Background
Inflammatory bowel diseases (IBD) are heterogenous diseases in terms of their clinical presentation including frequency and severity of flares.

We aimed to describe disease activity (DA) patterns in a Danish nationwide cohort including all IBD patients diagnosed < 18 years of age between 1996 and 2018.

Methods
Paediatric patients diagnosed with Crohn’s disease (pCD) or ulcerative colitis (pUC) were identified from national registers (Danish Civil Registration System, National Patient Registry, Danish National Prescription Registry and Danish Pathology Data Bank). DA was assessed in both incident and prevalent pIBD patients and categorised as mild-moderate in case of a course of topical corticosteroids, oral budesonide, or topical 5-ASA. Episodes were categorised as severe if they involved hospitalization, surgery, oral corticosteroids or biological therapy.

DA patterns in incident patients were assessed for 5- and 10-year periods from diagnosis as follows:

The 5-year DA pattern: A) No years with DA. B) 1 or 2 years with at least one episode of DA in each year. C) 3 or 4 years with at least one episode of DA in each year. D) All 5 years with at least one yearly episode of DA.

The 10-year DA pattern: A) Decreasing: First 5 years with 2 or more years with DA; following 5 years with 2 years or less with DA. B) Increasing: First 5 years with less than 2 years with DA; following 5 years with 2 or more years with DA. C) Chronic continuous: First 10 years with at least 8 years with DA. D) Quiescent: First 10 years with one year or less with DA. E) Chronic intermittent: DA pattern that does not meet the criteria above.

Results
1,965 pCD and 1,838 pUC incident patients were included. At diagnosis severe DA was found in 87% in pCD and 80% in pUC, in addition to 6.1% of pUC patients who had a colectomy the first year. After 5 years of disease the annual proportion of pCD and pUC with no DA 70% and 61%. This remained constant after 10 years (pCD:72% and pUC: 64%). In pUC, 12% and 16% required colectomy 5 and 10 years after diagnosis.

In prevalent pCD (N=2,515) and pUC (N=2,428) the proportion with no DA each calendar year was stable (pCD: 59%-73% and pUC: 50%-60%, prior to 2010). The majority of both pCD and pUC patients experienced one or two years with at least one episode of DA within the first five years (pCD: 51%, pUC: 53%). During the first 10 years from diagnosis, the decreasing DA pattern was the most common in both pCD and pUC (43% and 47%, respectively) (Figure).