Multiple Sclerosis (MS) is the most common disabling neurological disease of young adults. It most often appears when people are between 20 to 40 years old. However, it can also affect children and older people.
The course of MS is unpredictable. A small number of those with MS will have a mild course with little to no disability, while another smaller group will have a steadily worsening disease that leads to increased disability over time. Most people with MS, however, will have short periods of symptoms followed by long stretches of relative relief, with partial or full recovery. There is no way to predict, at the beginning, how an individual person’s disease will progress.
New treatments can reduce long-term disability for many people with MS. However, there are still no cures and no clear ways to prevent MS from developing.
What is Multiple Sclerosis?
Multiple sclerosis (MS) is a neuroinflammatory disease that affects myelin, a substance that makes up the membrane (called the myelin sheath) that wraps around nerve fibers (axons). Myelinated axons are commonly called white matter. Researchers have learned that MS also damages the nerve cell bodies, which are found in the brain’s gray matter, as well as the axons themselves in the brain, spinal cord, and optic nerve (the nerve that transmits visual information from the eye to the brain). As the disease progresses, the brain’s cortex shrinks (cortical atrophy).
The term multiple sclerosis refers to the distinctive areas of scar tissue (sclerosis or plaques) that are visible in the white matter of people who have MS. Plaques can be as small as a pinhead or as large as the size of a golf ball. Doctors can see these areas by examining the brain and spinal cord using a type of brain scan called magnetic resonance imaging (MRI).
While MS sometimes causes severe disability, it is only rarely fatal and most people with MS have a normal life expectancy.
What are plaques made of and why do they develop?
Plaques, or lesions, are the result of an inflammatory process in the brain that causes immune system cells to attack myelin. The myelin sheath helps to speed nerve impulses traveling within the nervous system. Axons are also damaged in MS, although not as extensively, or as early in the disease, as myelin.
Under normal circumstances, cells of the immune system travel in and out of the brain patrolling for infectious agents (viruses, for example) or unhealthy cells. This is called the "surveillance" function of the immune system.
Surveillance cells usually won't spring into action unless they recognize an infectious agent or unhealthy cells. When they do, they produce substances to stop the infectious agent. If they encounter unhealthy cells, they either kill them directly or clean out the dying area and produce substances that promote healing and repair among the cells that are left.
Researchers have observed that immune cells behave differently in the brains of people with MS. They become active and attack what appears to be healthy myelin. It is unclear what triggers this attack. MS is one of many autoimmune disorders, such as rheumatoid arthritis and lupus, in which the immune system mistakenly attacks a person’s healthy tissue as opposed to performing its normal role of attacking foreign invaders like viruses and bacteria. Whatever the reason, during these periods of immune system activity, most of the myelin within the affected area is damaged or destroyed. The axons also may be damaged. The symptoms of MS depend on the severity of the immune reaction as well as the location and extent of the plaques, which primarily appear in the brain stem, cerebellum, spinal cord, optic nerves, and the white matter of the brain around the brain ventricles (fluid-filled spaces inside of the brain).
Reference: National Institute of Neurological Disorders and Stroke (NINDS)
Last updated: May 26, 2016