What is Progressive Multifocal Leukoencephalopathy

By  ,  Onlymyhealth editorial team
Oct 11, 2012

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Quick Bites

  • PML is a rare infection of the brain.
  • It is caused by the JC virus.
  • People with a weakened immune system are prone to it.
  • Death usually occurs within 9 months.

Progressive multifocal leukoencephalopathy (PML) is a neurological disorder. It is marked by demolition of cells that produce the myelin, which is an oily substance that helps defend nerve cells in the brain and spinal cord, also known as central nervous system (CNS) white matter.

PML is caused by a virus called JC virus (JCV). It is named after the initials of the patient in whom it was first discovered.

The virus is far-flung, found in up to 85% of the general adult population. It stays inactive in healthy individuals and causes disease only when the immune system is severely weakened, due to some medical conditions such as HIV/AIDS, or haematological malignancies, and in organ transplant recipients who receive immunosuppressant medications to avoid rejection of the transplanted organ. Altogether, PML occurs in approximately one in 200,000 people. 

Brain Disorder

The term "progressive" in PML means that the disease keeps on getting worse and often extends to serious brain damage. The term "multifocal" means that JCV affects multiple parts of the brain.

However, it is possible for an individual with PML to have only one brain lesion instead of several lesions. The term "leukoencephalopathy" means that the disease affects mainly the white matter of the brain or myelin, although there are some rare cases in which the gray matter neurons are also involved.

The JC virus seems to cause no symptoms until it is reactivated. Symptoms may begin slowly and usually worsen continuously. They change depending on the infected part of the brain. In about two of three people, mental function descents quickly and increasingly, leading to dementia. Speaking becomes increasingly difficult. People may become partially blind. Walking may become difficult. Rarely, headaches and seizures occur.

Death is common within 1 to 9 months of when symptoms start, but a few people survive longer (about 2 years).

Image: Getty

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