Background To describe the clinical characteristics and explore the factors related to the MRI remission of sacroiliitis in patients with enthesitis-related arthritis (ERA). Methods Patients with ERA from 2018–2022 in our medical center were retrospectively reviewed, which identified according to Pediatric Rheumatology International Trials Organization (PRINTO) criteria. Demographics, clinical characteristics, examinations, and treatments were described. Univariate and multivariate logistic regression models were used to analyze the factors related to MRI remission of sacroiliitis in ERA. Results This retrospective study included 160 ERA patients (51.9% male) with a mean onset age of 9.2 ± 3.0 years. There were 144 cases (81.9%) with peripheral arthritis, and the hip, knee, and ankle joints were the most commonly involved joints. Enthesitis occurred in 48 cases (30.0%), and sacroiliitis occurred in 142 cases (88.5%) at diagnosis. Human leukocyte antigen (HLA)-B27 was positive in 33 cases (17.1%), and acute uveitis occurred in 3 cases (1.9%). The majority of patients (93.7%) were treated with disease-modifying anti-rheumatic drugs (DMARDs), and 60% with biologics. Among 62 patients with MRI-defined sacroiliitis, 27 (43.5%) cases showed improvement in the sacroiliac joint lesion after treatment. Multivariate logistic regression analysis showed that duration from onset to diagnosis of less than 3 months (OR = 3.609, 95% CI: 1.068–12.192) and active joints of more than 4 (OR = 4.916, 95% CI: 1.006–24.037) were independent factors. Conclusion We highlighted differences in ERA clinical characteristics. Patients with a shorter diagnosis time and more joint involvement improved more significantly in sacroiliac joint lesions after treatment.

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