Progressive Multifocal Leukoencephalopathy

March 3, 2005

Progressive Multifocal Leukoencephalopathy (PML) is a fatal viral disease of the Central Nervous System (CNS). PML is characterized by a demyelination in white matter, or destruction of the myelin sheath covering nerve cells of the brain. This demyelination occurs at the same time at numerous locations in the brain ( multifocal ).

Symptoms of PML include vision loss, speech disturbances, ataxia (inability to coordinate movements), paralysis, and ultimately coma resulting from multifocal brain lesions. PML is diagnosed on MRI scans by the characteristic multiple focal lesions of the white matter.

PML affects patients with suppressed immune systems. These include patients with cancers such as lymphatic myeloid leukemia (leukemia) or lymphatic lymphosarcoma (lymphoma).

As of today, researchers believe that the activation of the polyomavirus JC Virus, is the cause of the demyelination occurring in immuno-compromised patients and resulting in the development of PML. JC Virus is the human form of the polyomavirus , a family of viruses known to infect other mammals as well.

In humans, the JC Virus infects oligodendrocytes , destroying them quickly and in multiple locations (resulting in multifocal points seen on an MRI). As many as 80 percent of healthy adults could be carriers of the inactivated (latent) form of the JC Virus. The factors leading to viral activation are unknown. The relation, if any, between Tysabri, Avonex, and the activation of JC virus, remain to be established.

Christian CD Poncet, PhD

Multiple Sclerosis Association of America
706 Haddonfield Road, Cherry Hill, New Jersey 08002
(800) 532-7667

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