What is progressive multifocal leukoencephalopathy (PML)?

Progressive multifocal leukoencephalopathy (PML) is a rare brain infection caused by the JC virus (JCV) that typically results in death or severe disability. It is not uncommon for the virus to be present in the body, but when the immune system is weakened, it can be activated. The virus attacks the white matter of the brain, resulting in PML. Symptoms include clumsiness, progressive weakness and visual, speech and sometimes cognitive changes.

Progressive multifocal leukoencephalopathy (PML) is a disease that is caused by the reactivation of a common virus in the central nervous system of immune-compromised individuals. Polyomavirus JC (often called JC virus) is carried by a majority of people and is harmless except among those with lowered immune defenses. The disease occurs rarely in organ transplant patients; people undergoing chronic corticosteroid or immunosuppressive therapy; and individuals with cancer, such as Hodgkin's disease, lymphoma, and sarcoidosis. PML is most common among individuals with acquired immune deficiency syndrome (AIDS). Studies estimate that prior to effective antiretroviral therapy, as many as 5 percent of people with AIDS eventually developed PML. For them, the disease was most often rapidly fatal.

With current human immunodeficiency virus (HIV) therapy, which effectively restores immune system function, as many as half of all patients with HIV and PML survive, although they sometimes have an inflammatory reaction in the regions affected by PML. The symptoms of PML are the result of an infection that causes the loss of white matter (which is made up of myelin, a substance the surrounds and protects nerve fibers) in multiple areas of the brain. Without the protection of myelin, nerve signals can't travel successfully from the brain to the rest of the body. Typical symptoms associated with PML are diverse, as they are related to the location and amount of damage in the brain, and evolve over the course of several days to several weeks. The most prominent symptoms are clumsiness; progressive weakness; and visual, speech, and, sometimes, personality changes. The progression of deficits leads to a life-threatening disability and death over weeks to months.

This answer is based on source information from the National Institute of Neurological Disorders and Stroke.

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