Spondyloarthritis in First-Degree Relatives and Spouses of Patients with Inflammatory Bowel Disease: A Nationwide Population-based Cohort Study from Sweden.

Background and aims: Register-based research suggests a shared pathophysiology between inflammatory bowel disease [IBD] and spondyloarthritis [SpA], but the role of familial [genetic and environmental] factors in this shared susceptibility is largely unknown. We aimed to compare the risk of SpA in first-degree relatives [FDRs] and spouses of IBD patients with FDRs and spouses of matched, population-based, reference individuals.

Methods: We identified 147 080 FDRs and 25 945 spouses of patients with incident IBD [N = 39 203] during 2006-2016, and 1 453 429 FDRs and 258 098 spouses of matched reference individuals [N = 390 490], by linking nationwide Swedish registers and gastrointestinal biopsy data. Study participants were followed 1987-2017. Cox regression was used to estimate hazard ratios [HRs] of SpA.

Results: During follow-up, 2430 FDRs of IBD patients [6.5/10 000 person-years] and 17 761 FDRs of reference individuals [4.8/10 000 person-years] were diagnosed with SpA, corresponding to an HR of 1.35 [95% CI:1.29, 1.41]. In subgroup analyses, the increased risk of SpA was most pronounced in FDRs of Crohn's disease patients [HR = 1.44; 95% CI:1.34,1.5 6] and of IBD patients aged <18 years at diagnosis [HR = 1.46; 95% CI: 1.27, 1.68]. IBD patients' spouses also had a higher SpA rate than reference individuals' spouses, but the difference was less pronounced [4.3 vs 3.5/10 000 person-years; HR = 1.22; 95% CI:1.09, 1.37]. No subgroup-specific risk pattern was identified among spouses.

Conclusions: The observed shared familial risks between IBD and SpA support shared genetic factors in their pathogenesis. However, spouses of IBD patients were also at increased risk for SpA, reflecting the influence of environmental exposures or similarities in health-seeking patterns.