Myositis may refer to a number of muscle diseases, including polymyositis, dermatomyositis and inclusion body myositis (IBM). These are all rare conditions characterized by pain, inflammation, weakness and degeneration of the muscles, particularly those close to the torso. Most researchers suggest that myositis may be similar to autoimmune disorders, in which white blood cells and antibodies attack healthy tissues.

Myositis may affect children or adults. It typically affects women more often than men and African Americans more often than whites. In some instances, myositis can be fatal.

Polymyositis is the most common form of myositis. It affects many muscle systems, usually near the trunk. Dermatomyositis includes all of the muscle weakness characteristics of polymyositis as well as a rash. Inclusion body myositis (IBM) is similar to polymyositis but involves more muscle in the limbs. IBM is resistant to treatment. Myositis may also be associated with other conditions, such as cancer, viral infections and rheumatoid arthritis.

The cause of myositis is not known. Many researchers believe that genetic factors may trigger the white blood cells and antibodies in the blood to attack muscles after a viral infection or use of certain medication.

Symptoms of myositis usually develop slowly and may range from mild to severe in intensity. Weak muscles are the primary symptom and often lead to other symptoms, such as difficulty swallowing or shortness of breath. Other less serious symptoms include fever, fatigue and weight loss.

Diagnosis of myositis relies on physical examination and tests. Physicians look for signs of weak muscles. Typically, blood tests, muscle biopsies and electromyography (EMG) are used to rule out other conditions as the cause of the patient’s symptoms.

No methods of preventing myositis are known. The effectiveness of treatment varies among individuals. Corticosteroids are typically given in high initial doses. When symptoms go away, the dose is gradually reduced. Although adults usually continue taking corticosteroids for a long time or indefinitely, children typically stop therapy after two years. If corticosteroids do not work, other drugs may be used.