Abstract
Tumefactive appearing lesions on brain imaging can cause a diagnostic dilemma. We report a middle-aged man who presented with right-sided optic neuritis. A brain MRI showed enhancement of the right optic nerve, and non-enhancing white matter lesions including a 3 cm right frontal lesion with adjacent gyral expansion. Cerebrospinal fluid analysis showed five oligoclonal bands not present in serum. Glatiramer acetate was started for suspected tumefactive multiple sclerosis (MS). A follow-up brain MRI 6 months later showed persistence of the frontal gyral expansion. A brain biopsy led to the diagnosis of an oligodendroglioma, isocitrate dehydrogenase-mutant and 1 p/19q co-deleted (WHO grade II), managed with surgical resection and radiotherapy. Postoperative brain MRI showed a new enhancing periventricular lesion, making the choice of optimal disease-modifying therapy for MS challenging. This case highlights the possibility of coexistence of MS and oligodendroglioma, and emphasises the importance of a tissue diagnosis when atypical MS imaging features are present.