Progressive multifocal leukoencephalopathy

Progressive multifocal leukoencephalopathy (PML) is a rare and often fatal viral disease that damages the brain's white matter at multiple sites, causing progressive neurological deterioration. It is caused by the JC virus, a polyomavirus that remains dormant and harmless in most people under a functioning immune system, and causes disease only once immune defenses are severely compromised. PML occurs almost exclusively in severely immunocompromised people, most commonly those with AIDS, and also in patients on chronic immunosuppressive treatment such as transplant recipients or those treated for multiple sclerosis, psoriasis, rheumatoid arthritis or cancer. Symptoms develop over weeks to months and vary with the location of brain damage, including clumsiness, progressive weakness, and visual, speech or personality changes; some lesions cause alien hand syndrome. Between one-third and one-half of people with PML die within the first few months of diagnosis, and survivors often keep significant neurological disability.