Clinical Characteristics of Patients with Spondyloarthritis in Japan in Comparison with Other Regions of the World

Objective. To delineate clinical characteristics of patients with spondyloarthritis (SpA) in Japan in comparison to other areas of the world. Methods. Using the ASAS-COMOSPA (Assessment of Spondyloarthritis international Society–COMOrbidities in SPondyloArthritis) data, an international cross-sectional observational study of patients with SpA, we analyzed information on demographics, disease characteristics, comorbidities, and risk factors. Patients were classified by region: Japan, other Asian countries (China, Singapore, South Korea, Taiwan), and non-Asian countries (Europe, the Americas, Africa). Patient characteristics, including diagnosis and treatment, were compared. Results. Among 3984 patients included in the study, 161 were from centers in Japan, 933 from other Asian countries, and 2890 from other regions. Of patients with SpA in Japan, 42 (26.1%) had peripheral SpA, substantially more than in other countries. This trend was explained by the predominance of psoriatic arthritis (PsA) among Japanese patients with SpA. In contrast to the relatively low number in Japan, 54% of patients from other Asian countries had pure axial SpA (axSpA) without peripheral features. HLA-B27 testing, considered an integral part of the classification of axSpA, was performed in only 63.6% of Japanese patients with axSpA. More than half of Japanese patients with axSpA were classified using imaging criteria. Conclusion. In our study, there was a more substantial number of peripheral SpA cases observed in Japan compared to other parts of Asia and other regions of the world. Aside from ethnic differences, increasing recognition of PsA in Japan, as well as a potential underdiagnosis of axSpA due to the insufficient use of HLA-B27 testing, may partly explain regional discrepancies.