bone, joint, and muscle disorders
Autoimmune Myositis
Diagnosis of Autoimmune Myositis
Established criteria Muscle biopsy Doctors use the following criteria to make the diagnosis of autoimmune myositis: Muscle weakness at the shoulders or hips and thighs Sometimes a characteristic rash Increased levels of certain muscle enzymes (especially creatine kinase) in the blood, indicating muscle damage Abnormalities in electrical activity of muscles as measured by Electromyography or in appearance of muscles on a magnetic resonance imaging (MRI) scan Characteristic changes in muscle tissue obtained by biopsy and observed under a microscope (the most conclusive evidence) The diagnosis of autoimmune myositis is based on all of the information doctors gather, including symptoms, physical examination results, and all test results. Muscle biopsy is often done and is the most conclusive way to diagnose autoimmune myositis, especially when the diagnosis is not clear. Muscle biopsy is not usually necessary when people have characteristic skin changes of dermatomyositis. Other laboratory tests cannot specifically identify autoimmune myositis, but they can help doctors rule out other disorders, detect the risk of complications, and determine severity. Blood tests are done to detect levels of antinuclear antibodies (ANA) and other antibodies, which are present in most people who have autoimmune myositis. Although blood test results can help doctors diagnose autoimmune myositis, they alone cannot confirm a definite diagnosis of autoimmune myositis because sometimes the abnormalities they detect are present in healthy people or in people who have other disorders. MRI may also help the doctor select a site for biopsy. Special tests done on muscle tissue samples may be needed to rule out other muscle disorders. Doctors often suggest cancer screening for people age 40 or older who have dermatomyositis or for people age 60 or older who have polymyositis because these people may have unsuspected cancers. 