Progressive multifocal leukoencephalopathy (PML) is a neurological disorder. It is marked by demolition of cells that produce the myelin, which is an oily substance that helps defend nerve cells in the brain and spinal cord, also known as central nervous system (CNS) white matter.
PML is caused by a virus called JC virus (JCV). It is named after the initials of the patient in whom it was first discovered. The virus is far-flung, found in up to 85% of the general adult population. It stays inactive in healthy individuals and causes disease only when the immune system is severely weakened, due to some medical conditions such as HIV/AIDS, or haematological malignancies, and in organ transplant recipients who receive immunosuppressant medications to avoid rejection of the transplanted organ. Altogether, PML occurs in approximately one in 200,000 people.
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, since 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 life-threatening disability and death over weeks to months.
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