Multiple sclerosis is thought to be 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.
People with MS can expect one of four clinical courses of disease, each of which might be mild, moderate, or severe.
Relapsing-Remitting
Characteristics: People with this type of disease experience clearly defined flare-ups (also called relapses or exacerbations). These are episodes of acute worsening of neurologic function, followed by partial or complete recovery periods (remissions) that are free of disease progression. Frequency: Most common form of MS at time of initial diagnosis. Approximately 85 percent of people with MS begin with this form.
Primary-Progressive
Characteristics: People with this type of MS experience a slow but nearly continuous worsening of their disease from the onset of the disease, with no distinct relapses or remissions. However, there are variations in rates of progression over time, occasional plateaus and temporary minor improvements. Frequency: Relatively rare. Approximately 10 percent.
Secondary-Progressive
Characteristics: People with this type of MS experience an initial period of relapsing-remitting disease (see above) followed by a steadily worsening disease course with or without occasional flare-ups, minor recoveries (remissions), or plateaus. Frequency: Before the "disease-modifying" drugs became available, 50 percent of people with relapsing-remitting MS tended to develop this form of the disease within 10 years of their initial diagnosis. One goal of therapy is to slow the transition to secondary-progressive.
Progressive-Relapsing
Characteristics: People with this type of MS experience a steadily worsening disease from the onset but also have clear acute flare-ups (relapses), with or without recovery. In contrast to relapsing-remitting MS, the periods between relapses are characterized by continuing disease progression.
Frequency: Relatively rare. Approximately 5 percent.
