Multiple sclerosis (MS) is characterized by damage to the myelin sheath. This protective covering surrounds the nerves of the central nervous system (brain, optic nerves, and spinal cord). The damage is caused by inflammation. Damaged areas undergo gliosis (scarring). Lesions or scars (called plaques) may be scattered throughout the central nervous system. Plaques may be found:
- in areas around the ventricles in the brain
- around the optic nerves
- in the white matter in the optic nerves that control vision
- and in the white matter of the spinal cord, brainstem, cerebellum, and cerebral area of the brain.
Over the past several years, researchers have found evidence of damage in the gray matter of the brain as well.
The destruction of myelin interferes with nerve conduction. The symptoms of MS relate to this interruption of signaling between neurons (nerves of the central nervous system). Damage to the underlying axons is likely to cause irreversible disability and was originally believed to occur late in the disease. However, recent research has shown that axonal damage also occurs early in the disease.
It is not clear what exactly causes the damage that leads to MS. Various studies have shown evidence that points to a number of factors.
Researchers believe that a genetic predisposition may exist in the disease. MS is not caused by one gene. It is believed that several genes play a role in predisposing a person to the disease.
Immune System Attack
MS may be an autoimmune disorder. That means that your immune system mistakes your own cells for foreign invaders and attacks them. This is similar to an allergic response except that the allergen is part of your body. It is thought that the myelin loss associated with MS results from your immune system mistakenly attacking these tissues.
It is believed that the autoimmune response may be triggered by a bacterial or viral infection. Research is being done to study this theory.
Though the cause of MS is not clear, a number of risk factors exist.
Ancestry and Environment
MS affects up to 2.5 million people worldwide. It occurs most frequently in Caucasians of Northern European ancestry.
Areas that were settled or visited by Vikings and other northern European tribes have the highest rates of MS. The condition is also common in Europe, North America, certain areas of the Mediterranean, Australia, and New Zealand. It is relatively rare in Asia, Africa, and tropical countries.
Migration studies have also increased our knowledge of risk factors for MS. Migrants from high- to low-risk areas retain the risk of their birthplace if they are least 15 years old when they move.
Investigators have explored both noninfectious and infectious agents to explain the patterns of geographical variation in the occurrence of MS, such as sunlight and vitamin D. It has been found that the average annual hours of sunshine and the average December daily solar radiation at place of birth are strongly correlated with the presence of MS.
Researchers have also investigated the role of infectious agents in triggering MS, such as bacteria and viruses. Studies show the risk of developing MS is about 10 times greater in people who experienced an infection from the Epstein-Barr virus than those who did not. This risk increases about 20 fold in people who developed clinical mononucleosis.
According to the Mayo Clinic, in the general population, there is a 0.1% risk of developing MS. In fraternal twins, if one twin has MS, the other has a two percent risk of getting it. According to UCSF Multiple Sclerosis Center, in identical twins, if one twin has MS, the other has a 25 to 30 percent risk of getting it. This suggests that genes may play a role in developing MS. Since only a minority of identical twins both get MS, this also suggests that environment is a factor.
Specific genes have been isolated as potentially being able to explain susceptibility to the disease. These genes are currently being studied worldwide. Genes may also play a role in disease progression. For example, an individual’s ability to repair myelin and preserve their axons may be genetically determined.
Studies have shown that women are more likely to develop MS than men. Other studies have shown that hormonal changes that occur, for example, during the menstrual cycle and after delivering a child (postpartum period), may be linked to acute relapses of the disease.
While some researchers have concluded that men do not do as well as women in the long run, a more recent study indicates that although men may progress (worsen) faster, both genders ultimately have some degree of disability at the same age.
Finally, some investigators have suggested that those with a younger age of onset may have a better outlook. People diagnosed later in life often do not do as well over time. The reasons for this are not clear.