We followed a population-based cohort over a period of 10 years, including incident cases of both exposure and outcome and found a bidirectional relationship between pain and mental illness. Clinicians need to pay attention on both conditions, in patients seeking care due to mental illness or pain.

Background Widespread pain is a common comorbidity in several chronic diseases and is suspected to be caused by pain resulting from the underlying disease that has provoked a state of central sensitization. However, this argument is currently limited by evidence that has insufficiently captured the temporal nature of the relationship between diagnosis of the underlying disease and onset of widespread pain. The aim of this study was to investigate if patients with rheumatoid arthritis (RA), endometriosis or inflammatory bowel disease (IBD), have a higher risk of developing widespread pain (fibromyalgia or chronic widespread pain [CWP]). Methods Using the Swedish Skåne Healthcare register on health care consultation, a cohort of 889,938 adult patients were followed from 2007 to 2016 and incident cases of RA, endometriosis or IBD and of fibromyalgia and CWP were identified by registered diagnoses. Using Poisson regression, we calculated incidence rate ratios (IRR) adjusted for sex, age, education and propensity to seek health care. Results For patients with RA the IRR for later fibromyalgia was 3.64 (95% CI: 2.75–4.81) compared to patients without RA, for CWP it was 2.96 (95% CI: 1.81–4.86). For endometriosis patients the IRR for fibromyalgia was 2.83 (95% CI: 1.96–4.08) and for CWP 5.02 (95% CI: 3.10–8.13). IBD patients had an IRR = 2.32 (95% CI: 1.58–3.42) for fibromyalgia and 1.42 (95% CI: 0.93–2.17) for CWP. Conclusions This study shows that RA, endometriosis and IBD are all risk factors for later fibromyalgia and CWP, consistent with a hypothesis of central sensitization as an effect of a painful underlying condition. Significance We show that RA, endometriosis and IBD predisposes for later fibromyalgia and CWP, a common hypothesis previously difficult to verify due to lack of longitudinal data. The results inform further research regarding the aetiology of fibromyalgia and CWP and stress the need of clinical focus on the pain itself in chronic diseases with pain as a symptom.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.