Progressive multifocal leukoencephalopathy (PML), also known as progressive multifocal leukoencephalitis, is a rare and usually fatal viral disease 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.
Cause and epidemiology
The cause of PML is a type of polyomavirus called the JC virus (JCV), 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.
About five percent of AIDS patients develop PML. It is unclear why PML occurs more frequently in AIDS than in other immunosuppressive conditions; some research suggests that the effects of HIV on brain tissue, or on JCV itself, make JCV more likely to become active in the brain and increase its damaging inflammatory effects (Berger, 2003).
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.
PML is diagnosed by testing for JC virus DNA in cerebrospinal fluid or in a brain biopsy specimen. Characteristic evidence of the damage caused by PML in the brain can also be detected on MRI images.
There is no known cure. In some cases, the disease slows or stops if the patient's immune system improves; some AIDS patients with PML have been able to survive for several years, with the advent of highly active antiretroviral therapy (HAART).
AIDS patients who start HAART after being diagnosed with PML tend to have a slightly longer survival time than patients who were already on HAART and then develop PML (Wyen et al., 2004). A rare complication of effective HAART is immune reconstitution inflammatory syndrome (IRIS), in which increased immune system activity actually increases the damage caused by the infection; though IRIS is often manageable with other types of infections, it is extremely dangerous if it occurs in PML (Vendrely et al., 2005).
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