Abstracts

Rationale: Epilepsy surgery is the therapy of choice in patients with refractory focal epilepsy. The identification of an underlying lesion on magnetic resonance imaging (MRI) concordant to electrophysiological findings has the highest predictive value for a good surgical outcome. 20–40% of patients will still show no evidence of an underlying structural epileptogenic lesion and are referred to as non-lesional epilepsy and are considered to be less favorable candidates for epilepsy surgery. In the present study, we report the impact of the integration of a clinical MEG service on the surgical outcome in non-lesional refractory focal epilepsy (NLRFE).Methods: Between April 2006 and January 2010, 31 consecutive patients (mean age 34 years) with NLRFE underwent MEG study using a 275 axial gradiometer MEG system. For various reasons MEG results were not included in the preoperative consensus decision, as the acquired MEG data could not be analyzed in a timely fashion and/or were felt to still be experimental. In those patients the decision to undergo surgery was based on other non-invasive modalities. MEG data were analyzed retrospectively by an epileptologist who was blinded to the patients’ clinical data and had only access to samples of the patients’ interictal EEG. To determine if MEG results would have changed patient management, anonymized results of the standard presurgical evaluation for all patients were presented randomly to a multidisciplinary epilepsy surgery team at first blinded to MEG results and then with the MEG results. Correlations between MEG, intracranial EEG and clinical outcome were also evaluated.Results: Localized MEG spike source cluster was seen in 22 patients, bilateral MEG clusters in two, scattered spike sources in three, no or very few spikes in three and failed MEG study due to coregistration error in one. 17 patients proceeded to surgery with follow up duration of 1.5-7 years. Favorable outcome (Engel class I & II) was achieved in 11 patients while five patients had poor outcome (Engel class III and IV). One patient had class IV outcome after 6 years of near complete seizure freedom. In patients who had favorable outcome (Class I &II), MEG showed localized clusters corresponding to the resection area in nine patients (Mesial temporal (4), Frontal (2), Insula (2), Parieto-occipital (1)). Among patients with poor outcome (Class III & IV) three patients had MEG clusters that were not included in the resection margin. Based on consensus opinion, MEG would have changed the surgical decision by allowing direct surgery (2), decrease in the number of electrodes, mostly eliminating interhemispheric strips (5), Decrease in number of electrodes and change in grid position (2) and more importantly significantly changing the resection area in three patients who had poor outcome (18%).Conclusions: This study demonstrated the significant impact of MEG on surgical outcome of NLRFE. The integration of MEG in the surgical decision process would have significantly improved outcome in approximately 20 % of patients.