Progressive multifocal leukoencephalopathy as the first presentation of sarcoidosis.

Recognition of progressive multifocal leukoencephalopathy (PML) in patients with an established primary neuroinflammatory condition can be clinically challenging. Delayed or incorrect diagnosis may worsen the course of the disease and result in an inaccurate prognosis. We present an unusual case of a patient with a rapid decline in visual acuity, positive serum ACE and extensive lymphadenopathy who was found to have progressive subcortical lesions and cerebrospinal fluid PCR positive for John Cunningham virus supporting a coincidental diagnosis of PML. The prognosis of PML is affected by the associated condition. Establishing the diagnosis is important for an exact prognosis of the primary condition but also to allow early discontinuation of immunomodulatory treatment. Sarcoidosis-associated PML might have a similar aggressive course to that seen when associated with haematological malignancies. © BMJ Publishing Group Limited 2020. No commercial re-use. See rights and permissions. Published by BMJ.