Abstracts

Rationale: Resective surgery can lead to sustained seizure control in neocortical epilepsy but up to 60% of operated patients will suffer from recurrent or residual seizures after surgery. Reoperation can be particularly difficult because of the craniotomy defect and scarring from previous surgery. As magnetic fields are unaffected by the previous craniotomy or scarring, we studied the role of Magnetoencephalography (MEG) in the localization of recurrent or residual seizures after failed epilepsy surgery. Methods: We studied 14 patients with recurrent or residual seizures after neocortical epilepsy surgery who had an MEG study as part of the non-invasive evaluation for reoperation. MEG data was collected using a 275-channel whole head MEG system for a minimum duration of 60 minutes with simultaneous EEG recording. MEG data was analyzed using a single equivalent current dipole model applied to the earliest peak of each epileptic event as part of a standard clinical protocol. Results: Age at MEG ranged from 25-66 years (37.6±14.8). Previous surgery consisted of frontal cortical resection in 11 patients, insular resection, temporal neocortex and parietal operculum resection in one each. Magnetic resonance imaging (MRI) prior to the first surgery was normal in nine patients, cortical dysplasia, low grade tumour and encephalomalacia in one each and MRI was unavailable in two patients. Seizure recurrence after the first surgery occurred within one month in nine patients, 1-6 month in four patients and timing of recurrence was unknown in one patient. MEG spike sources localized adjacent to the margin of the previous resection in nine patients, remotely to a different brain region in one patient and to a hidden operculoinsular focus in four patients. Five patients underwent reoperation guided by the MEG results and surgery is pending in another five patients. Four patients had Engel class Īa outcome and ĪĪa outcome was achieved in one patient. Conclusions: MEG provides useful localizing information and should be part of the non-invasive workup after failed epilepsy surgery. False localization, hidden focus and recurrence at the margin of the previous resection are common reasons for unsuccessful epilepsy surgery. Specific patterns of MEG localization can provide useful information explaining the reason for surgical failure and guiding future surgery. Surgical resection including MEG cluster located adjacent to the margins of previous resections can achieve favorable outcome.