title: The sensitivity and significance of lateralized interictal slow activity on magnetoencephalography in focal epilepsy.
abstract: OBJECTIVE
  Asymmetric large-amplitude slow activity is sometimes observed on interictal electroencephalography (EEG) in epilepsy. However, few studies have examined slowing during magnetoencephalography (MEG) recordings, which are performed primarily to localize interictal spikes. Also, no prior investigations have compared the sensitivity of MEG to scalp EEG in detecting slow rhythms.
  
  
  METHODS
  We performed a retrospective cohort study of focal epilepsy patients who received MEG followed by surgical resection at our institution. We examined MEG, simultaneous EEG, and long-term EEG recordings for prominent asymmetric slow activity (delta-range, 1-4 Hz), and evaluated post-operative seizure outcomes.
  
  
  RESULTS
  We studied 132 patients with ≥ 1 year post-operative follow-up (mean, 3.6 years). Mean age was 27 (range, 3-68) years, and 55% of patients were male. Asymmetric large-amplitude slow wave activity was observed on interictal MEG in 21 of 132 (16%) patients. Interictal slowing lateralized to the hemisphere of resection in all but one (95%) patient. Among the 21 patients with interictal MEG slowing, 11 (52%) individuals had similarly lateralized EEG slowing, 7 patients had no EEG slowing, and 3 had bilateral symmetric EEG slowing. Meanwhile, none of the 111 patients without lateralized MEG slowing had asymmetric EEG slowing, suggesting significantly higher sensitivity of MEG versus EEG in detecting asymmetric slowing (χ(2)=63.4, p<0.001). MEG slowing was associated with shorter epilepsy duration with an odds ratio of 5.4 (1.7-17.0, 95% confidence interval). At last follow-up, 92 (70%) patients were seizure free (Engel I outcome), with no difference in seizure freedom rates between patients with (71%) or without (69%) asymmetric MEG slowing (χ(2)=0.4, p=0.99).
  
  
  CONCLUSIONS
  MEG has higher sensitivity than scalp EEG in detecting asymmetric slow activity in focal epilepsy, which reliably lateralizes to the epileptogenic hemisphere. Other uses of MEG beyond spike localization may further improve presurgical evaluations in epilepsy.
authors.affiliation: Department of Neurological Surgery, Vanderbilt University Medical Center, Nashville, TN, USA. Electronic address: englot@gmail.com.
authors.forename: Dario J
authors.initials: DJ
authors.lastname: Englot
authors.affiliation: Biomagnetic Imaging Lab, Department of Radiology and Biomedical Imaging, University of California, San Francisco, CA, USA.
authors.forename: Srikantan S
authors.initials: SS
authors.lastname: Nagarajan
authors.affiliation: Department of Neurological Surgery, University of California, San Francisco, CA, USA; Biomagnetic Imaging Lab, Department of Radiology and Biomedical Imaging, University of California, San Francisco, CA, USA.
authors.forename: Doris D
authors.initials: DD
authors.lastname: Wang
authors.affiliation: Department of Neurological Surgery, University of California, San Francisco, CA, USA; Biomagnetic Imaging Lab, Department of Radiology and Biomedical Imaging, University of California, San Francisco, CA, USA.
authors.forename: John D
authors.initials: JD
authors.lastname: Rolston
authors.affiliation: Biomagnetic Imaging Lab, Department of Radiology and Biomedical Imaging, University of California, San Francisco, CA, USA.
authors.forename: Danielle
authors.initials: D
authors.lastname: Mizuiri
authors.affiliation: Biomagnetic Imaging Lab, Department of Radiology and Biomedical Imaging, University of California, San Francisco, CA, USA.
authors.forename: Susanne M
authors.initials: SM
authors.lastname: Honma
authors.affiliation: Biomagnetic Imaging Lab, Department of Radiology and Biomedical Imaging, University of California, San Francisco, CA, USA.
authors.forename: Mary
authors.initials: M
authors.lastname: Mantle
authors.affiliation: Department of Neurological Surgery, University of California, San Francisco, CA, USA; Biomagnetic Imaging Lab, Department of Radiology and Biomedical Imaging, University of California, San Francisco, CA, USA.
authors.forename: Phiroz E
authors.initials: PE
authors.lastname: Tarapore
authors.affiliation: Department of Neurology, University of California, San Francisco, CA, USA; Biomagnetic Imaging Lab, Department of Radiology and Biomedical Imaging, University of California, San Francisco, CA, USA.
authors.forename: Robert C
authors.initials: RC
authors.lastname: Knowlton
authors.affiliation: Department of Neurological Surgery, University of California, San Francisco, CA, USA; Biomagnetic Imaging Lab, Department of Radiology and Biomedical Imaging, University of California, San Francisco, CA, USA.
authors.forename: Edward F
authors.initials: EF
authors.lastname: Chang
authors.affiliation: Department of Neurology, University of California, San Francisco, CA, USA; Biomagnetic Imaging Lab, Department of Radiology and Biomedical Imaging, University of California, San Francisco, CA, USA.
authors.forename: Heidi E
authors.initials: HE
authors.lastname: Kirsch
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D058256:Brain Waves / Q000502:physiology*; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D004569:Electroencephalography; D004828:Epilepsies, Partial / Q000175:diagnosis* / Q000503:physiopathology*; D005260:Female; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D015225:Magnetoencephalography; D008297:Male; D008875:Middle Aged; D012680:Sensitivity and Specificity; D055815:Young Adult
keywords: Epilepsy surgery; Epileptogenic zone; MEG; Magnetic source imaging; Slowing