Background/Purpose: Idiopathic Inflammatory Myopathies (IIM) comprise systemic autoimmune dosorders which may occur in patients with other conective tissue diseases (CTD). The coexistence of IIM with other CTDs suggests shared underlying pathogeic mechanisms, but may have clinical implications as well. Systemic sclerosis has been reported as the most common CTD overlaping with IIM. There is scarce information regarding prevalence, clinical and serologic features of IIM and concomitant CTD.
We assessed the prevalence of concomitant CTD in a cohort of patients with IIM and describe clinical and serological differences among patients with or without other CTDs.

Methods: We analyzed the MYOTReCSZ cohort, which includes 208 patients classified as IIM according to the ACR/EULAR and Connor´s criteria for anti-synthetase syndrome at a tertiary care center in Mexico City. The study was approved by the Institutional Ethics and Research committees. We analyzed the prevalence of concomitant CTDs and compared clinical and laboratory features between patients with or without CTD overlap using man whitney U test for numerical variables and G test for categorical variables.

Results: We found a total of CTD overlap in 22 (10.5%) patients. Among IIM patients with concomitant CTD, Rheumatoid Arthritis was the most common (10/208; 4.8%) followed by systemic lupus erythematosus (6/208; 2.8%), and finally Sjögren syndrome and systemic sclerosis (3/208; 1.4%). Interestingly, a family history of autoimmunity was more commonly detected in the concomitant CTD group (50% vs 15.3%; p=0.0002). Regarding serological features, there were no differences between the prevalence of antinuclear antibodies, however, homogeneous pattern was more common among IIM with concomitant CTD (54.5% vs 18.1%; p=0.0007) and fine granular pattern among IIM patients without concomitant CTD (43.4% vs 13.6%; p=0.001). Interestingly, among specific and associated IIM antibodies, anti Jo-1 was more common among patients with concomitant CTD (18.1% vs 3.6%; p=0.01). Among the clinical features, Raynaud phenomenon and alopecia were more commnonly registered among IIM patients with concomitant CTD (31.8% vs 11.5%; p=0.01 and 50% vs 23.6%; p=0.01 respectively). There were no differences regarding disease activity between the 2 groups.

Conclusion: This is the first comprehensive characterization of overlap CTD and IIM in a cohort of IIM patients. In contrast with previous reports, we found rheumatoid arthritis as the most common concomitant CTD, which might be explained based on prior used of historical cohorts and potential misclassification of patients. We provide the serological and clinical landmark related to this complex entities.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/clinical-and-laboratory-characterization-of-overlap-connective-tissue-disease-in-patients-with-idiopathic-inflammatory-myopathies-results-from-the-myotrecsz-cohort/