The relation of physical comorbidity and multimorbidity to fibromyalgia, widespread pain and fibromyalgia-related variables

Objective To investigate the relation of physical (non-psychological) comorbidity and multimorbidity to quantitative measures of fibromyalgia and musculoskeletal pain. Methods We studied 12, 215 patients in a research databank with quantitative measures of fibromyalgia related variables (FMV) that included binary determinations of fibromyalgia and widespread pain, and constituent variables of fibromyalgia diagnosis that included the widespread pain index (WPI), symptom severity score (SSS) and the polysymptomatic distress scale (PSD). We assessed 10 self-reported comorbid conditions, and covariates that included age, sex, body mass index, hypertension, smoking history and total household income. We used nearest neighbor matching and regression adjustment treatment effects models to measure the effect of comorbidities on FMV. Results We found a positive association between FMV and the probability of having each comorbid condition. Patients with ≥1 comorbidities had PSD, WPI and SSS increases of 3.0 (95% CI 2.7-3.3), 1.8 (95% CI 1.6-2.0) and 1.2 (95% CI 1.1-1.3) units, and an increase in FM prevalence from 20.4% to 32.6%. As the number of comorbid conditions present increased from 1 to 4 or more, PSD, WPI, SSS and fibromyalgia percent increased stepwise. For patients with ≥4 conditions, the predicted prevalence of fibromyalgia was 55.2%. Conclusion Fibromyalgia and FMV are associated with increase in the number of comorbidities, and the association can be measured quantitatively. However, the association with widespread pain and fibromyalgia may be an effect of man-made definitions of widespread pain and fibromyalgia, as comorbidity increases are also present with sub-syndromal levels of widespread pain and fibromyalgia