With MS can expect one of three clinical courses of disease, each of which might be mild, moderate, or severe.

• Relapsing-Remitting: People with this type of MS experience clearly defined flare-ups (also called relapses, attacks, or exacerbations). These are episodes of acute worsening of neurologic function. They are followed by partial or complete recovery periods (remissions) free of disease progression.This is the most common form of MS at time of initial diagnosis. Approximately 85% of people have this type of MS.

• Primary-Progressive: This type of MS is characterized with slowly progressive symptoms from the onset, without episodes of relapse or remission. For example, a typical patient may have leg weakness that starts slowly and progresses for months to years with spread eventually to the arms. These patients are, on average, more often male and a bit older, and have less abnormalities on MRI, as compared to patients with Relapsing Remitting or Secondary Progressive MS.

• Secondary-Progressive: This type of MS is characterizedby an initial period of relapsing-remitting disease, followed by a steadily worsening disease course with or without occasional flare-ups, minor recoveries (remissions), or plateaus.

About half 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 has significantly delayed by treatment.