Prevalence of other connective tissue diseases in idiopathic inflammatory myopathies

We sought to determine the prevalence of additional connective tissue diseases (CTDs) in patients with idiopathic inflammatory myopathies (IIM), and to study the muscle biopsy patterns in various clinico-serologic subsets of myositis. We undertook a retrospective cohort study of 648 patients with a histological diagnosis of IIM. The following was determined from the South Australian Myositis Database: presence of associated CTDs, histological details and presence of myositis-specific (MSA) or myositis-associated (MAA) antibodies. Among patients with IIM, a significantly greater proportion had systemic sclerosis 32/648 (4.9%) than mixed connective tissue disease (12/648, p = 0.003), primary Sjogren's syndrome (12/648, p = 0.003), systemic lupus erythematosus (10/648, p < 0.001) or rheumatoid arthritis (6/648, p = 0.0001). Polymyositis was the most common IIM diagnosis regardless of the presence or absence of CTD. MSA/MAA was more commonly detected in those with systemic sclerosis than those with IIM alone (OR 5.35, p < 0.005). The higher prevalence of SSc (compared with other CTDs) in IIM, together with the more frequent detection of autoantibodies in this group, suggests that these conditions may be linked.