Updated 15 February 2016

Progressive multifocal leukoencephalopathy

This is a brain disease.

What is progressive multifocal leukoencephalopathy?

Progressive multifocal leukoencephalopathy, or PML, is a disease that attacks the white matter of the brain. It is caused by a virus called JC virus (JCV). This is a very common human virus: most people (up to 80% of the general population) are infected with JCV, which is present in their kidneys – most people probably pick up the virus during childhood. In people with healthy immune systems, the virus is harmless.

It is only in people with suppressed immune systems that the JC virus gives rise to PML – including up to 4% of people with advanced HIV/Aids (CD4 counts below 100). It is a “late stage” disease; it generally only appears once immunity has been greatly compromised.

In a person with a weak immune system, the virus becomes active. In the brain, it destroys the myelin sheaths that cover nerve cells. The myelin sheath is a protective, fatty covering that acts as an insulator. Without it, the nerve cells die, causing lesions in the brain.


Symptoms of PML vary, and usually include three typical neurological features such as muscle weakness, gait disturbances and cognitive problems or dementia. These can manifest as:
  • muscle weakness and spasms
  • ataxia (inability to co-ordinate movements)
  • difficulty walking
  • difficulty swallowing
  • headaches
  • changes in personality
  • forgetfulness
  • poor concentration
  • confusion
  • loss of speech, or speech problems
  • blurred or double vision
  • loss of vision in one eye
  • paralysis or weakness on one side of the body
  • numbness in one arm or leg
  • seizures (rare)
  • coma

The variety of symptoms is explained by the wide distribution of lesions across different parts of the brain – it is “multifocal”.

How is PML diagnosed?

Tests are necessary to diagnose PML. These may involve:
  • Brain scan: The white matter of the brain can be examined using a CAT scan or MRI scan, which will show lesions or spots.
  • Brain biopsy: This is a risky procedure that is not often performed. It involves surgery to remove a small specimen of brain matter, which is then tested to confirm a diagnosis of PML.

How is PML treated?

There is currently no cure for PML. Although in some cases people have been known to survive with PML for several months or even years, in most cases the disease progresses rapidly, causing death within a few months. Doctors can only provide symptomatic and supportive treatment.

While there is no effective standard treatment for PML at present, research continues into this disease. In people with advanced HIV/Aids, PML can only be effectively halted or reversed by highly active antiretroviral treatment that leads to immune recovery and prevents further damage to the brain.

A drug called cytarabine (or cytosine arabinoside or Ara-C) is used most frequently to treat PML but without consistent results. Some reports have described remission of PML in individuals in which Ara-C is injected directly into the brain (or intrathecally). Ara-C blocks the replication of the virus by interfering with the virus DNA. It belongs to a group of drugs know as nucleoside analogues. Another drug called cidofovir, which is most commonly used to treat herpes virus infections such as cytomegalovirus, has also been used successfully in some cases in conjunction with alpha-interferon (a well known antiviral agent).

How can I help prevent PML?

PML is not preventable, because most people have already been infected by the JC virus. Currently, there are no medicines or treatments that can help to avoid PML.

Reviewed by Dr Eftyhia Vardas BSc(Hons), MBBCh, DTM&H, DPH, FC Path (Virol), MMed (Virol), Clinical Virologist, Director HIV AIDS Vaccine Division, Perinatal HIV Research Unit, Chris Hani Baragwanath Hospital, University of the Witwatersrand and senior lecturer, Department of Infectious Diseases, Faculty of Health Sciences, University of the Witwatersrand.


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