From The MS Information Sourcebook, produced by the National MS Society.
An exacerbation (also known as an attack, a relapse, or a flare) is a sudden worsening of an MS symptom or symptoms, or the appearance of new symptoms, which lasts at least 24 hours and is separated from a previous exacerbation by at least one month. The most common disease course in MS, called relapsing-remitting MS, is characterized by clearly defined acute exacerbations, followed by complete or partial recovery with no progression of the disease between attacks.
True Exacerbations Generally Last Days to Weeks
A true exacerbation of MS is caused by an area of inflammation in the central nervous system. This is followed by demyelination—the destruction of myelin, which is the fatty sheath that surrounds and protects the nerve fibers. Demyelination results in the formation of an abnormal area called a plaque and causes the nerve impulses to be slowed, distorted, or halted, producing the symptoms of MS. One example of an exacerbation of MS would be the development of optic neuritis, an inflammation of the optic nerve that impairs vision.
An exacerbation may be mild or may significantly interfere with the individual's daily life. Exacerbations usually last from several days to several weeks, although they may extend into months. It is generally accepted that a short course of corticosteroids will cause an exacerbation to be shorter and/or less severe.
Pseudoexacerbations Temporarily Aggravate MS Problems
Sometimes an increase in symptoms has nothing to do with the underlying MS, but is caused by factors such as fever, infection, or hot weather that can temporarily aggravate MS problems. This is referred to as a pseudoexacerbation. Once the triggering event is past—e.g., the body temperature returns to normal, the symptoms subside as well. Some people with MS report a worsening of their symptoms during or after periods of intense stress. Researchers are exploring the effects of stress on the immune system and its possible involvement in MS.
A remission does not mean that all the symptoms of MS disappear, but rather that a person with MS returns to the baseline that existed before the last exacerbation began.