The National Multiple Sclerosis Society describes MS as "an unpredictable, chronic disease of the central nervous system (the brain and spinal cord) in which inflammation and breakdown in the protective insulation (myelin sheath) surrounding the nerve fibers of the central nervous system occurs."

Like insulation on electrical wires, healthy myelin insures rapid transmission of nerve impulses. When myelin or nerve fibers are damaged, the messages from the brain, for example, to move a body part, and messages to the brain, for example, to interpret sensations, are not transmitted effectively. Body movement may be slow or uncoordinated and body sensations may be altered.

Multiple sclerosis is considered to be an autoimmune disease characterized by the inflammatory demyelination of neurons in the central nervous system (CNS). Autoimmunity is when the body's natural defenses (the immune system) mistakenly attack the body's own tissue. In MS, a person's own T Cells attack the myelin sheath surrounding the nerves.

Each person with MS has a unique set of symptoms depending on where in the central nervous system the demyelination and fiber damage occurs. Common symptoms include weakness, numbness, incoordination, loss of balance, visual problems, loss of bladder or bowel control, mood swings, cognitive problems, difficulty speaking, and fatigue.

Twice as many women as men suffer from this disease with the onset of symptoms generally occurring between the ages of 20 and 50. While the typical clinical course of multiple sclerosis is characterized by relapsing and progressive disability, there have been examples of subclinical cases of MS where the diagnosis is confirmed only by the presence of large, confluent, demyelinating plaques.

The following are important presentation categories:

Clinically Isolated Syndrome
A "clinically isolated syndrome" or CIS, can be thought of as the first attack of MS. It is a single clinical event that points to demyelination. For example, an attack of optic neuritis in one eye, or an episode of numbness on one side, would count as a clinically isolated syndrome.

Relapsing-Remitting
This is the most common phase of MS at time of diagnosis, affecting approximately 70% of MS patients - This group experiences clearly defined flare-ups (relapses), followed by partial or complete remissions between attacks that are free of disease progression.

Primary-Progressive
Affecting approximately 15% of MS patients, this group experiences a nearly continuous worsening of their disease from the onset with no relapses or remissions.

Secondary-Progressive
Fifty percent of the relapsing-remitting patients eventually develop this form of the disease within 10 years of diagnosis. This phase is characterized by an initial period of relapsing-remitting disease followed by steady worsening with or without flares or minor remissions.

Progressive-Relapsing
Affecting approximately 15% of MS patients, this group experiences a steady worsening of the disease from the onset but also have relapses with or without recovery. This stage is different from the relapsing-remitting stage because the periods between relapses are characterized by continuous disease progression.

It is important to note these four categories of MS since pharmaceutical products may be indicated for only one of these specific categories. The course of multiple sclerosis varies for each person. The diagnosis is based on the combination of problems, patterns of recurrence, which systems are impaired, and test results. It often takes years before a doctor can be certain of an MS diagnosis, and have some idea of how the disease will progress. (WebMD)