What are the causes and risk factors of multiple sclerosis (MS)?

What are the causes of multiple sclerosis (MS)?

The exact cause of MS is still unknown. It is thought to be an autoimmune disease that is also partly a neurodegenerative condition and is, therefore, not contagious.

An autoimmune disease refers to a condition wherein the immune system mistakenly attacks and damages its own cells whereas a neurodegenerative condition affects neurons in the brain and cause progressive nervous system dysfunction.

With MS, the immune system malfunctions and accidentally destroys myelin as a result. When this protective covering is damaged, nerve fibres are exposed and the messages in the form of nerve impulses travelling along the nerves, within the brain and to and from the brain and body, will be interrupted or blocked. Nerves themselves can also be damaged during this process and may die resulting in permanent damage and loss of function which in turn affect various areas of the body.

It is not yet clear as to why multiple sclerosis develops in some individuals and not in others. Experts believe that a combination of genetics, certain triggers and environmental factors may be responsible.

There are many theories regarding the exact causes of MS but none have yet been conclusively proven. 

The role of the immune system

It is evident from research that the immune system plays a role in the damage done to the spinal cord and brain tissue in patients suffering from MS, however, the specific target that the immune system attacks and the cells of the immune system involved in the resulting damage is not yet fully understood.

Experts and researchers have identified a number of plausible explanations for what these specifics may be. Theories include that a person’s immune system may be:

1. Fighting some form of an infectious agent, such as a virus that contains components that mimic functions of the brain, this is known as molecular mimicry.
2. Destroying specific brain cells because they are seen as unhealthy.
3. Identifying some brain cells as foreign invaders and acting to destroy them – this has been the generally accepted explanation for a number of years. 

Research has recently shown that the above explanations (one and two specifically) may have a role to play in the development of multiple sclerosis. There is a specialised barrier known as the blood-brain barrier (BBB), that separates the spinal cord and brain from one’s immune system. Should a break in this barrier occur, as is the case with some brain infections or brain cancers, this will expose the immune system to the brain. When this occurs, the brain might be interpreted as a foreign body by the immune system and subsequently attacked by it.

Risk factors

The following risk factors have been identified as increasing someone’s risk of developing MS:

• Gender – Women are twice as likely to develop MS than men.
• Age – MS can develop at any age, however, the most commonly affected people are between the ages of 15 and 60.
• Family history (genetic susceptibility) – Should a parent or close blood relative have MS, then one may be more likely to develop the condition themselves. This suggests that multiple sclerosis may, to some degree, be an inherited (genetic) condition.
  
  Studies of families have indicated that 15% of people with MS were found to have at least one family member with the condition². Current research suggests that there are a large number of genes and even hundreds of gene variations in a person’s genetic code (these are known as gene variants) that are able to combine to create a person’s vulnerability to the disease. Only some of these genes have been identified, the majority of which have been associated with the functioning of the immune system.
• Race – MS is more commonly observed in Caucasians, particularly those with a Northern European background. Those who have an African, Native American or Asian descent are thought to have the lowest risk.
• Specific autoimmune diseases – An individual may have an increased risk of developing MS if they have type 1 diabetes, thyroid disease, heart disease, IBD (inflammatory bowel disease) or have experienced a stroke.
• Infectious viruses and other factors – There are several viruses that have been identified in those suffering from MS, however, the virus that has been consistently linked to the development of MS is the EBV (Epstein Barr virus).
  
  Research shows that those who were infected with EBV at some point during their lives develop an immune system reaction that puts them at higher risk of developing MS. Bear in mind, research is still being conducted into whether EBV is a direct cause of MS but enough evidence of a relational link has been found to support this theory and as such, the Epstein Barr virus is considered a risk factor
  
  This evidence ties in with findings that link MS to some form of an immune system disturbance.
• Climate – MS is more commonly seen in countries that have more temperate climates, these include the northern states of America, New Zealand, Canada, Europe and south-eastern Australia.
• Vitamin D and sunlight³ – Experts have noted that people who spend more time in the sun have higher levels of vitamin D and have a lower chance of developing MS as a result. Experts believe that vitamin D plays a role in the functioning and regulation of the immune system, lowering a person’s risk of disease.
  
  Sunlight exposure ties in with climate which is influenced by where people are based. For example, those who are situated near the equator have a higher level of sunlight exposure and therefore, people from these regions have less of a risk of MS. Some studies of people with MS have also shown that those with higher levels of vitamin D suffer fewer relapses. 
• Smoking – The use of tobacco products increases a person’s risk of developing multiple sclerosis, with MS sufferers who use tobacco products exhibiting more lesions on the brain in comparison to non-smokers with the disease.

 

^References

^{3. Health Encyclopaedia Multiple Sclerosis (MS). Available: https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=85&contentid=P00790 [Accessed 29.08.2017]}

^{6. September 2015. Benefits of Exercise Training in Multiple Sclerosis. Available: https://www.ncbi.nlm.nih.gov/pubmed/26223831 [Accessed 28.08.2017]}