Multiple sclerosis

Aetiology

In time, the spinal cord, optic nerves, brainstem and cerebral hemispheres are affected by the disease. In the central nervous system, myelin is damaged in random patches called plaques or lesions. At the site of demyelination, blood vessels may become inflamed and inflammatory cells migrate into the plaque and cause further erosion of myelin. Eventually, scar tissue forms and neurons resume signalling, although the conduction of nerve impulses is impaired by the scar. Lesions occur in the white matter and show up as white patches when the brain is scanned using a technique called magnetic resonance imaging. Lesions are also characteristic of other brain diseases and to accurately diagnose MS, the brain scan must show at least four lesions - one of these lesions must be at least 6 mm in diameter and one must be located on the ventral surface of the cerebellum.

What happens in the brain?

At present, the cause of multiple sclerosis(MS) is not definitely known. However, many scientists believe MS to be an autoimmune disease, that is a disease in which the body launches an immune system attack upon its own tissues in a similar way that it would to a foreign body such as bacteria. It is believed that the white, fatty substance that insulates nerves (myelin) is attacked in this way in people with MS. This autoimmune attack may be initiated after exposure to an environmental trigger in people who are genetically susceptible to the disease. Therefore there may be genetic and environmental components to its aetiology.

Genetic components
Twin studies have provided evidence of a possible genetic component to the aetiology of the disease. For identical twins the likelihood that one twin will develop MS if the other twin has the disease is approximately 30%, whereas in non-identical twins it is approximately 4%. As identical twins inherit an identical gene pool, this evidence supports the idea that MS is caused partly by genetic factors. Researchers have correlated several genes with MS, and have found that people with MS inherit specific regions of certain genes more frequently than people who do not have the disease.

Environmental components
The observation that one identical twin has a 30% likelihood of developing MS if the other twin has the disease, rather than a 100% chance, suggests that the disease is not solely caused by genetic factors. However, the fact that some people with MS lesions remain asymptomatic throughout their lives may partly or fully account for this figure being lower than 100%.

MS therefore may be due to a combination of genetic and environmental factors that give rise to an autoimmune attack upon myelin.