Multiple sclerosis (MS) is a neurological disease that involves 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 or lesions. Sclerosis comes from the Greek word skleros, which means hard.
- Damage of axons, the fibers 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.
Autoimmune Disease Process
Multiple sclerosis is thought to be an autoimmune disorder. In autoimmune diseases, immune factors attack the body’s own cells. In the case of MS, the immune system attacks the tissues that make up myelin. The damage to myelin, and nerve fibers (axons), is caused by overactivated T cells. T cells are a type of white blood cells called lymphocytes.
Types of Multiple Sclerosis
Doctors generally group multiple sclerosis into four major disease course categories:
Click the icon to see an image depicting multiple sclerosis.
- Relapsing-Remitting MS. Relapse-remitting MS (RMSS) is the most common form of multiple sclerosis. About 85% of patients are first diagnosed with this type of MS. RMSS is marked by flare-ups (also called relapses or exacerbations) of symptoms followed by periods of remission when symptoms improve or disappear.
- Secondary-Progressive MS. Some patients with RMSS go on to develop secondary-progressive (SPMS). (For many patients, treatment with disease-modifying medications helps delay this progression.) In SPMS, the disease course continues to worsen with or without periods of remission or leveling off of symptom severity (plateaus).
- Primary-Progressive MS. About 10% of patients are diagnosed with primary-progressive MS (PPMS). In PPMS, symptoms continue to worsen gradually from the very beginning. PPMS has no relapses or remissions. There may be periods of occasional plateaus. This type of multiple sclerosis is more resistant to the medications typically used to treat the illness.
- Progressive-Relapsing MS. Progressive-relapsing MS (PRMS) is a rare form of MS, occurring in less than 5% of patients. It is progressive from the start with intermittent flare-ups of worsening symptoms along the way. There are no periods of remission.