Fibromyalgia in patients with ankylosing spondylitis: prevalence and utility of the measures of activity, function and radiological damage.

Objectives: To determine the prevalence of fibromyalgia (FM) in ankylosing spondylitis (AS). To evaluate the effect of FM on the measures of activity in AS. To analyse predictive factors in order to identify this group of patients.
Patients and Methods: A cross-sectional study based on 462 patients with definite ankylosing spondylitis included in the REGISPONSER. Sociodemographic data, clinical features, Bath AS disease activity index (BASDAI), Bath AS functional index (BASFI), Bath AS radiology index (BASRI), Stoke modified index (Sasss-m), laboratory data, Short-Format 12 (SF-12), AS specific quality of life (ASQoL), Fibromyalgia Impact Questionnaire (FIQ) and treatments used were all documented. To diagnose FM, the ACR 1990 criteria had to be fulfilled. All statistical tests were performed using STATA.
Results: The prevalence of fibromyalgia in all AS was 4.11%. Among the women with AS, the prevalence of FM increased to 10.83%. The BASDAI, BASFI and total BASRI were strongly influenced by the presence of FM. The inverse relationship between BASDAI or BASFI and total BASRI was taken to generate a ratio. Accordingly, if the patient presented BASDAI/BASRI ≥1.5 or BASFI/BASRI ≥1.08, the probability of having FM was very high.
Conclusions: There is an increased risk of FM in females with AS. The fact of having FM distorts the measures of activity and functional damage of AS. As a result, it is possible that some patients with AS and FM are being overtreated. The BASDAI/BASRI and BASFI/BASRI ratios are very useful to identify these patients.