title: High-frequency oscillations, extent of surgical resection, and surgical outcome in drug-resistant focal epilepsy.
abstract: OBJECTIVE
  Removal of areas generating high-frequency oscillations (HFOs) recorded from the intracerebral electroencephalography (iEEG) of patients with medically intractable epilepsy has been found to be correlated with improved surgical outcome. However, whether differences exist according to the type of epilepsy is largely unknown. We performed a comparative assessment of the impact of removing HFO-generating tissue on surgical outcome between temporal lobe epilepsy (TLE) and extratemporal lobe epilepsy (ETLE). We also assessed the relationship between the extent of surgical resection and surgical outcome.
  
  
  METHODS
  We studied 30 patients with drug-resistant focal epilepsy, 21 with TLE and 9 with ETLE. Two thirds of the patients were included in a previous report and for these, clinical and imaging data were updated and follow-up was extended. All patients underwent iEEG investigations (500 Hz high-pass filter and 2,000 Hz sampling rate), surgical resection, and postoperative magnetic resonance imaging (MRI). HFOs (ripples, 80-250 Hz; fast ripples, >250 Hz) were identified visually on a 5-10 min interictal iEEG sample. HFO rates inside versus outside the seizure-onset zone (SOZ), in resected versus nonresected tissue, and their association with surgical outcome (ILAE classification) were assessed in the entire cohort, and in the TLE and ETLE subgroups. We also tested the correlation of resected brain hippocampal and amygdala volumes (as measured on postoperative MRIs) with surgical outcome.
  
  
  RESULTS
  HFO rates were significantly higher inside the SOZ than outside in the entire cohort and TLE subgroup, but not in the ETLE subgroup. In all groups, HFO rates did not differ significantly between resected and nonresected tissue. Surgical outcome was better when higher HFO rates were included in the surgical resection in the entire cohort and TLE subgroup, but not in the ETLE subgroup. Resected brain hippocampal and amygdala volumes were not correlated with surgical outcome.
  
  
  CONCLUSIONS
  In TLE, removal of HFO-generating areas may lead to improved surgical outcome. Less consistent findings emerge from ETLE, but these may be related to sample size limitations of this study. Size of resection, a factor that was ignored and that could have affected results of earlier studies did not influence results.
authors.affiliation: EEG Department, Montreal Neurological Institute, Montreal, Quebec, Canada.
authors.forename: Claire
authors.initials: C
authors.lastname: Haegelen
authors.forename: Piero
authors.initials: P
authors.lastname: Perucca
authors.forename: Claude-Edouard
authors.initials: CE
authors.lastname: Châtillon
authors.forename: Luciana
authors.initials: L
authors.lastname: Andrade-Valença
authors.forename: Rina
authors.initials: R
authors.lastname: Zelmann
authors.forename: Julia
authors.initials: J
authors.lastname: Jacobs
authors.forename: D Louis
authors.initials: DL
authors.lastname: Collins
authors.forename: François
authors.initials: F
authors.lastname: Dubeau
authors.forename: André
authors.initials: A
authors.lastname: Olivier
authors.forename: Jean
authors.initials: J
authors.lastname: Gotman
mesh_terms: D000328:Adult; D001683:Biological Clocks / Q000502:physiology*; D001931:Brain Mapping; D058256:Brain Waves / Q000502:physiology*; D004569:Electroencephalography; D004833:Epilepsy, Temporal Lobe / Q000503:physiopathology* / Q000601:surgery*; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D016896:Treatment Outcome; D055815:Young Adult