Specific Imaging Findings

On CT, progressive multifocal leukoencephalopathy (PML) presents as patchy areas of low attenuation limited to the white matter, resembling vasogenic edema. MRI is the technique of choice showing high T2 signal of the lesions which are usually multifocal, commonly found in the parieto-occipital region, corpus callosum, and cerebellum, but they can be solitary and occur in any location. The lesions are characteristically of very low T1 signal, asymmetrical, and involving the subcortical Ufibers. In classic PML (cPML) enhancement is absent and hemorrhage is very unusual. At the center of active lesions, microcysts of very high T2 signal are sometimes seen. DWI shows a central low signal intensity core, surrounded by a rim of higher signal, with corresponding higher and lower ADC values, respectively. The advancing edge of reduced diffusivity represents active disease and may also contain incomplete T1 hyperintense rim. Perfusion studies show low rCBV of PML lesions. A severe reduction in NAA and increase in choline levels, along with lactate peak, is found on MRS. Inflammatory PML (iPML) lesions, which usually occur in the setting of immune reconstitution inflammatory syndrome (IRIS), show peripheral enhancement and/or mass effect. Advanced chronic stage of the disease demonstrates prominent atrophy of the involved white matter.

Pertinent Clinical Information

PML is an opportunistic infection caused by JC (after the patient John Cunningham) polyomavirus (JCV), which may affect any immunocompromised patient. The incidence has greatly increased with the AIDS epidemic and it also occurs as a side effect of monoclonal antibody medications. The clinical presentation includes gradually worsening hemiparesis, speech disturbances, limb incoordination, and visual impairment, without prominent dementia. The diagnosis suggested on imaging is confirmed by CSF polymerase chain reaction (PCR) for JC virus DNA. Therapy consists of cessation of immunosuppressive agents and restoration of the immune system.