Abstracts

Rationale: Routine scalp EEG cannot always distinguish whether generalized epileptiform discharges are the result of primary bilateral synchrony or secondary bilateral synchrony (SBS) from a focal origin. Magnetoencephalography (MEG) has superior spatial resolution compared to EEG and can sometimes indicate a focal origin for SBS, but the selection of appropriate patients for MEG can be difficult and the yield of MEG in this regard is unknown. Methods: Sixteen medically intractable epilepsy patients with suspected SBS were referred for MEG with magnetic source imaging (MSI). All had bilateral synchronous, widespread, and most often generalized spike-wave discharges on scalp EEG, plus some other clinical (e.g. seizure semiology) or MRI feature (e.g. focal lesion) suggesting focal onset and hence surgical candidacy. MEG and simultaneous EEG were recorded with a 275-channel whole-head system. Equivalent current dipoles were calculated according to a single dipole model, and superimposed on patients’ MRIs. Results: The mean patient age and duration of seizure history were 21 and 14.3 years, respectively. Focal MSI dipole clusters were demonstrated in 12 of 16 patients (75%). Eight patients had a focal abnormality on MRI, of which seven had co-localized dipoles on MSI. Frontal (N=8) and temporal (N=4) sites were most common. Selective anterior temporal lobectomy (including the dipole cluster) in the one patient for whom we have long-term follow-up resulted in significant improvement in seizure control (Engel II). Conclusions: These findings suggest an important role for MEG in the localization of focal onset in patients for whom there is a suspicion of SBS (based on clinical or MRI data). Identification of a focal seizure origin has significant implications for both resective and non-resective treatment of intractable epilepsy.