The aim of this study was to investigate whether peripheral arthritis together with disease activity independently contribute to functional impairment over time in patients with axSpA and to evaluate if there are contextual factors modifying this relationship.
Patients with axial spondyloarthritis from the ESPAXIA cohort were followed-up annually over a mean of 3.7 years. Physical function was assessed by the self-reported questionnaire BASFI, disease activity by ASDAS and peripheral arthritis was also recorded. Generalized estimating equations (GEE) were used to investigate longitudinal association between peripheral arthritis, ASDAS and BASFI as the outcome. Autoregressive models (adjusted for BASFI 1 year earlier) were run to allow for a truly longitudinal interpretation. Interactions between each of ASDAS and peripheral arthritis with contextual factors (age, gender, educational level, smoking, job type) were tested.
185 patients (77 % male, mean (SD) age 42 (13) years old and mean disease duration (SD) of 9.4 (9.6) years) were included. ASDAS and peripheral arthritis independently contributed to explaning BASFI over time. Contextual factors did not modify either of the relationships. A true longitudinal relation was proven with the autoregressive GEE model, showing that, adjusted for age, gender, spinal mobility and use of NSAIDs, an increase of one ASDAS unit led to a BASFI 0.48 units higher (ß 0.48 [95%CI 0.39, 0.57]), and the presence of peripheral arthritis, to a BASFI 0.44 units higher (ß 0.44 [95%CI 0.08, 0.8]).
Peripheral arthritis and higher disease activity independently lead to more functional impairment in axSpA over time. Contextual factors do not modify these relationships.