Abstracts

Rationale: Successful epilepsy surgery depends critically upon accurate localization of the epileptogenic zone. Magnetoencephalography (MEG) allows non-invasive detection of interictal spike activity in epilepsy, which can then be localized in 3D by magnetic source imaging (MSI) techniques. Nevertheless, the clinical value of MEG in the pre-operative epilepsy evaluation is not fully understood, as studies to date are limited by small sample size or a lack of long-term seizure outcomes.Methods: A retrospective cohort study was performed including focal epilepsy patients who received MEG for interictal spike mapping followed by surgical resection at our institution.Results: A total of 132 surgical patients were studied, with mean post-operative follow-up of 3.6 years (minimum 1 year). Dipole source modelling was successful in 103 (78%) patients, while no interictal spikes were observed in others. Among patients with successful dipole modelling, MEG findings were concordant with and specific to: i) the region of resection in 66% of individuals, ii) invasive electrocorticography (ECoG) findings in 67% of patients, and iii) the MRI abnormality in 74% of individuals. MEG lateralization was discordant in ~5% of cases. After surgery, 70% of all patients became seizure free (Engel class I outcome). Although 85% of patients with concordant and specific MEG findings became seizure-free, this outcome was achieved by only 37% of those with MEG findings that were non-specific or discordant with the region of resection (χ2 = 26.4, p < 0.001). MEG reliability was similar in patients with or without localized scalp EEG, and localizing MEG findings predicted seizure freedom with an odds ratio of 5.11 (2.23-11.8, 95% CI).Conclusions: MEG is an important tool for non-invasive interictal spike mapping in epilepsy surgery, including patients with non-localized findings on long-term EEG monitoring, and localized interictal spikes on MEG are associated with improved seizure outcomes.