We aim to report a series of patients who presented with an initial immune mediated neurological disorder and eventually diagnosed with Primary Brain Tumour (PBT)  within 5 years of the initial diagnosis and to explore a possible para neoplastic connection between the two.

Paraneoplastic neurological disorders (PND), classical or non classical are a common and well-established entity but not described with primary brain tumors (PBT) to date

Patients with a histopathological confirmed diagnosis of PBT, who also had an immune mediated neurological disorder with or without antibody positivity, within 5 years of the diagnosis of the PBT were identified by retrospective chart review. 6 such cases were included. The clinical, radiological data and other investigations were analysed.

Of the 6 patients, three had a final diagnosis of glioblastoma, wherein two had an initial presentation of CASPR2 and seronegative autoimmune encephalitis, 1.5 and 3 years preceding the final diagnosis, respectively while the third had concomitant AQP4 antibody positivity. One patient, diagnosed as primary CNS lymphoma, had an initial picture of MOG positive demyelination resistant to immune therapy with multiple relapses over 4-month period prior to the final diagnosis. One patient had an initial seronegative NMO spectrum disorder like presentation with optic neuritis and longitudinally extensive transverse myelitis, but finally diagnosed as H3K27 mutant diffuse midline glioma. One had undiagnosed myasthenia gravis, for 1 year prior to his subsequent diagnosis of Oligodendroglioma and post-surgical evaluation confirmed Acetylcholine receptor antibody positive myasthenia gravis.

Frequent relapses, chronic smouldering clinical course, inadequate response to optimal immunosuppression, atypical radiological features and inconsistent laboratory features are red herrings pointing to an atypical immune mediated disorder or a primary brain tumour. Identifying these can have far reaching therapeutic and prognostic implications. The dimension of a possible paraneoplastic association between the two requires further exploration.