UNIVERSITY OF MARYLAND MEDICAL CENTER: CLICK TO READ MORE"Multiple sclerosis (MS) is a disease of the central nervous system (CNS), the nerves that comprise the brain and spinal cord. Its cause is unknown, and it cannot be prevented or cured. It is not fatal, however, and great progress is being made in treating it and identifying underlying mechanisms that trigger this disease.

The primary characteristic of this disease is the destruction of myelin, a fatty insulation covering the nerve fibers. The end results of this process, called demyelination, are multiple patches of hard, scarred tissue called plaques. (Multiple sclerosis is well named. Sclerosis comes from the Greek word skleros, which means hard.)


Myelin is the layer that forms around nerves. Its purpose is to speed the transmission of impulses along nerve cells.

Another important feature in the disease is destruction of axons -- the long filaments that carry electric impulses away from a nerve cell -- which is now considered to be a major factor in the permanent disability that occurs with MS.
The symptoms, severity, and course of MS vary widely depending partly on the sites of the plaques and the extent of the demyelination. Experts generally group multiple sclerosis into two major symptom categories:

Relapsing-remitting.
Chronic-progressive MS, which is often subcategorized as primary-progressive, secondary-progressive, and progressive-relapsing MS.
Recent evidence is strongly suggesting that the disease process starts long before symptoms begin, and by the time symptoms appear, there are already signs of brain and spinal cord atrophy.

Relapsing-Remitting Multiple Sclerosis

Relapsing-remitting multiple sclerosis generally occurs in younger people and is the most common form of MS. It generally follows this course:

The characteristic feature of relapsing-remitting MS is the attack (also referred to as relapse, flare-up, or exacerbation), which is a bout of specifically MS symptoms (e.g., facial pain, Lhermitte's sign, or bladder instability) that lasts at least 24 hours (and typically several days). Such attacks are fairly mild in about half of patients with this form of MS. [For a description of symptoms see What Are the Symptoms of Multiple Sclerosis.]
The disease then goes into remission (when symptoms improve or disappear), usually for about four to eight weeks. At least 30 days should separate one attack from another to be considered a remission. Remission periods may be spontaneous or induced by immunosuppressive drugs. (A person with multiple sclerosis in remission may have subtle attacks and not realize it. For example, hands may be a little numb for a few days, or there may be slight awkwardness in gait or coordination.)
Remissions are almost always followed by relapses, in which symptoms flare-up or the patient experiences a period of deteriorating ability. The average number of relapses per year range from 0.14 to 1.1.
Some patients with relapsing-remitting MS can experience little or no progression for long periods of time, although by 25 years most patients have converted to a progressive phase [see below].

Chronic-Progressive Multiple Sclerosis

The term chronic-progressive multiple sclerosis is used to describe cases in which symptoms continue to worsen slowly without remission. About 20% of multiple sclerosis patients (usually those whose first symptoms occur after age 45) have the chronic-progressive form without first developing relapsing-remitting MS. Chronic-progressive MS generally follows a downhill course, but its severity varies widely. Three variants are commonly used to define this patient group:

Primary-Progressive MS (PPMS) -- progresses continuously and gradually without remission. It occasionally levels off, and minor improvement is even possible. This occurs in about 10% of patients, who tend to be older than average at the time of diagnosis.
Secondary-Progressive MS (SPMS) -- occurs after the initial relapsing-remitting phase in about half of patients during the first 10 years and nearly all of them within 25 years. It is follows a progressive course of nerve and muscle deterioration with occasional acute flare-ups, remissions, and plateaus.
Progressive-Relapsing MS (PRMS) -- is progressive from the start with acute symptom flare-ups and continued deterioration between relapses. It occurs in less than 5% of patients.
Because the natural courses of primary progressive and progressive relapsing MS are similar, some experts believe this distinction is unnecessary. "