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Original Investigation
October 2018

Assessment of the Utility of Ictal Magnetoencephalography in the Localization of the Epileptic Seizure Onset Zone

Author Affiliations
  • 1The Epilepsy Center at Cleveland Clinic Foundation, Cleveland, Ohio
  • 2Yale Comprehensive Epilepsy Center, School of Medicine, Yale University, New Haven, Connecticut
  • 3Yale Human Brain Mapping Program, School of Medicine, Yale University, New Haven, Connecticut
  • 4The Department of Pediatrics, Tohoku University School of Medicine, Sendai, Japan
JAMA Neurol. 2018;75(10):1264-1272. doi:10.1001/jamaneurol.2018.1430
Key Points

Question  What is the chance of recording a seizure and the localizing yield of seizures recorded during routine magnetoencephalography (MEG) studies?

Findings  In this medical record analysis of 44 patients, MEG provided unique and more focal localization information, including in some electroencephalography and interictal MEG silent cases and in some cases that were nonlocalizable otherwise. Extended-source localization was more suitable for analysis of ictal rhythms than commonly used single-point solutions.

Meaning  If feasible, ictal MEG should be sought and considered in presurgical evaluation of drug-resistant epilepsy.


Importance  Literature on ictal magnetoencephalography (MEG) in clinical practice and the relationship to other modalities is limited because of the brevity of routine studies.

Objective  To investigate the utility and reliability of ictal MEG in the localization of the epileptogenic zone.

Design, Setting, and Participants  A retrospective medical record review and prospective analysis of a novel ictal rhythm analysis method was conducted at a tertiary epilepsy center with a wide base of referrals for epilepsy surgery evaluation and included consecutive cases of patients who experienced epileptic seizures during routine MEG studies from March 2008 to February 2012. A total of 377 studies screened. Data were analyzed from November 2011 to October 2015.

Main Outcomes and Measures  Presurgical workup and interictal and ictal MEG data were reviewed. The localizing value of using extended-source localization of a narrow band identified visually at onset was analyzed.

Results  Of the 44 included patients, the mean (SD) age at the time of recording was 19.3 (14.9) years, and 25 (57%) were male. The mean duration of recording was 51.2 minutes. Seizures were provoked by known triggers in 3 patients and were spontaneous otherwise. Twenty-five patients (57%) had 1 seizure, 6 (14%) had 2, and 13 (30%) had 3 or more. Magnetoencephalography single equivalent current dipole analysis was possible in 29 patients (66%), of whom 8 (28%) had no clear interictal discharges. Sublobar concordance between ictal and interictal dipoles was seen in 18 of 21 patients (86%). Three patients (7%) showed clear ictal MEG patterns without electroencephalography changes. Ictal MEG dipoles correlated with the lobe of onset in 7 of 8 patients (88%) who underwent intracranial electroencephalography evaluations. Reasons for failure to identify ictal dipoles included diffuse or poor dipolar ictal patterns, no MEG changes, and movement artifact. Resection of areas containing a minimum-norm estimate of a narrow band at onset, not single equivalent current dipole, was associated with sustained seizure freedom.

Conclusions and Significance  Ictal MEG data can provide reliable localization, including in cases that are difficult to localize by other modalities. These findings support the use of extended-source localization for seizures recorded during MEG.