Differentials
Systemic lupus erythematosus (SLE)
SIGNS / SYMPTOMS
No differentiating symptoms or signs.
INVESTIGATIONS
Negative for anti-U1 ribonucleoprotein (U1 RNP).
High antibody titres to the Smith's antigen or to double-stranded DNA in the absence of anti-U1 RNP suggest SLE as the underlying diagnosis, whereas high titres to anti-U1 RNP suggest mixed connective tissue disease.
Polymyositis
SIGNS / SYMPTOMS
More pronounced muscle weakness than is typical of mixed connective tissue disease (MCTD); Raynaud's phenomenon uncommon.
INVESTIGATIONS
Negative for anti-U1 ribonucleoprotein (U1 RNP).
High antibody titres to the Mi-2 antigen or signal recognition particle suggest dermatomyositis or polymyositis rather than overlap syndrome.
High-titre antibodies to U1 RNP suggest MCTD; antibodies to the tRNA synthetase enzymes suggest antisynthetase syndrome.
Scleroderma
SIGNS / SYMPTOMS
Digital ischaemia and infarcts more common than in mixed connective tissue disease (MCTD); truncal sclerodermatous skin changes may also be seen, unlike in MCTD.
INVESTIGATIONS
Negative for anti-U1 ribonucleoprotein.
Antibody titres to anticentromere antibody suggest scleroderma spectrum disease rather than MCTD.
Anti-Ku and antipolymyositis/scleroderma point to a scleroderma overlap.
Idiopathic Raynaud's phenomenon
SIGNS / SYMPTOMS
Nail fold capillaroscopy normal, digital infarcts and necrosis not seen; abnormalities suggest underlying connective tissue disease instead.
INVESTIGATIONS
Negative for anti-U1 ribonucleoprotein.
Vasculitis
SIGNS / SYMPTOMS
Palpable purpura or livedo reticularis suggest small and medium vessel vasculitis that are only rarely seen in mixed connective tissue disease.
INVESTIGATIONS
Negative for anti-U1 ribonucleoprotein.
Antineutrophil cytoplasmic antibodies may be seen in primary small-vessel vasculitides.
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