Mixed connective tissue disease: overlap syndromes

Since the original description of mixed connective tissue disease (MCTD) as an apparently unique syndrome by Sharp and co-workers, the concept of MCTD has been highly controversial. In this chapter, a quarter of a decade later, we examine the evidence that MCTD is a distinctive entity rather than a haphazard association of clinical and serological features and that the presence of high titres of autoantibodies to U1RNP influences the expression of connective tissue disease in ways that are relevant to prognosis and treatment. Results of long-term clinical studies are presented, which show that the clinical phenotype of MCTD is robust and can be defined by classification criteria that show reasonable sensitivity and specificity. In addition, the chapter addresses the results of immunogenetic and serological studies that demonstrate that MCTD is quite distinctive from systemic lupus erythematosus and systemic sclerosis. Indeed, there is good evidence that the clinical and serological features of MCTD are not just a haphazard association but that these patients represent a distinctive subset of connective tissue disease in which the specific autoimmune response is relevant to clinical expression and to understanding the underlying pathogenesis.