Diffuse low grade glioma after the 2016 WHO update, seizure characteristics, imaging correlates and outcomes

The majority of patients with supratentorial diffuse grade II glioma present with seizures, which adversely affect quality of life. The exact mechanism of epileptogenesis is unknown and the influence of tumour characteristics, radiological and histological, are not well studied, particularly following the introduction of molecular genetics in the 2016 WHO reclassification of gliomas. We sought to define predictors of seizure development and outcome in low grade glioma.

A retrospective review of patients who underwent resection of a supratentorial grade II glioma in a single institution. All patients underwent surgery at initial presentation with the aim of maximal safe resection. Presenting symptoms and radiological variables were recorded, including eloquent location, cortical involvement, tumour margins and tumour volume. Extent of resection (EOR), surgery type (awake vs asleep) and seizure outcome were analysed. Using molecular genetics data the original histology was reclassified according to the 2016 WHO update.

63 patients were included, 45 (71%) presented with seizures. 36 (57%) had oligodendroglioma and 27 astrocytoma. IDH-1 mutation was present in 53 (84%). 18 (29%) had tumour in an eloquent location. 33 (73%) were Engel class I following surgery at median follow up of 43 months. 6 patients were Engel II, 6 class III. Complete and near total resection were associated with improved Engel class compared to subtotal resection. No factors such as age, tumour location, tumour margins or tumour molecular genetics (including IDH-1 mutation) predicted better seizure outcome. Updated histological subtype did not predict the presence of seizures at initial diagnosis, only tumour heterogeneousity on initial MRI (p = 0.043). More patients who underwent awake craniotomy with intraoperative mapping were Engel class 1 post-operatively than those operated under general anaesthetic (84% vs 65%). Tumour volume at presentation did not correlate with seizure outcome but impacts on the EOR.

Seizure outcome is directly related to EOR in low grade glioma, which can be predicted by the initial tumour volume. Tumour histological subtype, including updated molecular genetic classification did not predict seizure development or outcome in this series. The use of awake craniotomy results in greater EOR and improved Engel Class following surgery.