Mixed connective tissue disease (MCTD), or: Sharp syndrome, is an uncommon, specifically defined, overlap syndrome. MCTD should be suspected when additional overlapping features are present in patients appearing to have rheumatoid arthritis, SLE, scleroderma, or polymyositis. MCTD is characterised by very high titres of circulating antinuclear antibodies (ANA) to a ribonucleoprotein antigen (RNP).
Hand swelling, Raynaud’s syndrome, polyarthralgia, inflammatory myopathy, esophageal hypomotility, and pulmonary dysfunction are common. Diagnosis is by the combination of clinical features, ribonucleoprotein antibodies, and absence of antibodies specific for other autoimmune diseases.
Tests for antinuclear antibodies (ANA), RNP antibodies, and antibodies to ENA are done first. If results of these tests are compatible with MCTD (RNP antibodies very high, positive ANA!), tests for rheumatoid factors, Jo-1 antibodies, antibodies to the ribonuclease-resistant Smith component of ENA (Sm antibodies), and double-stranded DNA (dsDNA antibodies) are done to exclude other possible diagnoses.
Further evaluation depends on symptoms and signs; manifestations of myositis, renal involvement, or pulmonary involvement prompt tests of those organs (eg., creatine kinase, magnetic resonance imaging, electromyogram, or muscle biopsy for diagnosis of myositis).
Almost all patients have high titres of fluorescent ANA producing a speckled IFA pattern. Antibodies to ENA are usually present at very high titres. RNP antibodies are detectable, whereas Sm antibodies are absent. Rheumatoid factors are frequently positive, and titres are often high. The erythrocyte sedimentation rate (ESR) is frequently elevated.
Treatment of mixed connective tissue disease varies with disease severity and organ involvement but usually includes corticosteroids and sometimes additional immunosuppressants.