Myelin is a material that insulates and protects the spinal cord and brain’s nerve cells or neurons. Myelin is crucial for the overall health of your brain. Progressive multifocal leukoencephalopathy, otherwise known as PML, is a disease which targets cells that produce myelin. It is a disease of the white matter of the brain.
PML is caused by Polyomavirus JC (often called JC virus). This particular virus is carried by a majority of people and is, in many cases, completely harmless. The danger is for those with lowered immune defenses.
Progressive multifocal leukoencephalopathy is rare, but if there is a situation in which the immune system is greatly compromised, such as in patients with cancer or autoimmune conditions, or if one is being treated with biological therapies which allow JC virus reactivation, then PML becomes a possibility.
Progressive multifocal leukoencephalopathy is not a common disease. It occurs in approximately one in 200,000 people. In fact, PML is most common among individuals with HIV-1 infection / acquired immune deficiency syndrome (AIDS). Studies show that as many as 5 percent of people infected with HIV-1 eventually develop PML that is an AIDS-defining illness, prior to effective antiretroviral therapy.
Fortunately, however, current HIV therapy using antiretroviral drugs (ART), effectively restores immune system function. This has allowed as many as half of all HIV-PML patients to survive, even though there is a possibility they may sometimes have an inflammatory reaction in the regions of the brain which have been affected by PML.
To understand the term, let’s break it down.
"Progressive" in PML means that the disease continues to get worse, not better. It can lead to serious complications of the white matter of the brain, and severe brain damage. "Multifocal" means that more than one area of the brain can be affected. Mmultiple parts can be damaged. That being said, individuals can have only one brain lesion instead of several lesions.