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Progressive multifocal leukoencephalopathy (PML), also known as progressive multifocal leukoencephalitis, is a rare and usually fatal disorder that is characterized by progressive damage (-pathy) or inflammation (-itis) of the white matter (leuko-) of the brain (-encephalo-) at multiple locations (multifocal). It occurs almost exclusively in people with severe immune deficiency, e.g. transplant patients on immunosuppressive medications, or AIDS patients. Between 2 and 7 percent of AIDS patients develop PML.
PML is a demyelinating disease, in which the myelin sheath covering the axons of nerve cells is gradually destroyed, impairing the transmission of nerve impulses. It affects the white matter, which is mostly composed of axons in the outermost parts of the brain (cortex). Symptoms include weakness or paralysis, vision loss, impaired speech, and cognitive deterioration. PML is similar to another demyelinating disease, multiple sclerosis, but since it destroys the cells that produce myelin (unlike MS, in which myelin itself is attacked but can be replaced), it progresses much more quickly. Most patients die within four months of onset.
The cause of PML is a type of polyomavirus called the JC virus, after the initials of the patient in whom it was first discovered. The virus is widespread, found in at least 70 percent of the general population by some estimates, but usually remains latent, causing disease only when the immune system has been severely weakened. PML is diagnosed by testing for JC virus DNA in cerebrospinal fluid or in a brain biopsy specimen.
There is no known cure. In some cases, the disease slows or stops if the patient's immune system improves; with the advent of more effective AIDS treatments, some AIDS patients with PML have been able to survive for several years.
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