Progressive multifocal leukoencephalopathy (PML) is one of those “you either recognize it instantly or you miss it” diagnoses on Step exams—especially because the vignette often looks like stroke, MS, or a brain tumor at first glance. The trick is to anchor yourself on the risk factor (immunosuppression) + subacute focal deficits + non-enhancing white matter lesions—then use the answer choices to prove you’re right (or wrong).
Tag: Neurology > CNS Tumors & Infections
The Vignette (Classic Q-bank Style)
A 38-year-old man with HIV (CD4 count 55/µL) presents with 3 weeks of progressive right arm weakness, clumsiness, and difficulty speaking. He has no fever. Neuro exam shows right upper extremity weakness and expressive aphasia. MRI brain shows multiple asymmetric T2 hyperintense lesions in the subcortical white matter without mass effect and minimal/no enhancement.
Question: Most likely cause?
The Correct Answer: JC Virus Reactivation → PML
Why it fits
PML is caused by reactivation of JC virus (a polyomavirus) in the setting of impaired cell-mediated immunity.
High-yield anchors:
- Who gets it?
- Advanced HIV/AIDS
- Natalizumab (α4-integrin inhibitor), rituximab, other immunosuppressants/transplant
- Hematologic malignancy
- How it presents: Subacute, progressive focal neurologic deficits
- Hemiparesis, aphasia, visual field deficits, ataxia
- Often no systemic symptoms (no fever)
- MRI: demyelinating lesions in white matter
- Non-enhancing or faintly enhancing
- No mass effect (vs toxoplasmosis or tumor)
- Often parieto-occipital, but can be multifocal and asymmetric
- CSF: JC virus PCR positive (key test)
- Path: oligodendrocyte infection → demyelination
- Histology: enlarged oligodendrocyte nuclei with viral inclusions (classic teaching)
Management (Step-relevant)
- Restore immune function
- HIV: start/optimize ART
- Stop/reverse offending immunosuppressant when possible
- No reliably curative antiviral; outcomes hinge on immune reconstitution.
Why Every Distractor Matters (Systematic Breakdown)
Below are the most common “near-miss” options Q-banks love to pair with PML.
Distractor 1: Toxoplasma gondii encephalitis
Why they want you to pick it: AIDS + neuro deficits is a trap for toxo.
How to rule it out:
- MRI: classically multiple ring-enhancing lesions with edema/mass effect, often in basal ganglia
- Symptoms: headache, fever, seizures are more common than in PML
- CD4: typically <100/µL (overlaps with PML—so imaging and systemic signs matter)
High-yield tip:
If the stem says ring-enhancing with mass effect → think toxo (or lymphoma). If it says non-enhancing white matter lesions → think PML.
Distractor 2: Primary CNS lymphoma (EBV-associated)
Why it’s tempting: Also occurs in AIDS and can be multifocal.
How to rule it out:
- Imaging: typically enhancing mass lesion(s); may be ring-enhancing in AIDS
- Often periventricular; can have mass effect
- Clue words: “B symptoms,” pronounced intracranial pressure signs, more “tumor-like” course
- Testing: CSF EBV DNA can support diagnosis
Classic board distinction:
- Toxo: multiple ring-enhancing, basal ganglia, responds to empiric therapy
- CNS lymphoma: solitary or few lesions, periventricular, EBV+, thallium/PET more avid (advanced detail)
- PML: non-enhancing demyelination, no mass effect
Distractor 3: HSV encephalitis (HSV-1)
Why it’s tempting: Aphasia + neuro deficits.
How to rule it out:
- Tempo: more acute (hours–days), not progressive over weeks
- Systemic signs: often fever, altered mental status
- Localization: temporal lobes (limbic)
- CSF: lymphocytic pleocytosis, ↑ RBCs (hemorrhagic), HSV PCR+
Pearl:
Aphasia can happen in both, but HSV is encephalitis (AMS, fever, temporal lobe), whereas PML is focal demyelination (progressive deficits, minimal systemic symptoms).
Distractor 4: CMV encephalitis / CMV polyradiculopathy
Why it’s tempting: Advanced AIDS infections blend together.
How to rule it out:
- CMV encephalitis: confusion, lethargy, cranial nerve deficits; ventriculoencephalitis pattern
- CMV polyradiculopathy: back pain, urinary retention, ascending weakness (cauda equina picture)
- CD4: often <50/µL
- CSF: CMV PCR+
Exam move:
If the vignette emphasizes radicular pain + urinary retention + flaccid weakness, that’s CMV polyradiculopathy—not PML.
Distractor 5: Multiple sclerosis (MS)
Why it’s tempting: Demyelination + white matter lesions.
How to rule it out:
- Patient population: typically young women; not classically profound immunosuppression-related
- Course: relapsing-remitting; symptoms separated in time/space
- MRI: periventricular lesions, Dawson fingers; lesions may enhance when active
- CSF: oligoclonal bands, ↑ IgG index
Board-worthy comparison:
- PML: opportunistic demyelination (JC), progressive, immunosuppressed, CSF JCV PCR+
- MS: autoimmune demyelination, relapsing episodes, oligoclonal bands
Distractor 6: Acute disseminated encephalomyelitis (ADEM)
Why it’s tempting: Demyelinating and multifocal.
How to rule it out:
- Usually children/teens
- Follows viral infection or vaccination
- Encephalopathy is prominent (confusion, somnolence)
- MRI shows widespread demyelination; clinical course is typically monophasic
Distractor 7: Brain abscess (bacterial)
Why it’s tempting: Focal deficits + brain lesions.
How to rule it out:
- Fever, leukocytosis may appear
- Ring-enhancing lesion with surrounding edema; may show restricted diffusion on MRI (advanced)
- Often a source: otitis/mastoiditis, sinusitis, endocarditis
Rapid-Fire “PML vs. The World” Table
| Diagnosis | Typical Patient | Time course | MRI/CT pattern | Key clue |
|---|---|---|---|---|
| PML (JC virus) | HIV/AIDS, natalizumab, transplant | Subacute progressive | Non-enhancing white matter lesions, no mass effect | CSF JCV PCR+ |
| Toxoplasmosis | AIDS (CD4 <100) | Subacute | Multiple ring-enhancing, edema/mass effect | Basal ganglia; fever/seizures |
| Primary CNS lymphoma (EBV) | AIDS/transplant | Subacute | Enhancing lesion(s), often periventricular | EBV DNA in CSF |
| HSV encephalitis | Any (not necessarily immunosuppressed) | Acute | Temporal lobe involvement | Fever + AMS; HSV PCR+ |
| CMV polyradiculopathy | AIDS (CD4 <50) | Subacute | N/A (spine/roots) | Urinary retention + radicular pain |
| MS | Young adults | Relapsing-remitting | Periventricular plaques | Oligoclonal bands |
USMLE High-Yield Takeaways (What to Memorize)
- PML = JC virus reactivation infecting oligodendrocytes → demyelination
- Think PML in immunosuppressed patients with subacute progressive focal deficits
- MRI: multifocal asymmetric white matter lesions with minimal/no enhancement and no mass effect
- Confirm with CSF JC virus PCR
- Treatment principle: immune reconstitution (optimize ART; stop offending immunosuppressant)