Functional limitations due to axial and peripheral joint impairments in patients with ankylosing spondylitis: are focused measures more informative?

Objective: Functional limitations in ankylosing spondylitis (AS) may be due to peripheral joint or axial involvement. To determine if the Bath Ankylosing Spondylitis Functional Index (BASFI), an axial-focused measure, can detect limitations related to peripheral joint involvement equally as well as the Health Assessment Questionnaire modified for the spondyloarthropathies (HAQ-S), a peripheral arthritis-focused measure, and vice versa, we compared associations of each questionnaire with spinal and hip range of motion, peripheral arthritis, and enthesitis in patients with AS.

Methods: We examined patients every 4-6 months in this prospective longitudinal study. We used mixed linear models to analyze the associations between 10 physical examination measures and the BASFI and HAQ-S.

Results: We studied 411 patients for a median of 1.5 years (median 3 visits). In multivariate analyses, cervical rotation, chest expansion, lateral thoracolumbar flexion, hip motion, tender joint count, and tender enthesis count were equally strongly associated with the BASFI and HAQ-S. Peripheral joint swelling was more strongly associated with the HAQ-S. Individual items of the BASFI were more likely than items of the HAQ-S to be associated with unrelated physical examination measures (e.g., the association between difficulty rising from a chair and cervical rotation), which may have diminished the axial/peripheral distinction for the BASFI.

Conclusion: The BASFI and HAQ-S had similar associations with impairments in axial measures, while the HAQ-S had stronger associations with the number of swollen peripheral joints. The HAQ-S should be considered for use in studies focused on spondyloarthritis with peripheral joint involvement.