Diagnosing progressive multifocal leukoencephalopathy: clinical features, neuroimaging findings, confirmatory testing and proposed diagnostic algorithm.

Progressive multifocal leukoencephalopathy (PML) is an infectious disease of the central nervous system that is typically severe and classically occurs in the setting of overt immunocompromise. It is caused by the John Cunningham virus (JCV), a ubiquitous polyomavirus that only rarely leads to neurological complications. Prompt diagnosis of PML is essential for patient management, but can be complicated by disease development in individuals who are relatively immunocompetent, the lack of highly specific clinical symptoms, unfamiliarity with recently described neuroimaging signs and the imperfect sensitivity of assays used to detect JCV in cerebrospinal fluid (CSF). We review the supportive clinical features, neuroimaging findings and confirmatory testing for PML and propose an algorithm to facilitate accurate diagnosis in clinical practice.