Brain
PML: Subcortical, U-Fiber, and Silent
Fellowship-level neuroradiology teaching by E. Brooke Schrickel, MD. Open this topic in the interactive reading room →
Watch · concise explainerThe signature is what it doesn't do
Core clinical idea
In an immunocompromised patient, asymmetric confluent subcortical white-matter disease that hugs the U-fibers, spares the overlying cortex, makes essentially no mass effect, and doesn't enhance is PML until proven otherwise — say it, because it changes immunosuppression management and prompts JC virus testing.
Bottom line
Immunocompromised + cortex-sparing, no-mass-effect, non-enhancing white-matter disease = PML until proven otherwise — and know when it enhances.
Core workstation questions
- Is the patient T-cell immunocompromised, and on what (HIV/AIDS, transplant, hematologic malignancy, natalizumab/monoclonal antibody)?
- Does the lesion ride the U-fibers and spare the adjacent cortex?
- Is there truly no mass effect (or even negative mass effect from volume loss)?
- Is there enhancement, and if so could this be IRIS rather than progression?
- On natalizumab, do features favor PML over a new MS plaque (confluent subcortical vs periventricular, posterior fossa, infiltrating edge, no mass effect/enhancement)?
- Could a small early subcortical lesion be mistaken for an acute infarct?
What changes reporting / management
- Name the immune context in the report — it makes the PML call credible and is what the clinician acts on.
- Suggested phrasing: confluent asymmetric subcortical white-matter signal involving the U-fibers with relative sparing of overlying cortex, no mass effect, no enhancement — in this immunocompromised patient, concerning for PML.
- Read new enhancement in known PML as possible PML-IRIS (immune reconstitution), which can paradoxically worsen symptoms, rather than automatic progression.
- In an MS patient on natalizumab, flag PML-favoring features explicitly — the call can stop the drug.
Practical traps
- A small early subcortical PML lesion gets miscalled an acute infarct; cortical sparing plus a leading-edge diffusion pattern (not a vascular-territory shape) is the discriminator.
- Expecting no enhancement always — PML-IRIS and natalizumab-associated PML can show patchy/peripheral enhancement.
- Treating PML as periventricular like MS — PML is subcortical/confluent and rides the U-fibers, peripheral WM is relatively spared early.
Teaching pearls
- PML spares the cortex and rides the U-fibers; a cortical infarct doesn't.
- Restricted diffusion at the advancing edge, dead center — that's the active margin of PML.
- Big lesion, no mass effect, no enhancement — that combination is PML's signature.
- New enhancement in known PML may be IRIS, not progression.
- On natalizumab, a confluent posterior or subcortical lesion is PML until you've excluded it.
Source lectures
- Imaging of Progressive Multifocal Leukoencephalopathy
Educational material for radiology residents and neuroradiology fellows. Nothing here drives individual patient care, and it contains no patient data.