The subject of progressive multifocal leukoencephalopathy (PML) has become very topical for people with multiple sclerosis, though it can still be quite confusing for many people. To help, we have prepared this brief explanation:

What is PML?

Progressive multifocal leukoencephalopathy (PML) is a rare, but often fatal, disease in which there is extensive inflammation of the white matter of the brain. The course of the disease is very similar to MS, as there is destruction of the myelin sheath covering nerves. However, the progression of PML is much more rapid, with many people not surviving more than a few months after onset. 

What causes PML?

PML is caused by the JC virus (JCV), a generally harmless virus that infects between 70-90% of the general population. JCV has the potential to cross the blood-brain barrier and infect cells in the central nervous system. Following infection, the virus remains present in the body in a dormant state. However, in cases where the body becomes severely immunocompromised, such as via AIDS or medication, the virus can re-activate. If this occurs in the brain, this re-activation can lead to PML.

How is it linked to Multiple Sclerosis?

Cases of PML have been identified in people taking natalizumab (Tysabri) and more recently dimethyl fumarate (Tecfidera), strong immunosuppressants used in the treatment of MS. The incidence has been calculated at 1.5 cases of PML per 1000 people taking Tysabri (Kappos, 2011). Currently, the risk of developing PML is evaluated by blood tests to check for previous infection with JCV prior to beginning treatment with Tysabri. More recently, a more sensitive test to assess the risk of PML has been developed, which measures the levels of L-selectin, a marker on T cells (Schwab, 2013). It is thought that conducting both of these tests will minimise the chances of a person with MS developing PML.

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