Seizure clustering during epilepsy monitoring

Purpose: To identify risk factors associated with seizure clustering during epilepsy monitoring and to assess the effect of clustering on localization of the epileptogenic zone. Methods: Patients undergoing presurgical epilepsy monitoring at Montefiore Medical Center or Yale-New Haven Hospital were recruited. Seizure clustering was defined as three or more seizures within 24 h. Risk factors for seizure clustering were examined by using regression analysis. The effect of clustering on localization was examined by Student's t test evaluation of mean interseizure interval for consecutive concordant and discordant seizures. Results: Of 91 patients, clustering was present in 56 (61.5%). Variables significantly associated with clustering included a history of seizure clustering at home (p = 0.0003) and the presence of mesial temporal sclerosis (MTS) on magnetic resonance imaging (MRI; p = 0.0172). Clustering was present in nine of 10 patients with more than one seizure-onset zone. Ictal EEG localization was not associated with clustering, nor was rapid medication withdrawal. Mean interseizure intervals were not significantly different for concordant and discordant seizures. Conclusions: Seizure clustering during epilepsy monitoring is common. Risk factors include history of clustering at home, MTS on MRI, and possibly more than one seizure focus. In this study, seizures that occurred in clusters had as important a role in localization as did nonclustered seizures.