What is multiple sclerosis?
Multiple sclerosis (MS) is a chronic disease that affects the central nervous system (CNS). It causes inflammation and the destruction of myelin. Myelin surrounds nerve fibers and acts like insulation on a wire, preventing "short-circuits" that divert a nerve signal from having its desired effects. The "demyelination" process interferes with nerve impulse transmission, affects muscular control, and causes a variety of sensory, motor, and psychological symptoms. Damage to the myelin usually resolves with time and symptoms subside, but repeated attacks can lead to a continual process of demyelination and remyelination, which produces nerve fiber scarring and progressive disability.
The cause of MS is unknown. It is thought to be an autoimmune process triggered by a virus, environmental factors, and/or a genetic predisposition. Typically, MS first appears and is diagnosed when individuals are between 20 and 50 years of age, although it can occur in young children. It affects women more frequently 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. According to the National Multiple Sclerosis Society, MS affects about 2.1 million people worldwide. It is estimated that about 250,000 to 350,000 people in the U.S. have MS, according to the National Institute on Neurological Disorders and Stroke. The risk of developing this 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 component to the cause.
There is no single test that can conclusively diagnose MS. Instead, health practitioners look for a combination of factors to determine if a patient has MS. The factors are described in a document called the "McDonald Criteria," named for the doctor who chaired the 2001 panel of experts charged with establisheding criteria for an accurate diagnosis. The document is updated regularly as new research improves our understanding of the disease. Physicians will consult a patient's medical history and a variety of clinical and laboratory tests to aid in their diagnosis. In applying the criteria for diagnosing MS, a health practitioner must:
- Determine that the CNS 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
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 that are damaged in the CNS:
- Relapsing-remitting (RRMS)
- Secondary progressive (SPMS)
- Primary progressive (PPMS)
- Progressive-relapsing (PRMS)