Dr Mentzer and colleagues have taken an important first step in providing a laddered case definition for progressive multifocal leucoencephalopathy (PML) in patients treated with monoclonal antibodies (mAbs) which is based on the evidence supporting the diagnosis.1 While designated for PML occurring with mAbs, these case definitions should be equally applicable to PML arising with other conditions. In the decades following its initial description in 1958,2 the diagnosis of PML was dependent on demonstrating the characteristic histopathological triad of demyelination, bizarre astrocytes and enlarged oligodendroglial nuclei. The discovery of JC virus as the aetiology of PML in 19713 ultimately improved the diagnostic accuracy in tissue specimens. Despite descriptions of the characteristic lesions of PML on cranial MRIs shortly after the development of this technology,4 pathological confirmation remained a necessity until the introduction of PCR for JC virus DNA in the cerebral spinal fluid (CSF).5

PML essentially remained a medical curiosity that chiefly affected individuals with lymphoproliferative disorders until the AIDS pandemic.6 Through the mid 1990s, when effective antiretroviral therapy became available, PML was observed in as many as …