Progressive multifocal leukoencephalopathy (PML) is a life-threatening infection of the brain that can occur in people living with HIV. It is caused by a virus—the JC virus. The "JC" are the initials of the first patient to be diagnosed with PML. The virus is a polyomavirus, a family of viruses that also includes human papillomavirus (HPV).
The "progressive" in PML means that it continues to get worse and often leads to serious brain damage. The "multifocal" means that the JC virus causes disease in several different parts of the brain. The "leukoencephalopathy" (loo-ko-en-sef-a-lop-a-thee) means that the disease affects the white matter of the brain. More specifically, the JC virus infects cells in the brain called oligodendrocytes. These cells are responsible for producing myelin, a fatty substance that helps protect nerves in the brain. If too much myelin is lost and not replaced by oligodendrocytes, the nerves become damaged and eventually stop working correctly.
More than 85 percent of adults worldwide are infected with the JC virus, usually during early childhood. However, the virus only becomes active in people who have compromised immune systems. This includes people undergoing immune-suppressive chemotherapy for cancer and people with damaged immune systems due to HIV. Prior to the use of combination antiretroviral (ARV) therapy, it was estimated that between 3 and 7 percent of all people with AIDS developed PML. It usually occurs in people with very low CD4 cell counts (less than 100), but has been seen in some HIV-positive people with as many as 500 CD4 cells.
In the early years of the AIDS epidemic, PML was almost always progressive and fatal. Death usually occurred between one and four months after the first symptoms appeared. While a PML diagnosis today remains potentially rapid in its progression and possibly fatal, improvements in our ability to stabilize the immune system using HIV drugs has helped to improve the prognosis associated with this opportunistic infection.
Symptoms of PML include mental deterioration, vision loss, speech disturbances, ataxia (inability to coordinate movements), paralysis, and coma. In rare cases, seizures may occur. Because lesions and nerve damage can occur anywhere in the brain, the initial symptoms may be different among people with the disease.