What is the difference between MCTD overlap syndrome and UCTD?
• MCTD: first described by Sharp et al. in 1972, this is a very specific overlap syndrome characterized by a combination of manifestations similar to those seen in SLE, SSc, and inflammatory myopathy. Some patients have a severe erosive arthritis with Jaccoud’s deformity. The autoantibody pattern is a high-titer, speckled ANA, and U1-RNP (high titer). These patients lack other specific autoantibodies such as anti-Sm, anti-Sjögren’s syndrome antigen B (SS-B), anti-ds DNA, and anti-centromere. MCTD is 15 times more common in women than in men. The mean age at diagnosis is 37 years, with a range of 4 to 80 years. There is no apparent racial or ethnic predisposition.
• Overlap syndrome: these patients have features of more than one of the six classic systemic autoimmune rheumatic diseases (SLE, SSc, polymyositis [PM], dermatomyositis, rheumatoid arthritis [RA], and SS). Some overlap syndromes like MCTD and certain myositis overlap syndromes have specific autoantibody associations.
• UCTD: refers to a situation in which a patient has some of the clinical features of one or more of the systemic autoimmune rheumatic diseases in the setting of a positive ANA, but do not meet criteria for a more specific diagnosis. About 25% of patients referred to a rheumatologist may present as such. Over time, a similar percentage will develop additional clinical features that will allow a specific disease diagnosis, whereas the majority will remain undifferentiated.
