Fibromyalgia (FM) is common among patients with ankylosing spondylitis (AS), and its coexistence is believed to interfere with the measurement of patient-reported outcomes of disease activity and function in AS because of overlapping symptoms between the 2 diseases. This can confound clinical assessment and treatment decisions.
The aim of this study was to assess the relationship between the Fibromyalgia Symptom Scale (FSS) and its components, the Widespread Pain Index (WPI), and System Severity Scale with measures of disease activity, function, and patient-reported outcomes in AS.
We recruited 63 AS patients (aged ≥18 years) meeting the modified New York criteria, and Bath Ankylosing Spondylitis Disease Activity Index, Ankylosing Spondylitis Disease Activity Score-C-Reactive Protein, Routine Assessment of Patient Index Data 3, and Bath Ankylosing Spondylitis Functional Index questionnaires were administered to them. The presence of FM was determined using validated 2010 American College of Rheumatology diagnostic criteria for FM.
Twenty-eight of 63 patients (44.4%) with AS and FM had higher disease activity and greater impairment of functional ability compared with AS patients without FM. Using multiple linear regression estimates, there was no significant relationship of FSS scores with Bath Ankylosing Spondylitis Disease Activity Index (p = 0.36), Routine Assessment of Patient Index Data 3 (p = 0.50), Ankylosing Spondylitis Disease Activity Score-C-Reactive Protein (p = 0.24), Bath Ankylosing Spondylitis Functional Index (p = 0.42) scores, or erythrocyte sedimentation rate (p = 0.82) and C-reactive protein (p = 0.75).
Despite a high prevalence of FM in our patients with the diagnosis of AS, there was no relationship between FSS and measures of disease activity or function in AS, suggesting that FSS and its components could be a useful tool to assess FM in AS patients. Also, FM impairs functional ability in patients with AS.

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