A to Z of MS
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A to Z of MS PML (Progressive multifocal leukoencephalopathy)
Progressive multifocal leukoencephalopathy (PML) is rare viral disease of the brain. Susceptibility to infection has been identified as a potential side effect of treatment with natalizumab (Tysabri).
PML is caused by a mutation of a virus called the JC virus, which is normally kept under control by the immune system. If the immune system is severely affected, for instance by an immunosuppressive drug such as natalizumab, the virus can cause progressive damage and inflammation to the white matter of the brain.
The risk of PML increases with the length of treatments. In April 2011 the American Fedral Drug Administration (FDA) stated that the risk was 0.3 per 1,000 patients during the first two years of treatment. After three years, the rate was 0.9 per 1,000.
The manufacturers of natalizumab have developed a test which will show if someone has been exposed to the JC virus. If they have not, the risk of PML is very small. If someone tests positive, their risk of developing PML is higher although still small, and their condition needs careful monitoring. As the JC virus can be acquired, annual testing may be required.
As PML damages nerves, some of its symptoms are similar to those of MS, such as weakness, visual problems, impaired speech and cognitive problems. Although a serious and potentially fatal infection, awareness of the risk of PML means people receiving natalizumab are monitored for signs of the infection and treatment to slow or prevent further progression can be initiated quickly.
Treatment for PML involves plasma exchange in which a series of blood transfusions flushes natalizumab from the body and allows the immune system to attack the JC virus infection.