Epileptogenic zone localization using magnetoencephalography predicts seizure freedom in epilepsy surgery

Abstract

Objective: The efficacy of epilepsy surgery depends critically upon successful localization of the epileptogenic zone. Magnetoencephalography (MEG) enables noninvasive detection of interictal spike activity in epilepsy, which can then be localized in three dimensions using magnetic source imaging (MSI) techniques. However, the clinical value of MEG in the presurgical epilepsy evaluation is not fully understood, as studies to date are limited by either a lack of long-term seizure outcomes or small sample size.

Methods: We performed a retrospective cohort study of patients with focal epilepsy who received MEG for interictal spike mapping followed by surgical resection at our institution.

Results: We studied 132 surgical patients, with mean postoperative follow-up of 3.6 years (minimum 1 year). Dipole source modeling was successful in 103 patients (78%), whereas no interictal spikes were seen in others. Among patients with successful dipole modeling, MEG findings were concordant with and specific to the following: (1) the region of resection in 66% of patients, (2) invasive electrocorticography (ECoG) findings in 67% of individuals, and (3) the magnetic resonance imaging (MRI) abnormality in 74% of cases. MEG showed discordant lateralization in ~5% of cases. After surgery, 70% of all patients achieved seizure freedom (Engel class I outcome). Whereas 85% of patients with concordant and specific MEG findings became seizure-free, this outcome was achieved by only 37% of individuals with MEG findings that were nonspecific to or discordant with the region of resection (χ(2) = 26.4, p < 0.001). MEG reliability was comparable in patients with or without localized scalp electroencephalography (EEG), and overall, localizing MEG findings predicted seizure freedom with an odds ratio of 5.11 (95% confidence interval [CI] 2.23-11.8).

Significance: MEG is a valuable tool for noninvasive interictal spike mapping in epilepsy surgery, including patients with nonlocalized findings receiving long-term EEG monitoring, and localization of the epileptogenic zone using MEG is associated with improved seizure outcomes.

Keywords: Epilepsy surgery; Epileptogenic zone; Interictal spike; Magnetic source imaging; Magnetoencephalography.

Figure 1. Example of MSI dipole modelling with simultaneous MEG/EEG recordings

A) Recordings from selected MEG channels and simultaneous EEG in a 12-year-old female with drug-resistant focal epilepsy. A representative interictal spike is seen in both MEG and EEG recordings localizing to the right frontal/central region. B) Localization of single dipole sources corresponding to the spike in A, and similar spikes during the recording, shown as triangles with vector tails superimposed on T1-weighted anatomical MRI. C) Pre-operative T2-weighted axial MRI showing a subtle abnormal blurring of gray-white matter differentiation in the right anterior cingulate region (circle), proximal to the location of MEG dipoles. D) Post-operative T2-weighted axial MRI demonstrating the resection cavity. Neuropathological examination revealed FCD type IIA, and the patient remains seizure-free two years after surgery. EEG: electroencephalography; FCD: focal cortical dysplasia; MEG: magnetoencephalography; MRI: magnetic resonance imaging; MSI: magnetic source imaging.

Figure 2. MEG concordance with the area of resection, ECoG, and MRI

Shown is the number of patients with concordance between the region of MEG spike activity to three reference regions: i) the region of resection, ii) the epileptogenic zone delineated by ECoG, and iii) MRI abnormality. Cases are classified as concordant and specific, concordant but non-specific (same region, but >10% spikes also noted elsewhere), concordant lateralization only (same side, different region), and discordant lateralization (>50% spikes contralateral). Only 103 (78%) of 132 patients are included in this graph, as no spikes were modelled with MEG in 29 patients. ECoG: electrocorticography; MEG: magnetoencephalography; MRI: magnetic resonance imaging.

Figure 3. Relationship between MEG findings and seizure outcome

A) Post-operative seizure-freedom (Engel I outcome) was significantly more common in patients with concordant and specific MEG (85% seizure-free) than in patients with non-specific, lateralized-only, or discordant MEG (37% seizure-free overall, χ² = 26.4, p < 0.001). Among patients with no spikes modelled, 72% were seizure-free after surgery. B) MEG findings stratified by seizure outcome. For details of these categories, see Fig. 2 legend. N = 132 patients. MEG: magnetoencephalography.