Relapsing-Remitting Multiple Sclerosis (RRMS) is the most common disease course and is characterized by clearly defined attacks of new or increasing neurologic symptoms. These attacks – also called relapses or exacerbations – are followed by periods of partial or complete recovery (remissions). During remissions, all symptoms may disappear, or some symptoms may continue and become permanent. However, there is no apparent progression of the disease during the periods of remission.

At different points in time, RRMS can be further characterized as either active (with relapses and/or evidence of new MRI activity) or not active, as well as worsening (a confirmed increase in disability over a specified period of time following a relapse) or not worsening. An increase in disability is confirmed when the person exhibits the same level of disability at the next scheduled neurological evaluation, typically 6 to 12 months later.

Approximately 85 percent of people with MS are initially diagnosed with RRMS.

Relapsing-Remitting MS is defined by inflammatory attacks on myelin (the layers of insulating membranes surrounding nerve fibers in the central nervous system (CNS)), as well as the nerve fibers themselves. During these inflammatory attacks, activated immune cells cause small, localized areas of damage which produce the symptoms of MS. Because the location of the damage is so variable, no two people have exactly the same symptoms.

While RRMS is defined by attacks or relapses of new MS symptoms, progressive forms of MS involve fewer attacks:

• People with RRMS tend to develop more new brain lesions — also called plaques or scars — on magnetic resonance imaging (MRI) scans.
• People with RRMS, tend to have more inflammatory lesions on MRI (seen when gadolinium dye is used during the MRI)
• People with Primary Progressive MS (PPMS) tend to have more spinal cord lesions.
• In RRMS, women are affected two to three times as often as men; in PPMS, the number of women and men are approximately equal.

RRMS is generally diagnosed earlier than the progressive disease courses:

• Most people with RRMS are diagnosed in their 20s and 30s (although it can occur in childhood or later adulthood), while PPMS diagnosed during the 40s or 50s.
• The transition from RRMS to Secondary Progressive Multiple Sclerosis (SPMS) generally occurs in people who have been living with RRMS for at least 10 years.