Fatigue in Middle-Aged and Older Adults with Axial Spondyloarthritis: A Sex-Stratified Case-Control Study

Background: Fatigue is a common and disabling symptom in axial spondyloarthritis (axSpA), yet its magnitude relative to the general population and potential sex-specific differences remain insufficiently characterized, particularly in older adults. We therefore aimed to assess fatigue in adults aged ≥ 50 years with axSpA, using the Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-F) scale, to compare fatigue levels with age- and sex-matched controls, and to explore sex-specific differences and clinical factors associated with fatigue. Methods: We conducted an observational case-control study including consecutive patients with axSpA aged ≥ 50 years and control subjects frequency-matched by age and sex. Fatigue was assessed using the FACIT-F, and clinically relevant fatigue was defined as a FACIT-F score < 40. Case-control comparisons were stratified by sex, and sex-stratified multivariable linear regression models were applied. Results: The study included 173 patients with axSpA (120 men, 53 women; mean age: 64.2 years) and 383 controls. Clinically relevant fatigue was more frequent in women than in men (84.9% vs. 50.0%; p < 0.001). Women reported more severe fatigue than men (FACIT-F: 29.4 ± 10.4 vs. 37.4 ± 10.2; p < 0.001). In case-control comparisons, fatigue was greater in patients than in controls in both sexes, with descriptively larger differences among women. In sex-stratified multivariable analyses, the ASAS Health Index (ASAS-HI) was independently associated with fatigue in both men and women. In reduced models including age, BASDAI, and ASAS-HI, ASAS-HI remained independently associated with FACIT-F in both men (β: -1.74, 95% CI: -2.08 to -1.41) and women (β: -1.80, 95% CI: -2.35 to -1.26; p < 0.001 for both). BASDAI showed an additional independent association in women (β: -1.19, 95%: CI -2.09 to -0.30; p = 0.010), but not in men. Conclusions: Fatigue is highly prevalent and clinically relevant in adults aged ≥50 years with axSpA, with a clear sex-specific pattern. Women experience a greater fatigue burden, and comparisons with controls suggest a larger excess among women. Fatigue represents an important dimension of disease burden in axSpA, with stronger associations with overall health status than with conventional inflammatory measures.