Progressive multifocal leukoencephalopathy
Summary
Opportunistic JC virus–related demyelination in immunosuppressed patients: multifocal, asymmetric subcortical WM lesions with U-fibre involvement and typically little/no enhancement or mass effect.
Clinical
Subacute progressive focal neuro deficits (visual, language, motor, cognitive) in severe immunosuppression (HIV/AIDS, transplant, biologics).
Imaging features
MRI
- Multifocal, asymmetric T2/FLAIR hyperintense lesions in subcortical white matter.
- U-fibre involvement common (scalloped juxtacortical margin).
- Predilection: parieto-occipital > frontal; can involve cerebellar peduncles/brainstem.
- No / minimal mass effect.
- No / minimal enhancement classically.
- DWI: peripheral “leading edge” restriction (active demyelination rim) with higher ADC centrally (variable but helpful).
CT
- Hypodense subcortical WM lesions; often underestimates extent vs MRI.
Key differentials
- HIV encephalopathy (HIVE): symmetric deep/periventricular WM change + prominent diffuse atrophy; usually lacks U-fibre pattern.
- Toxoplasmosis / lymphoma: typically enhancing (often ring/solid), more oedema/mass effect; consider basal ganglia for toxo.
- PRES: vasogenic oedema, often symmetric posterior; clinical BP/trigger context.
Warning
PML-IRIS: after starting/optimising ART (or immune reconstitution) → lesions may enhance and swell; don’t discard PML just because enhancement appears.
