Respiratory muscle strength in radiographic axial spondyloarthritis: clinical significance and predictors

Introduction/objective Respiratory muscle function may be impaired in axial spondyloarthritis (axSpA) due to mechanical limitations of the thoracic spine and chest wall. However, the extent of respiratory muscle weakness and its clinical associations remain largely underexplored. Methods This comparative cross-sectional study included 51 patients with radiographic axSpA (r-axSpA) and 52 healthy controls. Maximal inspiratory (PImax) and expiratory (PEmax) pressures were measured using standardized protocols. Patients with r-axSpA also underwent assessments of pulmonary functions (forced vital capacity (FVC), forced expiratory volume in the first second (FEV1)), spinal pain, disease activity, spinal mobility, functional status, physical performance, quality of life, anxiety/depression, and fatigue. Subgroup analyses were performed based on the presence of respiratory muscle weakness (< 80% of predicted PImax and PEmax), and stepwise multiple regression was used to identify clinical predictors of PImax. Results Although the PImax (mean difference: - 8.6% [95% CI: - 21.7% to 4.6%]) and PEmax (mean difference: - 4.6% [95% CI: - 16.7% to 7.5%]) were lower in the r-axSpA group, the differences were not statistically significant (p > 0.05). Seventeen patients (33.3% of the total sample) with r-axSpA exhibited inspiratory muscle weakness. This subgroup demonstrated significantly poorer FVC, spinal pain at rest, spinal mobility, and quality of life (p < 0.05). Regression analysis revealed that pain at rest (beta = - 0.317, p = 0.013) and FVC (beta = 0.296, p = 0.022) independently predicted 19.2% of the variance in PImax. Conclusion Although respiratory muscle strength was mostly preserved in r-axSpA, a substantial subgroup exhibited inspiratory muscle weakness associated with adverse clinical outcomes. Identifying these patients may guide tailored interventions for optimal care.