Background Generalized pain in the form of chronic widespread pain (CWP) and fibromyalgia is common in the general population  and is also a frequent comorbidity in several diseases. Chronic pain from structural pathology (e.g. inflammation or injury) has been proposed as a mechanism behind generalization of pain, likely through the mechanism of central sensitization .
Objectives The aim was to study if patients with rheumatoid arthritis (RA), endometriosis or inflammatory bowel disease (IBD), diseases with a known component of pain from structural pathology and with diverse etiology, have an increased risk of later developing generalized pain.
Methods This prospective cohort study used the Skåne Healthcare Register, covering routinely collected data on all types of care in the region of Skåne, southern Sweden (population approx. 1.3 million). The cohort included all patients in specialized and primary care between 2004 and 2014. The primary outcome, later generalized pain, was defined as a diagnosis of pain, unspecified (ICD-10 R52.9) or fibromyalgia (ICD-10 M79.7) which follows the stricter ACR-10 criteria. We used Poisson regression to calculate the incidence rate ratio (IRR), we adjusted for sex and age at study inclusion.
Results During the study period 696,539 eligible individuals aged 18 or older living in the Skåne region and consulting health care at least once during the period were included in the study. After excluding patients with prevalent pain (n=1264) 695,275 individuals remained. Generalized pain was more common in women than in men (71% female). Out of 10,165 patients exposed to RA 5% later developed generalized pain, for endometriosis and IBD the corresponding percentage was 5% and 3% respectively. Among the 673,040 unexposed participants 2% developed generalized pain during the study period.
The IRR for later generalized pain in patients with RA was 2.5 (95% CI: 2.3 - 2.7) compared to the unexposed, for endometriosis patients the IRR was 2.4 (95% CI: 2.0 - 2.7) and for IBD-patients 1.7 (95% CI: 1.5 - 1.9) when adjusted for age and sex. Restricting the outcome to only the patients with fibromyalgia, rendered equal or higher IRR for all studied diseases.
Conclusions This study shows that pain from structural pathology, caused by diverse etiological factors, may predispose for later generalized pain. The understudied disease of endometriosis is a risk factor of almost equal magnitude as RA, which has, to the best of our knowledge not been shown before. These results have implications for both clinical practice, underlining the importance of optimizing pain control, and further research regarding the etiology of generalized pain and fibromyalgia.
Mansfield KE et al. A systematic review and meta-analysis of the prevalence of chronic widespread pain in the general population. Pain 2015.
Yunus MB. The prevalence of fibromyalgia in other chronic pain conditions. Pain Res. Treat. 2012.
Disclosure of Interest None declared
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