Idiopathic inflammatory myopathies (IIM) are traditionally identified as a group of disorders that target skeletal muscle due to autoimmune dysfunction. The IIM can be divided into subtypes based on certain clinical characteristics, and several classification schemes have been proposed. The predominant diagnostic criteria for IIM is the Bohan and Peter criteria, which subdivides IIM into primary polymyositis (PM), primary dermatomyositis (DM), myositis with another connective tissue disease, and myositis associated with cancer. However, this measure has been criticised for several reasons including lack of specific criteria to help distinguish between muscle biopsy findings of PM, DM, and immune-mediated necrotising myopathy, as well as the lack of identification of cases of overlap myositis (OM). Because of this issue, other classification criteria for IIM have been proposed, which include utilising myositis-associated antibodies and myositis-specific antibodies, as well as overlap features such as Raynaud’s phenomenon, polyarthritis, oesophageal abnormalities, interstitial lung disease, small bowel abnormalities such as hypomotility and malabsorption, and renal crises, amongst others. Indeed, the identification of autoantibodies associated with certain clinical phenotypes of myositis, in particular connective tissue disease-myositis overlap, has further helped divide IIM into distinct clinical subsets, which include OM and overlap syndromes (OS). This paper reviews the concepts of OM and OS as they pertain to IIM, including definitions in the literature, clinical characteristics, and overlap autoantibodies.
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