Progressive Multifocal Leukoencephalopathy



PML is a rare progressive disease of the nervous system. It is caused by a viral infection of the cells that produce myelin.
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PML is caused by a specific papovavirus, known as JC virus. Many people get this infection in childhood. It produces no illness and generally does not infect the nervous system. It reactivates later in life in people who have suppressed immune systems. The virus damages oligodendrocytes, the cells in the central nervous system (CNS) which produce myelin, the material that wraps around nerves. This impairs nerve function.

Risk Factors

PML is most common in people with suppressed immune systems. Suppressed immune systems may be the result of:
  • HIV/AIDS (most common)
  • Leukemia and lymphoma
  • Organ transplant
  • Cancer
  • Chronic steroid therapy
  • Rare inherited immunodeficiencies
  • Certain medications, such as natalizumab, a medication used to treat multiple sclerosis (MS)


Symptoms progress over weeks and may include:
  • Vision problems
  • Speech pronunciation problems
  • Coordination loss
  • Memory loss
  • Weakness in limbs
  • Behavioral changes
  • Changes in thinking
  • A loss of language capability— aphasia
  • Seizures
  • Sensory loss

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