What Causes Multiple Sclerosis?

The cause of multiple sclerosis (MS) is not known. Both environmental and genetic factors are thought to predispose a person to develop MS.

Multiple sclerosis results in the destruction of the myelin that surrounds the nerves of the CNS. Myelin is a fatty substance that insulates the nerves and allows them to transmit information to and from the brain. If the myelin is damaged, the transmitted information is not only delayed but also may be misinterpreted by the brain. The myelin destruction, also known as demyelination, is thought to be caused by the body's immune cells entering the CNS. The disruption of the normal barrier to the entry of these cells, termed the blood-brain barrier, leads to local swelling (known as edema). Also damaged are the nerve cell bodies (termed neuronal loss) or their prolongations (termed axonal loss). A plaque (area of inflammation, demyelination, axonal loss, edema or scarring) represents a typical multiple sclerosis lesion, or area of injury.

What initially triggers the immune system attack is not known. Microglia are cells in the CNS that take up fragments of myelin and present these fragments to the immune cells. In healthy individuals, this presentation of myelin fragments is not thought to trigger the immune cells to attack the CNS. In people with multiple sclerosis, this presentation of myelin fragments may trigger an exaggerated response by immune cells that leads to the formation of plaques around the blood vessels in the CNS.