Multiple sclerosis (MS) affects the function of the brain, spinal cord and nerves. It is caused by inflammation and destruction of myelin in small patches called plaques. Myelin is a white fatty tissue that surrounds nerve fibres and insulates them against short-circuits that can prevent nerve signals from having their desired effects. The "demyelination" process interferes with nerve impulse transmission, causing a wide variety of symptoms affecting sensation, movement and thought. Myelin damage occurs in attacks that usually resolve with time, allowing symptoms to subside, but repeated demyelination can damage the nerve fibres resulting in progressive and persistent disability.
MS is an autoimmune process thought to be triggered by a virus or environmental factors in those with a genetic predisposition. Typically, MS is first diagnosed when individuals are between 20 and 50 years of age, although it can occur in young children. It affects women two or three times more often than men, is more common in Northern European Caucasians than other ethnic groups and is seen in greater numbers in people who live in temperate climates than warm ones. Between 2 and 2.5 million people are affected worldwide with about 100,000 in the UK. The risk of developing the disease is estimated to be about 1 in 750 in the general population. In families with an affected member the risk rises to 1 in 40, and it is about 1 in 4 for the identical twin of an affected person, strengthening the notion of a genetic predisposition.
There is no single test that can conclusively diagnose MS. Instead, doctors will consider a patient's medical and family history together with a variety of clinical and laboratory tests to aid diagnosis. In 2001 an international panel chaired by Dr Ian McDonald recommended that to diagnose MS a doctor must:
- Determine that the central nervous system has been damaged in at least two places
- Confirm that the damage occurred at separate times, more than one month apart
- Rule out other conditions that cause a similar set of signs and symptoms
The McDonald criteria have been regularly updated to include radiological and laboratory tests that can help to make a more rapid diagnosis, for example following a single attack.
Once diagnosed, an individual may be classified as having one of several types of MS based on signs and symptoms, frequency of relapses, rate of disease progression and the number of areas of the central nervous system that are damaged.