Primary Central Nervous System Lymphoma with Exclusive Leptomeningeal Involvement: The Diagnostic Challenge of a Rare Presentation of an Uncommon Disease

Primary central nervous system lymphoma (PCNSL) is a rare and aggressive extra-nodal non-Hodgkin lymphoma (NHL). It must be confined to the brain, eyes, spinal cord or leptomeninges without systemic involvement at the time of diagnosis. Disease confined to the cerebrospinal fluid (CSF) is a rare form of presentation and poses a particular diagnostic challenge. We present the case of an 82-year-old man admitted to hospital because of an acute confusional state, later revealed to be due to PCNSL with exclusive leptomeningeal involvement. The diagnostic process was further impaired (or, perhaps, aided?) by the onset of a COVID-19 outbreak on the ward. LEARNING POINTS: Primary central nervous system lymphoma is a rare and aggressive form of non-Hodgkin lymphoma. Exclusive involvement of the cerebrospinal fluid (CSF) is a rare form of presentation of PCNSL. The diagnosis is usually histopathological but, in the absence of a solid lesion that can be biopsied, CSF cytology and flow cytometry may be sufficient for a definitive diagnosis.