Progressive multifocal leukoencephalopathy (PML) can be considered in patients with confluent white matter changes, and usually in patients with a degree of immunosuppression.

With the background of pulmonary sarcoidosis and recent azathioprine therapy, there were a number of potential triggers, although the disease clinically and radiologically worsened following cessation of AZA.

Despite several CSF samples being negative for JC virus, PML remained the favoured diagnosis and the decision was made to proceed to biopsy to confirm this, with positive results on both PCR and microscopy.

A good clinical and radiological response was seen following infliximab therapy, which has been used in sarcoidosis associated PML cases (reference available at https://doi.org/10.1093/braincomms/fcab292).