Long-Term Risk of Myocarditis in Patients With Inflammatory Bowel Disease: A Nationwide Cohort Study in Sweden

INTRODUCTION: Despite a suggested link between inflammatory bowel disease (IBD) and myocarditis, the association has not been well established. This study aimed to investigate the long-term risk of myocarditis in patients with IBD. METHODS: This nationwide cohort involved all patients with biopsy-confirmed IBD in Sweden (1969–2017) (n = 83,264, Crohn's disease [CD, n = 24,738], ulcerative colitis [UC, n = 46,409], and IBD-unclassified [IBD-U, n = 12,117]), general population reference individuals (n = 391,344), and IBD-free full siblings (n = 96,149) and followed until 2019. Primary outcome was incident myocarditis, and secondary outcome was severe myocarditis (complicated with heart failure, death, or readmission). Flexible parametric survival models were used to estimate adjusted hazard ratios (aHRs) and cumulative incidence of outcomes, along with 95% confidence intervals. RESULTS: During a median follow-up of 12 years, there were 256 myocarditis cases in patients with IBD (incidence rate [IR] = 22.6/100,000 person-years) and 710 in reference individuals (IR = 12.9), with an aHR of 1.55 (95% confidence interval 1.33–1.81). The increased risk persisted through 20 years after IBD diagnosis, corresponding to 1 extra myocarditis case in 735 patients with IBD until then. This increased risk was observed in CD (aHR = 1.48 [1.11–1.97]) and UC (aHR = 1.58 [1.30–1.93]). IBD was also associated with severe myocarditis (IR: 10.1 vs 3.5; aHR = 2.44 [1.89–3.15]), irrespective of IBD subtypes (CD: aHR = 2.39 [1.43–4.01], UC: aHR = 2.82 [1.99–4.00], and IBD-U: aHR = 3.14 [1.55–6.33]). Sibling comparison analyses yielded similar results. DISCUSSION: Patients with IBD had an increased risk of myocarditis, especially severe myocarditis, for ≥20 years after diagnosis, but absolute risks were low.