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.

Figure 1.. Localization of Seizure Onset Using Single Equivalent Current Dipole (sECD) Analysis

A, Detection and analysis of a seizure on a routine magnetoencephalography. The arrowhead indicates the seizure onset in the left parietal subset of contacts. The background is replaced by evolving admixed frequencies. B, Selection of a subset of magnetoencephalography sensors involved at seizure onset to increase the signal-to-noise ratio and efficient sECD fitting. The red circle is a schematic approximation of 30 to 60 sensors identified by a certified magnetoencephalographer and averaged for subsequent dipole fitting. The averaging is performed to maximize the contribution of the waveform of interest in dipole estimation and to eliminate/minimize other signals and weak fluxes contribution. C, Determining the fitting parameters of sECD of a spike at the onset of seizure. Top, The analysis was performed on the average magnetoencephalography signal segment (red) at a specific time point (vertical line). Bottom, The blue and red lines indicate outgoing and incoming magnetic fluxes, respectively. The green arrow indicates the location and direction of equivalent current dipoles. D, Coregistration of the results of sECD analysis after passing fitting parameters with T1-weighted magnetic resonance imaging (MRI). The yellow dot and line represent the location and the orientation of the dipole, respectively.

Figure 2.. L2 Minimum-Norm Estimate of a Narrow Band (MNE-fc)

A, A 10-second view of seizure onset in the right parietooccipital sensors. The low-frequency and high-frequency filters are set at 1 Hz and 40 Hz, respectively. The vertical line indicates the emergence of evolving paroxysmal fast activity at seizure onset. B, Morlet wavelet time-frequency decomposition of data. The red square indicates seizure onset. C, Demonstration of stability of current densities at the sensor level around the dominant frequency at onset (left) and in time with evolution (right). D, Concordant localization of seizure onset using single equivalent current dipole (sECD) analysis and L2 MNE-fc of seizure shown in A. E, The patient underwent 2 prior resections with early recurrences. The resection contained L2 MNE-fc activation but not the area pinpointed by sECD. The patient remains seizure free. The gray mesh indicates sites of previous resections. F, Modeling parameters of sECD were not met. Resection of the L2 MNE-fc area at the posterior edge of a previous resection led to seizure freedom. Color scales are normalized to the maximum current density per map in all panels.