Is there a clustering effect on electroencephalographic seizure localization?

Long-term video-EEG monitoring (LTM) is the gold standard for initial lateralization and localization of seizures in the workup for neurosurgical treatment of medically intractable epilepsy. Previous studies have yielded contradictory results as to whether seizures that occur in clusters tend to arise from the same brain region and may lead to the incorrect conclusion that seizures arise from a single focus. To determine whether seizure clustering affects localization in an LTM setting, the authors performed an observational study over 6 years at a large regional epilepsy center on those undergoing LTM for seizure diagnosis, characterization, or presurgical workup. Excluding repeat studies and LTMs with generalized or nonepileptic seizures resulted in 479 monitorings with 2774 focal seizures for analysis. Sequential pairs of consecutive focal seizures were classed as "concordant", "discordant," or "other," based on EEG localization. ANOVA analysis on the logarithm of the interseizure interval (LISI) among the three seizure pair groups showed no significant difference, p=0.47, nor did analysis defining concordance as lateralization to the same hemisphere (p=0.34). Analyses on subgroups with multifocal seizures, bilateral seizures, and extratemporal seizures all failed to show a significant difference. In conclusion, seizures have the same localizing value whether occurring in a cluster over a few hours or sporadically over a few days. This could potentially lead to shorter monitoring times.