The category of idiopathic inflammatory myopathies includes several entities involving progressive muscle weakness due to muscle inflammation of known etiology, primarily involving proximal muscles with a typically symmetrical distribution. There are elevated levels of various muscle-associated enzymes such as creatine kinase (CK), certain electromyographic abnormalities, and microscopic chronic inflammatory infiltrate in the affected muscle. Because of some degree of serological crossover with the rheumatoid-collagen diseases and also more recent finding of other autoantibodies, these myopathies are thought to be autoimmune disorders with as yet unknown etiology. The best-known entities in this group are dermatomyositis, polymyositis, and an uncommon entity called inclusion-body myositis. Also included are less well-defined conditions called myositis overlap and cancer-related myositis.

Diagnosis of the three major entities depends on muscle biopsy; each entity has a different pattern of inflammatory cell infiltration or other findings if the biopsy has a classic picture (if the classic picture is not present, interpretation is much more difficult). In addition, some of these entities have varying incidence of certain autoantibodies. One of these is Jo-1, an antibody directed against synthetase antigen. Jo-1 antibodies are found in about 33% of polymyositis, 33% of dermatomyositis, and 8% of myositis overlap patients. Other antibodies with even less frequency are anti-SRP (against signal recognition proteins) and anti-MAS. Although these antibodies are of little help in diagnosing muscle diseases as presently defined entities, the antibodies do help define patients with certain patterns of symptoms that possibly some day may be used to redefine autoimmune muscle diseases. For example, in patients who demonstrate synthetase (Jo-1) autoantibodies, there is an 87% incidence of fever, 62% of Raynaud’s phenomenon, 84% of myalgias, 94% of arthritis, 4% of distal weakness, 89% of interstitial lung disease, and 49% of carpal tunnel syndrome. In those with anti-SRP, there is no fever, 29% with Raynaud’s; 100% myalgias, no arthritis; 43% distal weakness; no interstitial lung disease; and 20% carpal tunnel syndrome. Anti-MAS is nearly always seen in alcohol-associated rhabdomyolysis, and not present in patients with Jo-1 or anti-SRP. Other than Jo-1, these autoantibodies are currently used more in research than clinical diagnosis; even Jo-1 has limited usefulness due to its poor sensitivity in currently defined muscle diseases.

Of some interest is significant incidence of ANA in most of the inflammatory myopathies (40% in polymyositis; 62% in dermatomyositis; 77% in myositis-collagen disease overlap; 31% in cancer-associated myositis; and 23% in inclusion-body myositis). There is a small incidence (less than 20%) of various ANA subgroups such as SS-A (Ro) in the myositis syndromes and an increased incidence of HLA DRw52 as well as specific DR antigens. However, the DR antigen incidence is not high enough for any one DR antigen to be either specific or diagnostic.