Abstracts

Rationale: High-frequency oscillations (HFOs) >80 Hz (ripples at 80-200 Hz and fast ripples [FRs] at >250 Hz) have been recorded in the intracranial electroencephalogram (EEG) from the epileptogenic brain. There has been no diagnostic modality to date to measure the epileptogenic zone directly. Interictal HFOs may be a valuable marker to localize the epileptogenic zone, removal of which is necessary and sufficient to achieve seizure freedom. We evaluated correlation between resection of the brain region with interictal HFOs and seizure outcome using automated detection of HFOs. Methods: We retrospectively analyzed 27 pediatric patients (2-18 years) with intractable epilepsy who underwent intracranial video EEG before tailored surgical resection between July 2005 and June 2008. Intracranial EEGs were sampled at 1 kHz using 52-124 channels. For each patient, we analyzed 10 epochs of 2-minute EEG during non-rapid-eye-movement sleep, which were separated from each other and seizure sections by ?1 hour. We calculated the rate (/min) of ripples and FRs for each channel in bipolar montage after automated detection by a custom-made software written in MATLAB (The MathWorks, USA). Subsequently we determined the channels with high-rate HFOs (ripples, FRs). We calculated HFO measures including resection ratio (= area of surgical resection / area covered by electrodes), HFO area ratio (= area with high-rate HFOs / area covered by electrodes) and HFO resection ratio (= area with high-rate HFOs within surgical resection / area with high-rate HFOs). We determined seizure outcome at 1 year after surgery according to the proposed outcome classification (classes 1-6, class 1 being the best) by ILAE in 2001. We compared these HFO measures between good (classes 1-2) and poor (classes 3-6) outcome groups by Wilcoxon s rank sum test. We also tested the effect of these HFO measures on seizure outcome by multivariate ordinal logistic regression analysis. We analyzed patients with HFOs in ?3 channels separately from those with HFOs in <3 channels. Results: Thirteen had good (classes 1-2) and 14 patients had poor seizure outcome (classes 3-6). Twenty-two patients (good outcome 11, poor 11) had high-rate ripples in ?3 channels. Nineteen (good 10, poor 9) had high-rate FRs in ?3 channels. HFO resection ratio (FRs) was higher in the good outcome group than the poor (median 100% vs. 75%, p = 0.04). Conversely, higher HFO resection ratio (FRs) predicted better seizure outcome (p = 0.02, odds ratio = 1.09 [1.01-1.17]). HFO area ratio (ripples) and HFO resection ratio (ripples) had no correlation with seizure outcome. The HFO measures for patients with HFOs in <3 channels did not have effect on seizure outcome. Conclusions: Interictal FRs are indicative of the epileptogenic zone if they are present in at least 3 channels. Interictal ripples are considered to reflect the irritative zone rather than the epileptogenic zone.