Abstracts

Rationale: Asymmetric large-amplitude slow activity is sometimes observed on interictal electroencephalography (EEG) in epilepsy, and may help lateralize the epileptogenic zone. However, few studies have examined the presence of slowing during magnetoencephalography (MEG) recordings, which are performed primarily to identify interictal spikes. Also, no prior investigations have compared the sensitivity of MEG to scalp EEG in detecting these slow rhythms.Methods: We performed a retrospective cohort study of focal epilepsy patients who received MEG followed by surgical resection at our institution. We examined MEG, simultaneous EEG, and long-term EEG recordings for prominent asymmetric slowing (1-4 Hz), and evaluated post-operative seizure outcomes.Results: We identified 132 patients who underwent MEG followed by resection, with ≥ 1 year post-operative follow-up (mean, 3.6 years). Mean age was 27 (range, 3-68) years, and 55% of patients were male. Asymmetric large-amplitude slow wave activity was observed on interictal MEG recordings in 21 of 132 (16%) patients. Interictal slowing lateralized to the hemisphere of planned resection in all but one (95%) patient. Among the 21 patients with interictal MEG slowing, similar lateralized EEG slowing was present in only 11 (52%) individuals, while 7 patients had no EEG slowing, and 3 had bilateral symmetric EEG slow activity. Meanwhile, none of the 111 patients without lateralized MEG slowing had asymmetric EEG slowing, suggesting significantly higher sensitivity of MEG versus EEG in detecting asymmetric slowing (χ2 = 63.4, p < 0.001). Epilepsy duration < 10 years was associated with an increased likelihood of MEG slowing with an odds ratio of 5.4 (1.7-17.0, 95% confidence interval). At last follow-up, 92 (70%) patients were seizure free (Engel I outcome), and there was no relationship between the presence of MEG slowing and seizure outcome (χ2 = 0.4, p = 0.99).Conclusions: MEG has higher sensitivity than scalp EEG in detecting asymmetric slow activity in focal epilepsy, which lateralizes to the epileptogenic zone. Other uses of MEG beyond spike localization may further improve pre-surgical evaluations in epilepsy.