Background  Ankylosing spondylitis (AS) is a chronic inflammatory disease that primarily involves the axial bones. Enthesitis, which is the most important pathological feature of AS, distinguishes it from other inflammatory joint diseases such as rheumatoid arthritis. However, there are few clinical studies on AS patients with enthesitis in China.

  Objective  To explore the significance of enthesitis in evaluating the condition and prognosis of AS patients by comparing the clinical manifestations of AS patients with or without enthesitis.

  Methods  A total of 1 083 AS patients with complete assessment of enthesitis were recruited from Smart phone Spondyloarthritis management system (SpAMS) from April 13, 2016 to January 15, 2019 in the Department of Rheumatology and Immunology of the First Medical Center of Chinese PLA General Hospital. The differences in demographic characteristics, clinical manifestations, laboratory data and disease severity between patients with or without enthesitis were analyzed.

  Results  Of the 1 083 patients with AS, there were 890 males and 193 females with the average age of 29.0 (24.0, 35.0) years. There were 304 patients with enthesitis and 779 patients without enthesitis. The age of disease onset was younger in patients with enthesitis than patients without enthesitis (Md IQR: 20.0 17.0, 25.0 years vs 21.0 17.0, 27.0 years, P=0.022). Compared with patients without enthesitis, patients with enthesitis had more frequent involvement of anterior chest wall (52.0% 156/300 vs 43.7% 330/755, P= 0.015) and heel (38.3% 115/300 vs 31.1% 235/755, P=0.025) during the course of disease. Peripheral arthritis (22.7% 69/304 vs 12.2% 95/779, P ＜ 0.001) were more common in patients with enthesitis. AS patients with enthesitis had higher degree of general back pain (MdIQR: 3.0 2.0, 4.0 vs 2.0 1.0, 4.0, P ＜ 0.001), dorsalgia at night (MdIQR: 2.0 1.0, 4.0vs 2.0 1.0, 4.0,P=0.001), limitation of thoracic dilatation (21.4% 65/304 vs 14.9% 116/779, P=0.010), Patient Global Assessment (MdIQR: 3.0 2.0, 5.0 vs 2.0 1.0, 4.0, P ＜ 0.001), Physician Global Assessment (MdIQR: 2.0 2.0, 3.0 vs 2.0 1.0, 3.0, P=0.024), Bath Ankylosing Spondylitis Metrology Index (MdIQR: 1.0 0.0, 3.0 vs 1.0 0.0, 3.0, P=0.049, Bath Ankylosing Spondylitis Disease Activity Index (MdIQR: 2.7 1.5, 4.0 vs 1.9 1.0, 3.1, P＜0.001), Bath Ankylosing Spondylitis Functional Index (MdIQR: 1.5 0.4, 2.9 vs 1.2 0.2, 2.4, P=0.008) and the Assessment of Spondyloarthritis international Society Health Index (MdIQR: 6.0 3.3, 9.0 vs 5.0 2.0, 8.0, P ＜ 0.001) than those without enthesitis. Logistic regression analysis showed that history of anterior chest pain (OR=1.428, 95% CI: 1.084-1.881, P=0.011) and peripheral arthritis (OR=2.166, 95% CI: 1.521-3.086, P ＜ 0.001) were risk factors for the enthesitis.

  Conclusion  Compared with patients without enthesitis, AS patients combined with enthesitis have a younger age of onset and a greater risk for occurrence of peripheral arthritis. AS patients with enthesitis have more severe disease activity and body disfunction, so clinicians should pay more attention to it.