The term “myositis” refers to a general inflammation or swelling of the muscles. Many people have experienced sore muscles after vigorous exercise,but this is temporary and improves with rest. Other conditions that can cause muscle weakness and pain include infection, muscle injury from medications, inherited diseases, electrolyte imbalances, and thyroid disease.

More often, however, the term myositis is used to refer to a set of autoimmune diseases involving chronic inflammation of the muscles. This often occurs together with other symptoms, such as pain, fatigue, trouble swallowing, and others. This type of myositis is also known as idiopathic inflammatory myopathy (IIM). Visit our glossary for an extensive list of myositis terms and definitions.

Types of Myositis

The disease is highly variable and has been classified into a number of forms, including: 

Inflammatory Myopathies

Inflammatory myopathies are autoimmune diseases, meaning the body’s immune system, which normally fights infections and viruses, is misdirected and begins to attack the body’s own normal, healthy tissue. Inflammatory myopathies are rare diseases. All forms combined affect an estimated 75,000 people in the United States. While it is still unclear what causes myositis, some scientists believe certain individuals have a genetic predisposition to develop an autoimmune disease, which is triggered by an environmental exposure to some trigger, such as infection, virus, toxin, or sunlight.


Symptoms of weakness, swelling, and muscle damage often appear gradually. Long before patients are diagnosed, they may have trouble getting up from a chair, climbing stairs, or grasping objects with their hands. Patients may fall, find it difficult to reach their arms up, have difficulty swallowing, or other symptoms. In addition to these symptoms, it’s common for patients to experience other complicating conditions.


Myositis is often difficult to diagnose, because many physicians are unfamiliar with the disease and its symptoms. A typical diagnosis process for myositis patients begins with a medical history and physical examination. It may also include blood tests, muscle and skin biopsies, and a variety of other diagnostic tests.

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Treatment and Disease Management

Myositis is a rare disease, so it is also difficult to conduct adequate research to test new treatments. There is a lot of confusion among the medical community over how to effectively manage patients with myositis. Nevertheless, myositis is a serious illness that, in most cases, needs to be treated aggressively. With inadequate or no treatment, myositis can cause significant disability and even death. There is no cure for any of the forms of myositis.

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