Posterior fossa progressive multifocal leukoencephalopathy: first presentation of an unknown autoimmune disease.

We present a case of a 57-year-old man who presented with progressive cerebellar dysarthria and cerebellar ataxia. Additional investigations confirmed the diagnosis of progressive multifocal leukoencephalopathy (PML) in the posterior fossa. This is a demyelinating disease of the central nervous system, caused by an opportunistic infection with John Cunningham virus. PML has previously been considered a lethal condition, but because of careful monitoring of patients with HIV and of patients using immunosuppressive drugs it is discovered in earlier stages and prognosis can be improved. Our patient had no known immune-compromising state, but further work-up revealed that the PML was most likely the first presentation of a previous untreated autoimmune disorder: sarcoidosis.