MS is thought to be caused by a combination of genetic and environmental factors

Last updated: 14th October 2019

What causes MS?

A substance called myelin protects the nerve fibres in the central nervous system, which helps messages travel quickly and smoothly between the brain and the rest of the body.

In MS, the immune system, which normally helps to fight off infections, mistakes myelin for a foreign body and attacks it. Researchers do not know what triggers the immune system to attack myelin, but it is thought to be a combination of genetic and environmental factors.

Genetic factors

MS is not thought to be a hereditary disease. However, the risk of getting MS is higher in relatives of a person with the disease than in the general population, especially in the case of siblings, parents and children.

However, there is only around a two per cent chance of a child developing MS when a parent is affected. It is important to consider that identical twins, who have the same genetic material, do not always both have MS. There is around a 20-30% chance of someone developing MS if their identical twin is affected. This is why researchers agree that MS is not simply a genetic disease.

Specific genes have been linked with MS. Most are genes that influence specific components of the immune system. These are also the genes that seem to contribute more significantly than others to the susceptibility of the disease. The search for MS genes is important because their discovery will provide vital information on which biological mechanisms influence the disease. This will lead to a better understanding of what causes MS and to the development of new approaches to treatment and prevention.

Environmental factors

Various environmental factors – infectious and non-infectious – have been proposed as risk factors for MS.

MS is more common in people who live further away from the equator. The reason for this is not clear, but decreased sunlight exposure has been linked with a higher risk of MS and there is growing evidence that a lack of vitamin D is linked to increasing prevalence in a range of conditions including MS. As we get most of our vitamin D through exposure to sunlight, low sun exposure and subsequent vitamin D insufficiency has been proposed as one explanation of this effect. This effect may also explain the recent observation that there is an excess MS risk in people born in April and May, and a reduced risk in those born in October and November.

Many microbes (particularly Epstein Barr Virus) have been proposed as potential triggers for MS, but none have been proven. Age at exposure to infection seems to play an important role, and it has been shown that moving at an early age from one location in the world to another alters a person’s subsequent risk of MS.

Smoking is another environmental factor that seems to be strongly associated with MS.