Primary diffuse leptomeningeal melanomatosis presenting as chronic meningitis and dense spinal plaques.

Primary diffuse leptomeningeal melanomatosis (PDLM) is an exceptionally rare and aggressive central nervous system melanocytic tumour, often masquerading as inflammatory or infective leptomeningeal disease. A woman in her early 60s presented with hydrocephalus and diffuse leptomeningeal enhancement, initially treated as neurosarcoidosis. This progressed, despite immunosuppression, and repeated lumbar punctures yielded no cerebrospinal fluid (CSF) ('dry taps'). Intradural biopsy identified a melanocytic neoplasm with a GNA11 mutation. Fewer than 25 cases of isolated PDLM have been reported, and misdiagnosis is common. MRI typically shows T1-hyperintense leptomeningeal lesions but CSF is often non-diagnostic. Repeated failure to obtain CSF is a potential clinical clue to extensive leptomeningeal tumour infiltration. PDLM should be considered in atypical, treatment-refractory leptomeningeal disease. Recognising its characteristic imaging features and obtaining early biopsy are important to avoiding diagnostic delay.