= rapidly progressive fatal demyelinating disease in patients with impaired immune system (chronic lymphocytic leukemia, lymphoma, Hodgkin disease, carcinomatosis, AIDS, tuberculosis, sarcoidosis, organ transplant)

Prevalence: 4% of AIDS patients

Etiology: reactivation of ubiquitous JC papovavirus

Pathophysiology: destruction of oligodendrogliocytes leading to areas of demyelination + edema

Histo: intranuclear inclusion bodies within swollen oligodendrocytes (viral particles in nuclei); lysis of oligodendrocytes → demyelination; absence of significant perivenous inflammation

• progressive neurologic deficits, dementia, ataxia, spasticity
• speech, motor and visual disturbances; normal CSF fluid

Location: frontoparietal >temporo-occipital cerebral hemispheres; white matter tracts in cerebellum, brainstem, deep gray matter; ? predilection for parieto-occipital region

Site: subcortical white matter spreading centrally into brainstem, deep gray matter

• bilateral white matter lesions (92%); confluent (94%); discrete (67%) in periventricular region + centrum semiovale + subcortical white matter
• gray matter lesions in thalamus + basal ganglia (from involvement of traversing white matter tracts)
• sparing of cortical gray matter
• mild cortical atrophy (up to 69%)
• ventricular dilatation (50%)
• NO contrast enhancement

CT:

• multicentric confluent white matter lesions of low attenuation with scalloped borders along cortex
• NO mass effect, NO edema

MR:

• hypointense lesions on T1WI
• patchy hyperintense lesions of white matter away from ependyma in asymmetric distribution on T2 + FLAIR
• NO enhancement

Prognosis: death usually within 6 months

DDx in early stages: primary CNS lymphoma