Introduction Progressive Multifocal Leukoencephalopathy (PML) is a rapidly progressive demyelinating disorder of the central nervous system, caused by reactivation of JC virus under conditions of cellular immunosuppression. Stereotactic brain biopsy provides a gold standard for diagnosis. Though PCR for the JC virus in the CSF is the method of first choice, the sensitivity is reported as only 74% to 82%, with specificity from 96% to 100%.
Case presentation We report a 59-year old man with a previous history of biopsy-proven pulmonary fibrosis and 5 years of treatment with Azathioprine 75 mg per day and Prednisolone 5 mg daily, who presented with unusual symptoms of loss of colour vision over 6 months. His imaging was suggestive of PML, but JC virus PCR in the CSF was negative. However, brain biopsy confirmed the diagnosis of PML through histological evidence of demyelination and visualisation of viral inclusions. Despite terminating the immunosuppression, and treating with Cidofovir and Mirtazapine, the disease rapidly advanced and was fatal within a few months.
Conclusion Patients with low dose immunosuppression are at risk of PML, and this diagnosis cannot be ruled out by a negative PCR for the JC virus in the CSF.