What is progressive multifocal leukoencephalopathy (PML)?

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What is Progressive Multifocal Leukoencephalopathy (PML)?

PML is a rare, often fatal demyelinating disease of the central nervous system caused by reactivation of the JC virus (JCV) in immunocompromised individuals, characterized by lytic infection and destruction of oligodendrocytes leading to progressive white matter destruction. 1

Pathophysiology

PML occurs when the ubiquitous JC polyomavirus—which establishes asymptomatic latent infection in approximately 60% of the European population—undergoes adaptive genetic mutations to become a neurotropic, pathogenic strain. 1 This mutated virus produces lytic infection of oligodendrocytes, astrocytes, and neuronal cells in the brain, resulting in progressive demyelination with multiple foci of white matter lesions. 2

The virus typically resides benignly in the gut, kidneys, bone marrow, and lymphoreticular system but transforms into a pathogenic form almost exclusively in the setting of severe immunosuppression. 1

Clinical Presentation

PML presents with subacute onset evolving over weeks with progressive neurological deterioration, distinctly different from acute MS relapses: 1

Characteristic symptoms include:

  • Aphasia
  • Behavioral and neuropsychological alterations
  • Retrochiasmal visual deficits (not optic neuritis)
  • Hemiparesis
  • Seizures

This contrasts sharply with MS symptoms (diplopia, paresthesia, paraparesis, optic neuritis, myelopathy) which develop over hours to days and typically stabilize or resolve spontaneously.

At-Risk Populations

Historically: HIV/AIDS patients (3-5% develop PML, classified as AIDS-defining illness), hematologic malignancies, organ transplant recipients, and other severely immunocompromised states. 2

Modern era: PML has emerged as a significant complication of immunomodulatory monoclonal antibody therapies, particularly:

  • Natalizumab (for multiple sclerosis)
  • Rituximab
  • Efalizumab (withdrawn from market due to PML risk)

3

As of June 2015, overall PML incidence in natalizumab-treated patients was 3.96 per 1,000 patients globally. 1

Diagnostic Criteria

Definitive PML diagnosis requires either: 1

Option A (Clinical diagnosis):

  • Clinical presentation consistent with PML
  • JCV DNA detected in CSF (via quantitative PCR)
  • MRI findings suggestive of PML

Option B (Pathological diagnosis):

  • Histopathology showing PML on brain biopsy
  • JCV presence confirmed by electron microscopy and/or immunohistochemistry

Critical caveat: A negative CSF JCV PCR does not exclude PML, particularly in early disease when viral loads may be below assay sensitivity. In one study, 16 of 28 natalizumab-associated PML patients initially had undetectable JCV DNA. 1 If clinical suspicion remains high, CSF testing should be repeated over several months or brain biopsy performed. 1

MRI Characteristics

FLAIR is the preferred imaging sequence due to PML's subcortical location. 1

Typical PML lesions: 1

  • Size and distribution: Large lesions (>3 cm) in unifocal, multifocal, or widespread distribution
  • Location: Subcortical rather than periventricular; affects U-fibers extending into gyrus; cortical gray matter involvement in 50% of cases
  • Borders: Irregular shape with ill-defined borders toward white matter, sharp borders toward cortical gray matter
  • Evolution: Continuous, sometimes rapid volume increase to contiguous and non-contiguous regions confined to white matter tracts
  • Mass effect: Absent even in large lesions (unless inflammatory response present)
  • T2/FLAIR: Diffuse hyperintensity with punctate microcystic appearance
  • T1: Slightly hypointense at onset with decreasing signal over time (no reversion to isointense)
  • Contrast enhancement: Less than half show enhancement at presentation, typically patchy or punctate; rim enhancement possible at leading edge

Prognosis and Management

PML remains an untreatable disease with poor outcomes. 4 Early detection followed by rapid cessation of causative immunosuppression (e.g., natalizumab) and plasma exchange to restore immune function appears associated with better prognosis. 1 However, current management focuses entirely on immune reconstitution rather than direct antiviral therapy, as no effective treatment exists. 4, 5

The disease is frequently fatal, making prevention through risk stratification (particularly anti-JCV antibody testing and index levels in natalizumab-treated patients) the most critical intervention. 1

References

3
Research

Progressive multifocal leukoencephalopathy: an unexpected complication of modern therapeutic monoclonal antibody therapies.

Clinical microbiology and infection : the official publication of the European Society of Clinical Microbiology and Infectious Diseases, 2011

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