These syndromes occur when patients meet criteria for the diagnosis of more than one of the six classic systemic autoimmune rheumatic diseases (SLE, SSc, PM, dermatomyositis, RA, and Sjögren’s syndrome). Although the features of both diseases may occur concurrently, usually one disease predominates clinically. As many as 25% of patients with features of a connective tissue disease have or will develop an overlap syndrome. Some overlap syndromes such as MCTD and certain myositis overlap syndromes have specific autoantibody associations.
Sjögren’s syndrome is the most common overlap and is seen with RA, SLE, SSc, PM, MCTD, primary biliary cirrhosis (PBC), necrotizing vasculitis, autoimmune thyroiditis, chronic active hepatitis, mixed cryoglobulinemia, and hypergammaglobulinemic purpura.