What Is Progressive Multifocal Leukoencephalopathy?

Medically Reviewed by Carmelita Swiner, MD on March 02, 2023
3 min read

Progressive multifocal leukoencephalopathy (PML) is a disease that attacks part of your brain. It happens if your body can't fight off disease the way it should.

It damages your brain's white matter -- cells that make a substance called myelin. This protects your nerves, and losing it can make it harder for you to move, think, and feel sensations.

PML is a very serious illness that can be fatal.

It's caused by a virus called the JC virus. Most adults carry it, and it usually doesn't cause any health problems. But it can if you have a weak immune system -- if your body's natural defenses against illness aren't working right.

The first signs of PML can be different from person to person, depending on the nerves that are damaged first. But they often include:

  • Clumsiness or loss of coordination
  • Difficulty walking
  • Facial drooping
  • Loss of vision
  • Personality changes
  • Trouble speaking
  • Weak muscles

Sometimes, PML can also cause seizures.

About 1 person out of every 200,000 will get PML. That works out to a total of about 4,000 people a year in the United States and Europe combined.

It happens most often to people who have AIDS, which attacks the immune system. But people who have some types of cancer or take drugs that keep their body from rejecting a transplanted organ also have a higher risk.

People who take medication for multiple sclerosis, which attacks the central nervous system, or other immune system problems, like rheumatoid arthritis or lupus, may be at risk too. U.S. safety officials have said that PML may be a possible side effect of some drugs used to treat multiple sclerosis after a small number of people who used them got it.

If your doctor thinks you might have PML, they'll scan your brain with a magnetic resonance imaging machine (MRI). This uses powerful magnets and radio waves to make a detailed picture. They'll look for lesions -- spots of damaged tissue -- that show the disease is there.

If an MRI doesn't give a clear picture, they might do a brain biopsy. They'll take a small sample of tissue from your brain to look at under a microscope for signs of the disease.

Your doctor might also order a lumbar puncture (spinal tap) to take a sample of the fluid around your brain and spinal cord for testing.

The best way to treat PML is to fight whatever has made your immune system weak. You might take drugs that attack the virus that causes AIDS (HIV) or avoid drugs that affect your immune system. You also might need to avoid treatments like chemotherapy, which can put you at higher risk of infections.

Researchers are trying to find other drugs to fight the JC virus, but none has been approved for widespread use.

White matter killed by the virus doesn't grow back, so your symptoms may be permanent. Many people who have had PML live with the effects of nerve damage. These can be similar to the issues caused by a stroke and can include paralysis and memory loss.