What is myositis?

Definition

Myositis is a general term for a group of rare chronic conditions characterized by inflammation of the skeletal muscles. This inflammation can cause
muscle weakness. Myositis refers to the inflammatory myopathies, including
polymyositis, dermatomyositis, inclusion-body
myositis, and juvenile myositis. It is thought that all these disorders are autoimmune diseases. Inflammatory myopathies can also be caused by
certain medications or by exposure to a toxic substance; these myopathies are
usually not chronic and resolve once the harmful substance is removed.

Causes

It is not known what causes myositis. It is believed that an environmental
factor, such as a viral infection, triggers myositis in people who might be
genetically predisposed to the condition. The damage in myositis is caused by the
body’s own immune
system, as white blood cells and antibodies
attack the muscle and, in some cases, the skin.

Risk Factors

Generally, women are affected more often than men, although inclusion-body myositis affects twice as many men as women. Polymyositis is observed in persons between twenty and sixty years of age, whereas inclusion-body myositis is more common after age fifty years. Children can develop dermatomyositis. African Americans are at higher risk for myositis, while the lowest rates of myositis are reported in persons of Japanese origin.

Symptoms

Common symptoms of the inflammatory myopathies include muscle weakness, sometimes with muscle pain, that lasts for more than a few weeks; general tiredness and fatigue; difficulty climbing stairs, standing up from a seated position, or reaching up; and difficulty swallowing. Additional symptoms for the various myopathies include a variety of skin symptoms (such as a rash or scaly, dry, and rough skin) in dermatomyositis; and hardened lumps of calcium (calcinosis) under the skin in juvenile dermatomyositis. Unlike other inflammatory myopathies, the muscle weakness in inclusion-body myositis is often asymmetrical.

Screening and Diagnosis

Myositis varies from person to person and can often resemble other diseases,
such as scleroderma or lupus. Tests used to help confirm a diagnosis include a
physical exam; tests of muscle strength; magnetic resonance
imaging (MRI) scan; an electromyogram (EMG); blood tests,
including erythrocyte sedimentation rate, creatinine kinase, and antinuclear
antibodies; and muscle and skin biopsies.

Treatment and Therapy

Treatment for myositis generally includes rest, physical therapy, and the use
of anti-inflammatories (corticosteroids as first-line therapy
and methotrexate, hydroxychloroquine, and azathioprine), and intravenous
immunoglobulin. If left untreated, inflammatory myopathy can cause permanent
damage.

Prevention and Outcomes

Because the cause of myositis is unknown, there is no known way to prevent the condition. To lessen the severity of dermatomyositis, however, persons with the condition should avoid excessive exposure to the sun, which can worsen any dermatomyositis-associated skin rashes.

Bibliography

Isenberg, D. A., et al. “International Consensus Outcome Measures for Patients with Idiopathic Inflammatory Myopathies: Development and Initial Validation of Myositis Activity and Damage Indices in Patients with Adult Onset Disease.” Rheumatology 43, no. 1 (January, 2004): 49-54.

Kagen, Lawrence J., ed. The Inflammatory Myopathies. Totowa, N.J.: Humana Press, 2009.

Marieb, Elaine N. Essentials of Human Anatomy and Physiology. 8th ed. San Francisco: Pearson/Benjamin Cummings, 2006.

Murphy, Kenneth, Paul Travers, and Mark Walport. Janeway’s Immunobiology. 7th ed. New York: Garland Science, 2008.

Parker, James N., and Philip M. Parker, eds. Myositis: A Medical Dictionary, Bibliography, and Annotated Research Guide to Internet References. San Diego, Calif.: ICON Health, 2004.