Specific Imaging Findings

Patchy and ill-defined, usually large areas of “lacy” enhancement within the white matter are characteristic of immune reconstitution inflammatory response (IRIS) on post-contrast MR imaging. These enhancing lesions may be focal or scattered throughout different areas of the brain and leptomeningeal enhancement may also be observed. There is usually associated white matter T2 hyperintensity, low T1 signal, increased diffusivity and minimal to mild mass effect. The new imaging findings frequently occur within or adjacent to the isolated pre-existing lesions, and may be associated with progressive multifocal leukoencephalopathy (PML).

Areas of abnormal signal and contrast enhancement do not necessarily correspond to each other and enhancement may be absent. Overall, the imaging findings in IRIS are diverse and frequently atypical so that this entity should be considered whenever an unusual MRI appearance is encountered in patients treated with HAART.

Pertinent Clinical Information

IRIS presents as clinical deterioration and imaging disease progression during immunologic recovery while the HIV infection is effectively treated with HAART. Criteria for diagnosis include prior response to antimicrobial therapy, return of original symptoms or new inflammatory syndromes after initiation of HAART, and negative CSF cultures. The patients at greatest risk have a low CD4+ T-cell count prior to the initiation of HAART and show a rapid decrease in plasma HIV RNA during the first 3 months of therapy. IRIS may also occur following discontinuation of natalizumab in patients with multiple sclerosis.