Background/Purpose: Systemic sclerosis (SSc) and idiopathic inflammatory myositis (IIM) are two systemic autoimmune connective tissue diseases with predominant effects on skin and muscle respectively . Although they generally occur as distinct disease entities, some features of SSc and IIM may be seen together in “overlap syndromes” or in mixed connective tissue disease (MCTD). The aim of this study was to determine the occurrence of SSc in IIM, and vice versa, in patients without features of MCTD or overlap syndromes.  We also sought to compare the incidence of SSc in IIM, with that of rheumatoid arthritis (RA), systemic lupus erythematosus (SLE) and primary Sjogrens syndrome (pSS).

Methods: The South Australian (SA) Myositis Database has registered all patients with biopsy-proven IIM in SA subsequent to 1980. All adult muscle biopsies performed in SA are reported in a central diagnostic laboratory and are subjected to peer review. The criteria used to diagnose IIM are those based on current published literature. Myositis-associated antibodies (MAA) and myositis-specific antibodies (MSA) have been measured on 138 patients with myositis. The South Australian Scleroderma Register similarly registers all patients in SA diagnosed with limited or diffuse SSc. Fisher’s exact T-test was used for statistical comparisons.

Results: The incidence of SSc in IIM was 18/426 (4.2%). A greater proportion of IIM patients had SSc (18/426) than RA  (2/426, p=0.003) or MCTD (4/426, p=0.004).There was a trend towards a greater proportion of IIM patients having SSc than SLE or pSS (each 8/426, p=0.07). There was no difference in the proportion of females amongst patients with IIM alone (231/426) compared with IIM and SSc (13/18, Fishers exact T test p =0.15. Of the 18 patients with SSc and IIM, 14 had polymyositis, 3 dermatomyositis, 2 inclusion body myositis and one had necrotizing myopathy. Four patients had antibodies to PM-Scl  (3 of these had polymyositis, one had dermatomyositis). Antibodies to other MSA or MAA were detected as follows: Ku (n=2), MI-2 (n=1), PL12 (n=1), Jo-1 (n=1) and Ro52 (n=2). Of the 177 patients on the SA Scleroderma Register, 22 had suspected IIM (incidence of IIM in SSc =12.4%). Of these 22 patients, 18 had elevations in creatinine kinase levels, and ten had biopsy confirmation of IIM.  

Conclusion: Although the population incidence of RA, SLE and pSS is greater than SSc, in IIM, there is a greater incidence of SSc and SSc is the commonest autoimmune connective tissue disease in IIM. The co-occurrence of these two conditions raises the possibility of shared disease susceptibility.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/idiopathic-inflammatory-myositis-is-associated-with-an-increased-incidence-of-systemic-sclerosis/