Abstracts

Rationale: High frequency oscillations (HFO) are spontaneous activity observed in EEG whose descriptions and mechanisms are still evolving. Their characteristics span a wide range in frequency, amplitude, and duration. Their occurrence often correlates with epileptogenicity, although it remains unclear if HFO reflect pathophysiology or are epiphenomena. Recent works have demonstrated that fast activity progressively increases before seizure onset - suggesting the potential use as a marker for a preictal state. Removal of the areas generating HFO is associated with better postsurgical outcomes. To investigate HFO as biomarkers of epileptogenic tissue, we have examined changes of HFO activity before and during complex partial seizures in humans. Methods: Intracranial recordings from ten patients diagnosed with mesial temporal and neocortical onset partial seizures undergoing presurgical evaluations were analyzed. For HFO detection we applied an automated method that consists of identifying at least 4 consecutive oscillations with amplitudes above 10 µV and at least two times larger than the average amplitude of oscillations of the surrounding background. The analysis was performed on bipolar filtered EEG (80-200Hz) signals. To detect short, transient HFO events rather than long continuous high frequency activity, a limitation of a maximum of 100ms was applied for each event duration. The results of detection were compared between channels and for 2-minutes periods (interictal: 2 hours, 1 hour and 30 minute before onset; preictal: 2 minute before onset; ictal: immediately after seizure onset). Results: During interictal periods, distributions of the rate of HFO across electrodes remains consistent, and is similar for interictal periods taken relative to consecutive recorded seizures for each patient. In some instances the distribution of the interictal activity differs from the distribution of HFO during the preictal periods. The distribution of HFO during preictal periods is also consistent between consecutive seizures. During precictal periods, the number of HFO is increased compared to interictal levels. These levels increase further during ictal periods. The maximum number of HFO during the ictal period often correlates with the area of seizure onset as indicated by visual analysis of experienced neurologists. Consistent distributions of HFO activity between interictal and ictal periods corresponds to the better postsurgical outcomes. Conclusions: Significantly different distributions of HFO across elecrodes during interictal and ictal periods correlating with suboptimal surgical outcomes could indicate that in some instances different regions can be responsible for generation of preictal and ictal HFO. It remains to be determined whether interictal/preictal and ictal HFO are different processes; however the consistency across time suggests similar substrates. Whether or not localized HFO represent seizure onset zones or irritative secondary regions remains. Detailed investigation of spatiotemporal dynamics of fast activity could help elucidate the role of HFO in ictogenesis. Funded by NIH NS075020