Introduction:

Multiple sclerosis (MS) is a disease of the central nervous system (CNS), the nerves that comprise the brain and spinal cord. It has two major features:

• Destruction of myelin, a fatty insulation covering the nerve fibers, is the main characteristic of MS. 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.)
• Destruction of axons, the long filaments that carry electric impulses away from a nerve cell, is also a major factor in the permanent disability that occurs with MS.

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

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

Chronic-progressive MS is often subcategorized as primary-progressive, secondary-progressive, and progressive-relapsing.

Recent evidence suggests that the disease process starts long before symptoms begin. By the time symptoms appear, there are often already signs of brain and spinal cord atrophy. The cause of MS 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.

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:

• Most patients first experience a single attack of symptoms called a clinical isolated syndrome, which typically occurs between the ages of 20 - 40 years. Once a second attack occurs, the patient is considered to have relapsing-remitting multiple sclerosis.
• 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 (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.
• The disease then goes into remission (when symptoms improve or disappear), usually for about 4 - 8 weeks. To be considered in remission, attacks need to be separated by at least 30 days. 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 has a period of deteriorating ability.

About 20% of patients with relapsing-remitting MS experience little or no progression after a first attack for long periods of time, although by 25 years most patients have converted to a progressive phase.

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:

• Secondary-Progressive MS (SPMS). SPMS is the natural evolution of relapsing-remitting MS and develops in about half of patients during the first 10 years and nearly all of them within 25 years. It follows a progressive course of nerve and muscle deterioration with occasional acute flare-ups, remissions, and plateaus.
• Primary-Progressive MS (PPMS). PPMS progresses continuously and gradually from the first onset of symptoms and has no remissions. 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. This type of multiple sclerosis is more resistant to the medications typically used to treat the illness.
• Progressive-Relapsing MS (PRMS). PRMS is progressive from the start with acute symptom flare-ups, but may have some relapses with continued deterioration between them. 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.