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

Abstract

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.

Multifrequency Dynamics of Cortical Neuromagnetic Activity Underlying Seizure Termination in Absence Epilepsy

“…However, MEG signals are usually unaffected by the skull and skin, which leads to a higher spatial resolution than that of EEG ( Babiloni et al, 2009 ). Several studies have reported that MEG can localize epileptiform activity more accurately than EEG and provide reliable localization in the brain cortex ( Alkawadri et al, 2018 ; Tamilia et al, 2019 ; van Klink et al, 2019 ). Moreover, MEG has been increasingly used in the evaluation of epileptogenic foci before epileptic surgery because of its non-invasive precise localization ( De Tiege et al, 2017 ; Tamilia et al, 2017 ).…”

Distributed source analysis of magnetoencephalography using a volume head model combined with statistical methods improves focus diagnosis in epilepsy surgery

“…4). Unlike previous studies 6,7,[17][18][19][20][21][22] on focus diagnosis using DS analysis, all 19 cases analyzed were consecutive surgical cases, and their number was sufficient for statistical analysis ( Supplementary Table S2). We improved the DS analysis by introducing statistical methods.…”

“…Based on this principle, by constraining the dipole orientation parallel to cortical surface and modelling current distribution limited to the cortical surface, it is possible to precisely and efficiently calculate current distribution. In previous MEG studies 6,7,[17][18][19][20][21][22] that used DS analysis for focus diagnosis, a cortical surface model was used as the source space in the forward model ( Supplementary Table S2). However, when examining actual surgical cases, either (i) the brain structure was deformed by lesions caused, for example, by a tumor or trauma; (ii) the tumor had an epileptogenicity (e.g., dysembryoplastic neuroepithelial tumors and gangliogliomas); (iii) the epileptic focus was in a deep area, such as the medial, basal, and subcortical regions; or (iv) the focus had a complex histological structure.…”

“…It was clinically shown that the cDS analysis using the volume head model remained accurate in the deep area. In previous studies 6,7,[17][18][19][20][21][22] , brain lesions were mostly superficial, and the source space was calculated from the cortical surface model. This is the rare study to assess clinically the efficacy of the volume head model as the source space in epileptic focus diagnosis of actual surgical cases ( Supplementary Table S2).…”

Interictal and Ictal MEG in presurgical evaluation for epilepsy surgery

“…A study by Alkawadri et al [19] comparing interictal vs ictal MEG vs EEG reported that in 44 ictal MEGs, equivalent current dipole (ECD) analysis was possible in 29 patients (66%), of whom 8 had no clear interictal discharges in either MEG or EEG. Lobar concordance between ictal and interictal dipoles was seen in all the remaining 21 cases, in whom interictal activity was detected.…”