Abstracts

Rationale: The contribution of localization data from clustered seizures during presurgical evaluation remains controversial. A cluster effect whereby seizures occurring within 8 hrs of a prior seizure are less likely to confer independent localizing data than seizures with longer interseizure intervals has been described but not uniformly replicated. Herein we evaluate the cluster effect in seizures included in the European database (Epilepsiae).Methods: Data consisted of continuous EEG monitoring from patients admitted to University Medical Center Freiburg 02/02-11/09. For each pair of consecutive seizures, the interseizure interval (ISI) was calculated, and the pair was classified as concordant (same localization); discordant (different localization); or indeterminate (the onset of one or both seizures was nonlocalizable). Indeterminate seizure pairs were eliminated from analysis. The relation between ISI and seizure concordance was determined using T-test for overall ISI and univariate analysis for individual ISI ranges. Because individual patients might affect the results of the univariate analysis, analysis of variance (ANOVA) was performed to analyze the relationship among ISI and patient variables and seizure concordance.Results: 1282 seizures from 96 patients (of whom 32.3% underwent intracranial monitoring) were evaluated. Of 1024 seizure pairs for which both seizures were localizable, 892 (87.1%) were concordant and 132 (12.9%) were discordant. Mean ISIs for concordant seizure pairs was 8.39 hrs vs. 14.65 hrs for discordant pairs (p=0.008). We repeated the analysis including only subjects with both concordant and discordant seizure pairs (N= 56). Of 503 seizure pairs in this group (73.8% concordant and 26.2% discordant), mean ISI was 9.7 hrs for concordant seizure pairs and 14.6 hrs for discordant pairs (p=0.022). We then categorized the 503 seizures pairs into 5 intervals based on ISIs (0-2 h, 2-4h, 4-8h, 8-24h, >24h). In this analysis, shorter interseizure interval remained associated with a higher chance of seizure concordance (% concordance for each group: 81.25, 71.76, 69.13, 68.42, 61.7, p=0.016).Conclusions: The previously described cluster effect was demonstrated again in this study in that shorter ISIs significantly increased concordance among seizure pairs, although the time frame is slightly longer than the 8 hours previously described. Further work will examine seizure semiology and localization in relation to clustering. Aknowledgments: EU project 'Epilepsiae', Grant 211713. Supported as part of the Cluster of Excellence 'BrainLinks-BrainTools' by the German Research Foundation (DFG) within the framework of the German Excellence Initiative.