Abstracts

Rationale: High Frequency Oscillations (HFO), spontaneous activity observed in EEG, have been recently considered to be a more reliable biomarker of epileptogenicity than interictal spikes. However oscillations in the frequency range of HFO are also reported from normal tissue (hippocampus, parahippocampus, neocortex). We investigated which types of HFO, ripples or fast ripples are more commonly seen within the seizure onset zone (SOZ). We assess whether any characteristics of these HFO are specific for pathological high frequency oscillations. Methods: An automated method based on the existing criteria used for visual identification was applied for detection of HFO activity in the 80-200Hz frequency range (ripples). This method was also adapted to detect fast ripples in the 200-400Hz range by optimization of sensitivity and specificity to visual identification of these patterns by an expert. Intracranial recordings from grid and depth electrodes (sampled at 10,000Hz) from 5 consecutive patients undergoing presurgical evaluation were analyzed. Records include patients with mesial and neocortical onset epilepsy. HFO events detected during 2h windows before seizure onset (interictal period) and during ictal period were compared in terms of rate, duration, frequency, amplitude, energy, and other features characterizing the shape of the HFO. Comparisons between interictal and ictal periods and between channels within and outside the seizure onset zone were also performed.Results: Automated detection of HFO in both frequency ranges allows one to analyze longer periods of EEG data. The number of fast ripple events is lower than the number of ripple events during interictal and ictal periods. In the four patients with localized onset seizures, the seizure onset was always within the area of fast ripples activity. One patient was inconclusive because of multifocal onset seizures. Distribution of ripples has a wider spread across channels during both interictal and ictal periods than fast ripples. Channel distribution of fast ripples is much more consistent between interictal and ictal period than for ripples. Based on clustering analysis of events from all channels and all patients, the most common characteristics of HFO for channels within the SOZ are events of high frequency and high amplitude, with an average duration of 60ms.Conclusions: Taking advantage of the automatic procedure it was possible to investigate relatively long epochs and numerous events and their characteristics. The quantitative analyses indicate stronger associations between fast ripples and the region of seizure onset supports the hypothesis that fast ripples reflect pathological activity, which could be associated with ictogenesis.