Clinical characteristics, organ damage and survival in idiopathic inflammatory myopathies: a long-term retrospective cohort study.

To compare the clinical phenotype, organ damage, and survival of patients with antisynthetase syndrome (ASyS) and other idiopathic inflammatory myopathies (OIIM) in a long-term, single-center retrospective cohort. We retrospectively analyzed patients with idiopathic inflammatory myopathy who fulfilled the Bohan and Peter criteria, the EULAR/ACR classification criteria and had available myositis specific autoantibody results. This cohort comprises patients followed in our autoimmune connective tissue disease clinic between 1991-2024. Demographic, clinical, laboratory, autoantibody, organ damage, and survival data were compared between ASyS and OIIM groups. Myositis Damage Index was used for assessing organ damage at the sixth month of follow-up and the last visit. Of the 122 patients, 35 (28.7%) were classified as ASyS (85.7% female) and 87 as OIIM (59.8% female). Median age at diagnosis was 47 and 46 years, respectively. The mean follow-up duration of the overall cohort was 84.48 ± 77.95 months. Among patients with ASyS, anti-Jo-1 was the most frequent autoantibody detected (88.6%), followed by anti-PL-7 in 8.6% and anti-KS in 2.8%. Mechanic's hands, arthritis, and interstitial lung disease were more frequent in the ASyS group. Dysphagia was more frequent in the OIIM group (ASyS: 8.6%; OIIM: 37.9%, p = 0.003). Anti-Ro-52 positivity was more common in ASyS than in OIIM (ASyS: 42.9%, OIIM: 21.8%, p = 0.034). C-reactive protein, leukocyte, and neutrophil levels were higher in the ASyS group at baseline. The median Myositis Damage Index score at the sixth month was nominally higher in ASyS than in OIIM (median MDI: 3 vs. 2, p = 0.046). However, MDI scores were comparable between the two groups at the last assessment. In autoantibody-based subgroup analyses, higher early damage scores were observed in ASyS than in anti-TIF1γ-positive patients (median MDI = 3 vs 1.5, p = 0.033). Interstitial lung disease (ILD) was more frequently present in ASyS, whereas malignancy was more frequent among anti-TIF1γ-positive patients. The mortality rate was 14.3% in the ASyS group and 14.9% in the OIIM group. Survival analysis did not reveal a statistically significant difference at the end of the follow-up between ASyS and OIIM groups. In this long-term retrospective cohort, patients with ASyS showed a distinct clinical phenotype compared with OIIM. Sixth-month MDI scores were nominally higher in ASyS and may partly reflect the higher burden of ILD; however, MDI subdomain analyses were not available. No statistically significant difference in survival was detected between groups, although this finding should be interpreted cautiously because of the small number of deaths. Overall, these findings should be considered exploratory and hypothesis-generating and require confirmation in larger multicenter prospective cohorts.