Objectives To investigate the frequency of psychiatric disorders before and after onset of paediatric-onset immune-mediated inflammatory diseases (pIMID).

Study design In a nationwide study from 1996 to 2018, we investigated psychiatric disorders in patients with paediatric-onset inflammatory bowel diseases, autoimmune liver diseases and rheumatic diseases, using Danish national healthcare and population registers. Each case was matched with up to 10 controls from the background population. The cumulative incidence for psychiatric disorders prior to pIMID onset in patients was compared with controls. Cox proportional regression was used to estimate adjusted HRs (aHR) with a 95% CI between cases and controls after the index date.

Results We included 11 208 cases (57% female) and 98 387 controls. The median age at disease onset was 12.5 years (IQR 8–15) and follow-up time 9.8 years (IQR 5–15). We found an association between psychiatric disorders before index date and a diagnosis of subsequent pIMID (OR 1.3, 95% CI 1.2 to 1.4). Notably, after index date, cases also had an increased risk (aHR 1.6, 95% CI 1.5 to 1.7) of psychiatric disorders compared with controls. This risk was increased for all groups of psychiatric disorders. Female patients had an increased risk of suicide attempt after index date (aHR 1.4, 95% CI 1.1 to 1.8).

Conclusion Patients with pIMID are at increased risk for a broad spectrum of psychiatric disorders both before and after onset of pIMID. The results support the need for awareness of psychiatric morbidity in this young patient group and the need for coordinated healthcare for those with comorbid states.