MS is thought to be an autoimmune disease that affects the central nervous system (CNS). The CNS consists of the brain, spinal cord, and the optic nerves. Surrounding and protecting the nerve fibers of the CNS is a fatty tissue called myelin, which helps nerve fibers conduct electrical impulses.

In MS, myelin is lost in multiple areas, leaving scar tissue called sclerosis. These damaged areas are also known as plaques or lesions. Sometimes the nerve fiber itself is damaged or broken.

Myelin not only protects nerve fibers, but makes their job possible. When myelin or the nerve fiber is destroyed or damaged, the ability of the nerves to conduct electrical impulses to and from the brain is disrupted, and this produces the various symptoms of MS.

People with MS can expect one of four clinical courses of disease, each of which might be mild, moderate, or severe.

• Relapsing-Remitting
  Characteristics: People with this type of MS experience clearly defined flare-ups (also called relapses, attacks, or exacerbations). These are episodes of acute worsening of neurologic function. They are followed by partial or complete recovery periods (remissions) free of disease progression.
  Frequency: Most common form of MS at time of initial diagnosis. Approximately 85%.

• Primary-Progressive
  Characteristics: People with this type of MS experience a slow but nearly continuous worsening of their disease from the onset, with no distinct relapses or remissions. However, there are variations in rates of progression over time, occasional plateaus, and temporary minor improvements.
  Frequency: Relatively rare. Approximately 10%.

• Secondary-Progressive
  Characteristics: People with this type of MS experience an initial period of relapsing-remitting disease, followed by a steadily worsening disease course with or without occasional flare-ups, minor recoveries (remissions), or plateaus.
  Frequency: 50% of people with relapsing-remitting MS developed this form of the disease within 10 years of their initial diagnosis, before introduction of the “disease-modifying” drugs. Long-term data are not yet available to demonstrate if this is significantly delayed by treatment.

• Progressive-Relapsing
  Characteristics: People with this type of MS experience a steadily worsening disease from the onset but also have clear acute flare-ups (attacks or relapses), with or without recovery. In contrast to relapsing-remitting MS, the periods between relapses are characterized by continuing disease progression.
  Frequency: Relatively rare. Approximately 5%.

The long-established criteria for diagnosing MS are:

1.      There must be objective evidence of two attacks (i.e. two episodes of demyelination in the central nervous system).  An attack, also known as an exacerbation, flare, or relapse, is defined clinically as the sudden appearance or worsening of an MS symptom or symptoms, which lasts at least 24 hours.  The objective evidence comes from findings on the neurologic exam and additional tests.

2.      The two attacks must be separated in time (at least one month apart) and space (indicated by evidence of inflammation and/or damage in different areas of the central nervous system).

3.      There must be no other explanation for these attacks or the symptoms the person is experiencing.