MS is an autoimmune disease that affects the central nervous system (CNS). The CNS consists of the brain, spinal cord, and the optic nerves. Surrounding and protecting the nerve fibers of the CNS is a fatty tissue called myelin, which helps nerve fibers conduct electrical impulses.

In MS, myelin is lost in multiple areas, leaving scar tissue called sclerosis. These damaged areas are also known as plaques or lesions. Sometimes the nerve fiber itself is damaged or broken.

Myelin not only protects nerve fibers, but makes their job possible. When myelin or the nerve fiber is destroyed or damaged, the ability of the nerves to conduct electrical impulses to and from the brain is disrupted, and this produces the various symptoms of MS. These include fatigue, difficulty walking - even paralysis, vision difficulties, dizziness and vertigo, spasticity, pain, numbness and tingling, bowel and bladder problems, cognitive difficulties, depression, and sexual dysfunction.

People with MS can expect one of four clinical courses of disease, each of which might be mild, moderate, or severe.

Relapsing-Remitting, 85%

People with this type of MS experience clearly defined flare-ups (also called relapses, attacks, or exacerbations). These are episodes of acute worsening of neurological function. They are followed by partial or complete recovery periods (remissions) free of disease progression.

Primary-Progressive, 10%

People with this type of MS experience a slow but nearly continuous worsening of their disease from the onset, with no distinct relapses or remissions. However, there are variations in rates of progression over time, occasional plateaus, and temporary minor improvements.

Secondary-Progressive

People with this type of MS experience an initial period of relapsing-remitting MS; followed by a steadily worsening disease course with or without occasional flare-ups, minor recoveries (remissions), or plateaus. 50% of people with relapsing-remitting MS developed this form of the disease within 10 years of their initial diagnosis, before introduction of the "disease-modifying" drugs. Long-term data are not yet available to demonstrate if this is significantly delayed by treatment.

Progressive-Relapsing, 5%

People with this type of MS experience a steadily worsening disease from the onset but also have clear acute relapses (attacks or exacerbations), with or without recovery. In contrast to relapsing-remitting MS, the periods between relapses are characterized by continuing disease progression.