Respiratory Function and S-index in Radiographic Axial Spondyloarthritis

Radiographic axial spondyloarthritis (r-axSpA) is a systemic, chronic inflammatory rheumatic disease strongly associated with HLA-B27, primarily affecting the sacroiliac joints and the axial skeleton. Its prevalence in the general population ranges between approximately 0.1% and 1.4%. In patients with r-axSpA, involvement of the costovertebral and costosternal joints, as well as the entheses in the thoracic region, may lead to progressive ankylosis over time. This process results in reduced thoracic mobility, decreased chest expansion in the early stages, and a predominantly restrictive pattern of respiratory dysfunction. Previous studies have investigated the relationships between chest expansion, aerobic capacity, and spirometric parameters with disease activity, functional status, and spinal mobility, as assessed by the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), Bath Ankylosing Spondylitis Functional Index (BASFI), and Bath Ankylosing Spondylitis Metrology Index (BASMI), in individuals with r-axSpA. Maximum inspiratory pressure (MIP) is widely used for the static assessment of inspiratory muscle strength and has been evaluated in conjunction with chest expansion and spirometric parameters in several studies. In contrast, the S-index is a relatively novel parameter that reflects inspiratory muscle strength dynamically and has not yet been investigated in patients with r-axSpA. Although previous studies have demonstrated moderate to strong correlations between S-index and MIP, these measures represent different physiological aspects of respiratory muscle function and therefore do not yield identical results. The present study aims to compare chest expansion, spirometric parameters, and S-index values between individuals with r-axSpA and healthy controls. In addition, within the r-axSpA group, the relationships between these parameters and clinical indices (BASDAI, BASFI, and BASMI) will be examined. Furthermore, the discriminative ability of chest expansion, spirometric parameters, and the S-index in distinguishing individuals with r-axSpA from healthy controls will be evaluated. As a distinctive aspect of this study, S-index values will be analyzed both in comparison between groups and in relation to spirometric parameters, chest expansion, and clinical indices in patients with r-axSpA. This study is expected to provide novel evidence regarding the potential role of the S-index in the assessment of pulmonary function in patients with r-axSpA.