Abstracts

Rationale: High-frequency oscillations (HFO) >80 Hz (ripples at 80-250 Hz and fast ripples at >250 Hz) are recorded over the epileptogenic brain in ictal and interictal electroencephalogram (EEG). We previously reported that HFO within ictal onset and ictal symptomatogenic zones are valuable for localizing the epileptogenic zone (Epilepsia 2007;48:286-296). The distribution of interictal HFO is more extensive and complex than that at ictal onset. Interictal HFO analysis is potentially relevant because it is not uncommon to experience seizure recurrence after resection of ictal onset zone with HFO only. In this study, we evaluated the concordance between distribution of interictal HFO and resection area as well as the presence of ripples vs. fast ripples to determine which factors can be used to predict seizure-free outcome. Methods: We retrospectively analyzed HFO in 21 pediatric patients with intractable neocortical epilepsy with single focus who had undergone intracranial video EEG using 52-124 channels (subdural and depth electrodes) sampled at 1 kHz between July 2006 - June 2008. We divided them into 3 groups based on seizure outcome (good; seizure free since surgery, moderate; seizure free in the last 6 months, poor; not seizure free). For each patient, we analyzed 20-minute EEG during non-REM sleep >1 hour preceding and/or following a seizure. We high-pass filtered the EEG at 80 Hz or 250 Hz by Insight II (Persyst, USA) and detected periods with increase in root-mean-square amplitude of HFO beyond 3 inter-quartile ranges over median by MATLAB (The MathWorks, USA). We calculated occurrence rate /min of HFO >80 Hz lasting >50 ms and HFO >250 Hz lasting >20 ms for each channel to represent HFO distribution. We did nominal logistic regression analysis to determine concordance between HFO distribution and resection area, which we compared with seizure outcome. Results: There were 8, 8 and 5 patients with good, moderate and poor seizure outcome, respectively. Nine (43%) of 21 patients had a significant concordance between resection area and distribution of HFO >80 Hz. Among them, 3 (33%) had good outcome, 3 (33%) had moderate outcome and 3 (33%) had poor outcome. Six (29%) of 21 had a significant concordance between resection area and distribution of HFO >250 Hz. Among them, 4 (67%) had good outcome, 1 (20%) had moderate outcome and 1 (20%) had poor outcome. Three (38%) of 8 patients with good outcome, 3 (38%) of 8 with moderate outcome and 3 of 5 (60%) with poor outcome showed significant concordance between resection area and distribution of HFO>80 Hz. Four (50%) of 8 patients with good outcome, 1 of 8 (13%) with moderate outcome and 1 of 5 (20%) with poor outcome showed significant concordance between resection area and distribution of HFO >250 Hz. Conclusions: Interictal HFO >250 Hz (fast ripples) were better indicators for good seizure outcome than HFO >80 Hz (ripples), when their distribution was concordant with resection area. Interictal HFO, especially fast ripples, in residual brain regions might play a significant role for seizure recurrence.