scherg, Clin Neurophysiol 2009; 120:1220-24). Here, we compared the spatial features of HFO and CFA in 3 patients who underwent neocortical resection. Methods: We performed post hoc analysis of the subdural ictal EEG recordings using the BESA software (MEGIS GmbH, Germany). For presurgical localization, Ad-Tech platinum electrodes (contact diameter 2.3 mm, inter-contact distance 10 mm) had been used to acquire the data at a sampling rate of 1000 Hz. The seizures were visually examined initially using the conventional frequency setting (CF: 1-70 Hz bandpass filter; 60 Hz notch; normal time scale 10 sec/page). Using a bipolar montage, the electrical onset was temporally localized tentatively to those channels showing the earliest ictal change (rhythmic activity or repetitive spikes with or without preceding slow transients). Afterwards, the seizures were visually examined for the HFO using the high frequency setting (HF: 70-300 Hz bandpass filter; expanded time scale 3 sec/page). Definitive electrical onset was then temporally localized to the channels showing the earliest HFO. For subsequent analysis, a reformatted montage comprised of the above ictal channels and their neighboring channels was used. Fast Fourier transform-derived peak power spectra were obtained for a 1-sec epoch at the seizure onset over the 1-70 Hz and the 70-300 Hz range. Spatial distribution of the seizure onset in terms of CFA and HFO was then localized to those channels showing ≥10% of the peak spectral power on the CF and HF settings respectively. Results: Eight neocortical seizures from 3 patients were analyzed (Table). Electrical onset preceded the clinical onset in all the seizures. Median frequency of HFO was 97 (range 70-136) Hz and that of CFA was 1.7 (range 0.83-21) Hz. Using a linear mixed model, the seizure onset based on HFO was found to be significantly smaller in spatial distribution compared to the onset based on CFA (mean=-13.37, SD=3.99, p=0.006). Although both HFO and CFA occurred in the same channels to some extent, this spatial overlap was not significant (p=NS, Fisher’s exact test). Conclusions: HFO at neocortical seizure onset occur in the range of ripple oscillations, and have a significantly more focal distribution than CFA. In conjunction with the prevalent notion that the HFO localize the seizure focus and participate in seizure initiation (Worrell et al., Brain 2004; Traub et al., Epilepsia 2001), our findings suggest that defining the seizure onset in terms of HFO could result in a more accurate and smaller seizure onset zone, leading to a feasible surgical target. Secondly, the lack of significant spatial overlap between HFO and CFA suggests that they are separate phenomena, perhaps regulated by different neuronal network mechanisms. Future studies will correlate these findings with surgical outcome.