What lies beneath the tent? JC-virus cerebellar granule cell neuronopathy complicating sarcoidosis

A 49-year-old, HIV-negative woman with sarcoidosis presented with a subacute unilateral cerebellar syndrome. A brain MRI revealed a hyperintense lesion without mass effect in the left cerebellar hemisphere, but no pathology above the tentorium. Steroid therapy for presumed neurosarcoidosis was ineffective and the patient deteriorated progressively. Cerebellar biopsy showed abnormal granule cells and demyelination. Immunocytochemistry confirmed the diagnosis of progressive multifocal leucoencephalopathy (PML) with JC (John Cunningham) virus granule cell neuronopathy. The patient succumbed to progressive brainstem dysfunction despite treatment with cidofovir. Although rare, PML should be considered in all patients with impaired cell-mediated immunity and unexplained neurological dysfunction, even in the absence of HIV infection.