Abstracts

Rationale: Neocortical epilepsy surgery currently fails to achieve post-operative seizure freedom in 20-40% of cases. It is not fully understood why surgical interventions in some patients are unsuccessful. Comparing patient data to normative maps, which describe healthy spatial and population variability, can identify abnormalities which could relate to surgical failure. Here we propose three possible mechanisms that could contribute to a poor surgical outcome; (1) failure to resect abnormal tissue, (2) removing abnormal tissue, but failing to resect other, more abnormal regions, and (3) failure to sufficiently decrease the overall abnormality of the brain, leading to the reorganisation of the epileptogenic network. In this study we develop markers of these mechanisms, validating them against patient outcomes.    _x000D_ _x000D_ Methods: Resting-state eyes-closed MEG recordings were acquired for 70 healthy controls and 32 patients with refractory neocortical epilepsy. Relative band power spatial maps for five frequency bands were computed using source localised recordings of healthy controls. Patient and region-specific bandpower abnormalities were estimated as the maximum absolute z-score across all five frequency bands using healthy data as a baseline. Resected regions were identified using post-operative T1 MRI. We hypothesised that our mechanistically interpretable markers would discriminate patients with post-operative seizure freedom (ILAE 1 vs ILAE 2+). _x000D_ _x000D_ Results: Mechanisms of surgical failure discriminate surgical outcome groups (Mechanism 1: AUC=0.80, p=0.003, Mechanism 2: AUC=0.68, p=0.053, Mechanism 3: AUC=0.64, p=0.096), performing as well as commonly collected clinical demographics. Leveraging all mechanisms simultaneously demonstrated that 95% of ILAE 2+ patients exhibited negative markers in at least one of the three proposed mechanisms. In contrast, of those patients with positive markers for all three mechanisms, 80% were ultimately seizure-free. _x000D_ _x000D_ Conclusions: The mapping of abnormalities across the brain is important for a wide range of neurological conditions. Here we have demonstrated that interictal MEG bandpower abnormalities has merit for the localisation of pathology, and improving our mechanistic understanding of epilepsy. Our mechanisms of surgical failure, in addition to others, could be used by future studies to construct predictive models of surgical outcome, aiding clinical teams during patient pre-surgical evaluations.  _x000D_ _x000D_ Funding: T.O. was supported by the Centre for Doctoral Training in Cloud Computing for Big Data (EP/L015358/1). P.N.T. and Y.W. are both supported by UKRI Future Leaders Fellowships (MR/T04294X/1, MR/V026569/1).