Multiple sclerosis (MS) occurs because of damage to the nerve fibres of the central nervous system. Your central nervous system consists of the brain and spinal cord and is responsible for controlling every action, conscious and unconscious, of your body.
Myelin
When you perform an action, your brain sends messages to the appropriate part of your body through the nerve fibres in your spinal cord. These nerve fibres are covered by a substance called myelin. Myelin insulates the nerve fibres and helps carry messages to and from your brain quickly and smoothly. In MS, the myelin around your nerve fibres becomes damaged. This disturbs the messages coming to and from your brain.
Autoimmune condition
MS is an autoimmune condition. This means your immune system mistakes the myelin for a foreign substance and attacks it. The myelin becomes inflamed in small patches (called plaques or lesions), which can be seen on an MRI scan. This process is called demyelination.
Demyelination disrupts the messages travelling along nerve fibres. It can slow them down, jumble them, accidentally send them down a different nerve fibre, or stop them from getting through completely.
When the inflammation goes away, it can leave behind scarring of the myelin sheath (known as sclerosis) and sometimes damage to the underlying nerve cell.
Why do people develop multiple sclerosis?
It is not understood what causes the immune system to attack myelin, although there are several theories. Most experts agree that MS is probably caused by a combination of genetic and environmental factors. This means it's partly due to genes you inherit from your parents and partly due to outside factors that may trigger the condition.
Genetic factors
MS is not defined as a genetic condition because there is no single gene that causes it. It's not directly inherited, although research has shown people who are related to someone with MS are more likely to develop it.
Researchers have found that if one twin develops MS then the second twin has around a one in four chance of also developing MS.
The chances of a brother, sister, or child of a person with MS also developing MS themselves is less than 1 in 30.
It's possible that different combinations of genes make developing MS more likely, and research into this is continuing. However, genetic theories cannot explain the wide variation in occurrences of MS throughout the world.
Sunlight and vitamin D
Research into MS around the world has shown that it's more likely to occur in countries far from the equator. For example, MS is relatively common in the UK, North America and Scandinavia, but rare in Malaysia or Ecuador.
It’s possible that people living further from the equator are exposed to less sunlight and, therefore, have less vitamin D in their bodies. Some studies have found a link between lower levels of vitamin D and incidence of MS.
Some researchers have suggested that vitamin D supplements may reduce the risk of MS. However, this has not been proven.
Viral infection
Another theory is that MS may be the result of viral infection of the nervous system and /or the immune system.
The idea is that the virus lies dormant for many years and then periodically ‘re-awakens’, triggering an autoimmune response against the nervous system.
This could explain the relapse-remission nature of most cases of MS.
A virus called the Epstein-Barr virus (EBV) is known to act in this way, but there is currently no firm evidence that EBV, or any other virus, is responsible for MS.
Problems with blood flow
A new and controversial theory is that some cases of MS may actually be due to problems with the flow of blood inside the body.
The idea is that some people may have narrowing of veins inside their brain and spinal cord and the blood supply from the brain and spine has trouble returning to the heart (known as cerebrospinal venous insufficiency).
This could lead to a build-up of tiny iron deposits inside nerve tissue, which may damage the nerves and /or trigger an immune response.
Some studies have found higher-than-expected levels of cerebrospinal venous insufficiency in people with MS, but others have not.
Further research is ongoing looking at larger groups of people and using more sophisticated brain imaging scanning.
