title: Seizure lateralization during EEG monitoring in patients with bilateral foci: the cluster effect.
abstract: OBJECTIVE
  To determine whether seizures that occur in clusters are more likely to reflect activity of the same focus than are seizures that are widely separated in time.
  
  
  METHODS
  EEG monitoring data from 14 patients with bilateral independent seizure onsets were analyzed. Twelve of the 14 patients had surface recordings only, and two had implanted electrodes. Interseizure intervals (ISIs) for 151 seizure pairs were measured. Seizure onsets were classified as right hemispheric, left hemispheric, or indeterminate. Seizure pairs were classified as concordant for hemisphere of onset, discordant, or indeterminate. The relation between seizure-pair concordance and ISI was examined by using univariate analysis and analysis of variance (ANOVA).
  
  
  RESULTS
  Both seizures originated from the same hemisphere in 61 (75%) of 81 seizure pairs with ISIs <8 h, compared with 28 (55%) of 51 seizure pairs with ISIs >8 h (p < 0.015). The cluster effect was not more pronounced for ISIs <2 h. ANOVA demonstrated that the relation between ISI and seizure concordance was not a result of the variability in seizure rates among patients. In three patients, the presence of bilateral foci was not demonstrated until more than five seizures were recorded.
  
  
  CONCLUSIONS
  Seizures that occur after an ISI of <8 h are more likely to come from the same side as the previous seizure than are those with longer ISIs. Thus clustered seizures should not be given the same weight as seizures widely separated in time. In addition, more than five seizures may sometimes be needed to adequately assess patients being evaluated for epilepsy surgery.
authors.affiliation: Comprehensive Epilepsy Management Center, and Department of Neurology, Montefiore Medical Center, Albert Einstein College of Medicine, Bronx, New York 10467-2490, USA.
authors.forename: S R
authors.initials: SR
authors.lastname: Haut
authors.forename: A D
authors.initials: AD
authors.lastname: Legatt
authors.forename: C
authors.initials: C
authors.lastname: O'Dell
authors.forename: S L
authors.initials: SL
authors.lastname: Moshé
authors.forename: S
authors.initials: S
authors.lastname: Shinnar
mesh_terms: D000704:Analysis of Variance; D002540:Cerebral Cortex / Q000503:physiopathology*; D016000:Cluster Analysis; D004567:Electrodes, Implanted; D004569:Electroencephalography / Q000379:methods / Q000706:statistics & numerical data*; D017029:Epilepsy, Complex Partial / Q000175:diagnosis / Q000503:physiopathology; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D008991:Monitoring, Physiologic; D012640:Seizures / Q000175:diagnosis* / Q000503:physiopathology