The exact cause of the rare autoimmune disorders dermatomyositis (DM), polymyositis (PM) and inclusion body myositis (IBM) is not known. It is possible that a variety of factors play a role in causing these diseases. These factors include changes in the body's immune system and viral agents.

The immune system is a set of organs and cells that work together to defend the body from harmful foreign substances referred to as "antigens." The immune system produces antibodies to fight antigens. Some antibodies are produced to combat one specific antigen. Other antibodies are produced to combat anything that is recognized as "not self" -- that is, not part of the body itself.

In autoimmune diseases, the immune system mistakes some part of the body as a harmful foreign agent. Consequently, the immune system begins attacking its own body. With myositis, the immune system mistakes muscle fibers as antigens and begins attacking them.

When an antibody attaches to an antigen, the antibody begins the process of destroying it. Because this process is seldom strong enough to completely destroy the antigen, other cells are needed to finish the task. The immune system also produces these cells, which are known as T-cells. When the antibody attaches to the antigen, these cells are triggered to join the attack. In myositis, these inflammatory cells surround, invade and destroy the muscle fibers. Weakness develops as the muscle fibers are destroyed.

Some scientists have proposed that a virus may trigger the abnormal immune response. PM has been known to develop as a complication of an HIV infection and of a human T-lymphocyte 1 infection. These cases would support a "viral trigger" theory.

Some scientists have also theorized that a group of viruses known as enteroviruses can cause myositis. Of these viruses, Toxoplasma, Borrella (lyme disease bacterium) and Coxsackievirus have been associated with DM and PM.

Despite the association of a virus with some cases of DM and PM, there is no strong evidence to support this theory. Most cases of these diseases do not have an identifiable viral trigger. Consequently, additional research is needed to determine what role, it any, viruses play in DM, PM and IBM.

Further research will reveal more about dermatomyositis, polymyositis and inclusion body myositis. As the cause of these diseases is more clearly understood, better treatment -- and perhaps even a cure -- will emerge.

References

First published March 1, 2000
Last updated August 1, 2003
Copyright © 2000 Accordant Health Services, Inc. All Rights Reserved.

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