An MS attack (also called an "exacerbation", "relapse", or "flare-up") is a sudden worsening of MS symptoms, or the appearance of new symptoms that lasts for at least 24 hours.
MS attacks are thought to be associated with the development of new areas of damage in the brain. Exacerbations are characteristic of relapsing-remitting MS, in which attacks are followed by periods of complete or partial recovery with no apparent worsening of symptoms.
An attack may be mild or its symptoms may be severe enough to significantly interfere with life's daily activities. Most exacerbations last from several days to several weeks, although some have been known to last for months.
When the symptoms of the attack subside, an individual with MS is said to be in remission. However, MRI data have shown that this is somewhat misleading because MS lesions continue to appear during these remission periods. Patients do not experience symptoms during remission because the inflammation may not be severe or it may occur in areas of the brain that do not produce obvious symptoms.
Research suggests that only about 1 out of every 10 MS lesions is perceived by a person with MS. Therefore, MRI examination plays a very important role in establishing an MS diagnosis, deciding when the disease should be treated, and determining whether treatments work effectively or not. It also has been a valuable tool to test whether an experimental new therapy is effective at reducing exacerbations.
Reference: National Institute of Neurological Disorders and Stroke (NINDS)