Extraparenchymal Brain Lesions in Anti-NMDAR Encephalitis Leading to Primary Leptomeningeal Lymphoma: A Rare Evolution with Diagnostic Implications.

Anti-N-methyl-d-aspartate receptor encephalitis is an autoimmune disorder typically associated with normal or intraparenchymal brain magnetic resonance imaging (MRI) findings. We herein report a patient who initially presented with extraparenchymal brain lesions and later developed primary leptomeningeal lymphoma (PLML), an uncommon sequence that may inform clinical vigilance and diagnostic refinement. A 49-year-old woman presented with a headache and dysarthria. MRI revealed hyperintensity along the right central sulcus and fluid accumulation in the right frontal epidural space. Anti-NMDAR antibodies were detected in the cerebrospinal fluid, confirming the diagnosis of anti-NMDAR encephalitis. She was treated with steroids, intravenous immunoglobulin, plasma exchange, and cyclophosphamide, which resulted in a full recovery. Two years later, she presented with cerebellar symptoms and was diagnosed with PLML via a brain biopsy. Atypical extraparenchymal brain lesions were present at disease onset. Despite successful initial treatment, the patient later developed PLML, suggesting a potential paraneoplastic relationship or immunological continuum between the two diseases. his case illustrates the diagnostic challenges posed by atypical neuroimaging findings in anti-NMDAR encephalitis and the need for a longitudinal follow-up. Clinicians should consider neoplastic etiologies when imaging or the clinical course deviates from the norm, even if anti-NMDAR antibodies are positive.