Functional relevance of radiographic spinal progression in axial spondyloarthritis: results from the GErman SPondyloarthritis Inception Cohort

Objectives: The aim of the study was to investigate the functional relevance of the development of structural damage in the spine in patients with early axial spondyloarthritis (axSpA). Methods: Altogether, 210 patients with early axSpA (symptom duration ⩽10 years) who completed a 2-year clinical and radiographic follow-up in the GErman SPondyloarthritis Inception Cohort were included. An association between structural damage in the spine [modified Stoke AS Spine Score (mSASSS)] and functional status (the BASFI) or spinal mobility (the BASMI) was assessed in a longitudinal linear mixed model analysis; both unstandardized (β) and standardized (βstand) regression coefficients were calculated. Results: There was an association between mSASSS and BASFI: β = 0.05 (95% CI: 0.03, 0.08) and βstand = 0.20 (95% CI: 0.11, 0.59) adjusted for disease activity parameters (the BASDAI and CRP), the presence of definite radiographic sacroiliitis and sex. An association between mSASSS and BASMI was stronger: β = 0.08 (95% CI: 0.05, 0.11) and βstand = 0.41 (95% CI: 0.25, 0.57) adjusted for the same parameters. These data indicate that, over time, an increase of 20 or 12 mSASSS points would be responsible for an increase of one BASFI or one BASMI point, respectively. Disease activity (BASDAI) also showed a significant association with BASFI [β = 0.79 (95% CI: 0.71, 0.86) and βstand = 0.71 (95% CI: 0.63, 0.77)] and BASMI [β = 0.22 (95% CI: 0.15, 0.30) and βstand = 0.28 (95% CI: 0.18, 0.37)]. Conclusion: Structural damage in the spine and disease activity are both determinants of the functional status and spinal mobility in early axSpA.