Risk of Rheumatoid Arthritis in Patients with Endometriosis: A Nationwide Population-Based Cohort Study.

Abnormalities in the immune system of endometriosis has been demonstrated and may reflect the chronic inflammatory response or the autoimmune reaction to the presence of ectopic endometrial tissue. Rheumatoid arthritis (RA) is a chronic inflammatory joint disease of an autoimmune nature. The study aimed to investigate the risk of incident RA in patients with endometriosis. A total of 17,913 patients with endometriosis and 17,913 unaffected controls matched by age, index year, and Charlson Comorbidity Index (CCI) score were enrolled between 2000 and 2012. Patients were followed until the end of 2013 using Taiwan's National Health Insurance Research Database, at which time participants who developed RA were identified. Cox regression analysis was used to calculate the hazard ratio (HR) with a 95% confidence interval (CI) of RA incidence rate between patients with endometriosis and unaffected controls. Patients with endometriosis were associated with an increased risk of incident RA compared with unaffected controls after adjusting for age, CCI score, and hormonal and surgical treatments (3.56 vs. 1.30 per 10,000 person-years, HR: 3.71, 95% CI: 2.91-5.73). Among these adjusted variables, hormonal and surgical treatments were treated as time-dependent covariates. Stratification analyses also revealed similar risk associations linking endometriosis to subsequent RA in all stratified age and CCI score subgroups (adjusted HR all >1, although not all were significant) Patients with endometriosis was associated with an increased risk of incident RA. Additional prospective studies that take into account genetic vulnerability and environmental exposures are warranted to confirm this relationship.