This case demonstrates the imaging appearance of immune reconstitution inflammatory syndrome (IRIS) complicating progressive multifocal leukoencephalopathy (PML) in an AIDS patient with a low CD4 count.

The first scan demonstrated background PML changes. The progressive appearance and new enhancement seen on the second MRI, particularly given the clinical setting of worsening symptoms after initiation of therapy, is consistent with IRIS. The subsequent improvement following treatment with steroids (on the third followup MRI) supports this.

IRIS is a phenomenon of paradoxic worsening of clinical and/or radiological inflammatory illness which is sometimes seen in AIDS patients who are commenced on antiretroviral therapy (ART), usually in the setting of low CD4 count/high viral load.

It is thought to occur due to restoration of the previously suppressed inflammatory immune response, and has been described in patients with various AIDS related illnesses, including TB, cryptococcus, CMV, Kaposi’s sarcoma – as well as PML.