[Idiopathic inflammatory myopathies from the viewpoint of rheumatologists]

Idiopathic inflammatory myopathies (IIMs) are a group of inflammatory muscle disorders of unknown etiology; these include polymyositis (PM), dermatomyositis (DM), and inclusion body myositis. Extra-muscular manifestations such as dermatitis, arthritis, interstitial lung disease (ILD), cardiomyopathy, and enteropathy are occasional complications in patients with PM/DM. Several myositis-specific autoantibodies (MSAs) have been discovered in IIMs; these can help predict clinical characteristics, response to treatment, and prognosis. For example, anti-aminoacyl-tRNA synthetase (ARS) antibodies, including Jo-1 antibody (Ab) and anti-melanoma differentiation-associated gene 5 (MDA-5) Ab, have been associated with the manifestation of ILD in PM and DM. Anti-MDA5 Ab-associated ILD has a 1-year survival rate of 50-60%; however, short-term prognosis is relatively good in anti-ARS Ab-associated ILD. Fatal outcome occurs remarkably often within the first 6 months of anti-MDA5 Ab-associated ILD. Therefore, intensive treatment should be administered to patients harboring anti-MDA-5 Ab or showing hyperferritinemia in ILD with DM. In addition, corticosteroid occasionally induces myopathy, which is an issue arising in PM/DM treatment. Some experts recommend combination therapy of corticosteroid and an immunosuppressive agent as a first-line treatment for myositis in PM/DM. Methotrexate and azathioprine are commonly used immunosuppressive agents for myositis in western countries. Immunosuppressive agents are steroid-sparing, serving to mitigate corticosteroid-related side effects, thus making combination therapy an effective treatment option. Preventing the progression of physical dysfunction is of prime importance to patients with PM/DM. Dermatologists, neurologists, and rheumatologists should therefore work together to care for these patients before muscular and extra-muscular involvement develop progressively and irreversibly.