Epidemiology and clinical characteristics of systemic sclerosis overlap syndrome (SSc-OS), and the factors significantly associated with SSc-OS in Thai patients with systemic sclerosis

Objective: To investigate the epidemiology and characteristics of systemic sclerosis (SSc) overlap syndrome (SSc-OS). Methods: This study included patients enrolled in the Siriraj Systemic Sclerosis Cohort registry during November 2013 to September 2019. SSc-OS was defined as SSc patients who also met criteria for rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), dermatomyositis (DM), polymyositis (PM), Sjogren, and/or antiphospholipid antibody syndrome. Baseline and 1-year characteristics were compared between SSc and SSc-OS. Results: 185 patients (age 50.3 ±11.4 years, 85.4% female, disease duration 2.7 years, 75.1% diffuse cutaneous subset, 75.6% anti-Scl-70 positivity) were included. The incidence and prevalence rate of SSc-OS was 3.2/100 patient-years and 17.8%. Regarding SSc-OS, 12.4%, 2.2%, 1.1%, 1.6%, and 0.5% of patients were classified as SSc-RA, SSc-SLE, SSc-PM, SSc-RA-SLE, and SSc-SLE-PM. SSc-OS had a higher prevalence of limited cutaneous subset (lcSSc), usual interstitial pneumonia, finger contractures, ESR >20 mm/hr., globulin >3.5 g/dL, rheumatoid factor, anti-citrullinated peptide antibody, and antiphospholipid antibodies. LcSSc subset (OR: 11.3, 95%CI: 2.0-62.6) and globulin >3.5 g/dL (OR: 6.2, 95%CI: 1.6–23.6) were associated with SSc-OS. Conclusion: SSc-OS is associated with the lcSSc subset. RA is the most common overlap syndrome. LcSSc patients with globulin >3.5 g/dL are associated with SSc-OS.