Progressive multifocal leukoencephalopathy

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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.

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, with 86% of the general population presenting antibodies, but it usually remains latent, causing disease only when the immune system has been severely weakened.

About 2-5% 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.[1]

See also recent announcement by Roche US Pharm regarding PML and CellCept http://www.fda.gov/cder/drug/early_comm/mycophenolate.htm

There are case reports of PML being caused by pharmacological agents, although there is some speculation this could be due in part to the existing impaired immune response or ‘drug combination therapies’ rather than individual drugs. These include rituximab,[2] infliximab,[3] natalizumab,[4] chemotherapy,[5] corticosteroids,[6] and various transplant drugs such as tacrolimus.[7]

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 from the outermost parts of the brain (cortex). Symptoms include weakness or paralysis, vision loss, impaired speech, and cognitive deterioration. PML destroys oligodendrocytes and produces intranuclear inclusions. 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. The median survival of patients with PML as a complication of AIDS is 6 months. In 10% of patients, survival exceeds 12 months. The longest reported survival is 92 months from the onset of illness.

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