Mixed connective tissue disease.

Mixed connective tissue disease (MCTD) was first described in 1972 as a disease syndrome with overlapping features of systemic sclerosis, systemic lupus erythematosus (SLE) and polymyositis associated with antibodies to RNAse sensitive extractable nuclear antigen. When the antigen was subsequently characterized as polypeptides on the U1 ribonuclear protein component of the splicesosome (U1RNP), MCTD became the first rheumatic disease syndrome to be defined by a serologic test. Clinical features include a high frequency of Raynaud's syndrome, swollen hands, sclerodactyly, arthritis, polymyositis and interstitial lung disease. Over the last 30 years there has been a continuing debate as to whether MCTD constitutes a 'distinct clinical entity'. Here, I will review the pathological, immunogenetic and clinical features of MCTD and conclude that the debate remains unresolved. The early misconception that it has a relatively good prognosis has not stood the test of time with long-term follow-up studies. These have identified a tendency for MCTD to evolve into SLE or systemic sclerosis and highlighted pulmonary hypertension and scleroderma renal crisis as important causes of death. Providing it is realized that our appreciation of the clinical features associated with anti-U1RNP have evolved over time, MCTD remains a useful concept in clinical practice. Whether it can be credited with the term 'disease' awaits the demonstration of common etiopathological events underlying the development of antibodies to U1 RNP and their associated clinical features.