Multiple Sclerosis

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What is Multiple Sclerosis?

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Multiple sclerosis (MS) is a disease of the central nervous system (brain and spinal cord). As shown in the diagram below, nerves in the body are covered with a fatty sheath called myelin. Myelin considerably increases the speed that nerve signals (impulses) travel down the axons. A thin myelinated axon transmits impulses at anything from 5 to 30 meters per second, whereas an unmyelinated one transmits them at 0.5 to 2 meters per second. It does this by both insulating and containing the nerves.

In MS, there are multiple areas of damage to the myelin sheath (demyelination) within the brain and spinal cord. This causes conduction problems within the central nervous system. The peripheral nervous system is not usually involved but may be subtly. MS can manifest in different ways, including:

Relapsing remitting;
Primary progressive;
Secondary progressive (when relapsing and remitting disease becomes progressive); and
Progressive relapsing (a combination of relapsing remitting and progressive from the outset).

Who gets Multiple Sclerosis?

MS is one of the most common causes of chronic neurological disability in adults, and affects approximately one million people worldwide. The disease usually starts in early adult life (20–45 years) and causes progressive disability over several decades. MS is more common in women, who outnumber males by a ratio of 2:1.

The prevalence of MS is directly proportional to distance from the equator. At 50–65 degrees latitude (north or south), the prevalence is 60–100 per 100,000 people. At the equator, MS is a rarity. The incidence, however, is very variable.

Approximately one in 3000 Australians are affected by the disorder.

Predisposing Factors

Multiple sclerosis The exact cause of MS is not entirely understood, but it is thought to be due to a number of genetic, environmental and immune factors. It is hypothesised that individuals with a genetic predisposition who are infected with certain viruses induce an immune response that attacks their own nerve cells.

Risk factors for MS therefore include:

Latitude: Latitude is more strongly related to MS than any other risk factor. Risk of MS increases when an individual lives further away from the equator. If you migrate to a high-risk area before the age of 15, you will have the same high risk as those who were born in the area. Therefore, your risk of MS reflects that of the area in which you spent your first 15 years.
Sex: MS affects females more than males, but the reason for this difference is unknown.
Family history: If you have a first degree relative with MS, you are 15 times more likely than the general population to suffer from MS (this is still a very low risk). Twins from the same egg (monozygotic) are more likely to have MS than those from separate eggs (dizygotic) if one twin is affected.
Race: MS presents more often in populations of northern European ancestry.
Vitamin D Status: Low levels of vitamin D exposure result in an increased risk of MS.
Smoking: smokers have an increased risk of developing MS
Epstein-Barr Virus: Exposure to EBV increases the risk of MS
Diet: It is thought that diets high in animal fats may increase the risk of MS, due to observations that MS is uncommon in coastal communities where large amounts of fish are consumed.

Progression

Most patients have a relapsing/remitting disease, where they have discrete attacks of symptoms (such as loss of vision and limb sensations/weakness) that last weeks to months and then completely resolve. These attacks may not have a clear precipitant, and on average tend to occur once every two years. These attacks or relapses should be distinguished from minor exacerbations of previous symptoms which can occur with increases in body temperature. Later in the course of the disease, patients may not fully recover from attacks and experience progressive disability. This is the secondary progressive form of disease, which occurs in 50% of patients after 15 years. The majority of MS cases will eventually evolve into the secondary progressive form.

Some patients may have only one or two attacks and may fully recover from these attacks. In other patients, the disease will progress and produce severe disability, or even death.

Other patients have primary progressive disease, which shows a rapid decline in function without any acute attacks. This occurs in approximately 20% of patients and affects males and females equally. However, males with this disorder have the worst prognosis, as spinal cord involvement leads to early disability.

The final type of MS is progressive relapsing, affecting 5% of patients. This is an overlap of the former types, characterised by a progressive course from the onset but with additional superimposed acute attacks.

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