Prevalence and Prognosis of Mild Inflammatory Bowel Disease: A Population-Based Cohort Study 1997-2020.

Background And Aims: The inflammatory bowel diseases (IBD), ulcerative colitis (UC) and Crohn's disease (CD), are heterogenous diseases ranging from mild to severe. We aimed to describe the prevalence and prognosis of mild IBD in an unselected population-based patient cohort.

Methods: We identified all individuals diagnosed with IBD during 1997-2020 in North Denmark (n=4,607). Patients with mild disease, based on treatment history within the first year, were followed for progression to moderate-severe disease, based on a composite outcome of immunomodulators, biologic therapies, IBD-related hospitalization, and/or surgery. We used time-to-event analysis to calculate probabilities of progression based on IBD subtype, age, sex, and duration of mild IBD.

Results: Among 2,315 individuals with initial mild IBD, 24.5% (n=474) with UC and 46% (n=174) with CD progressed to moderate-severe disease during follow-up. Of the total IBD population, 52.3% of patients with UC and 86.8% with CD progressed during follow-up. Individuals <18 years at diagnosis were the most likely to progress. The 10-year probability of progressing to moderate-severe UC was 26% after 1 year with mild disease, 19% after 5 years, and 12% after 10 years. The probability of progressing to moderate-severe CD was 53% after 1 year with mild disease, 34% after 5 years, and 33% after 10 years.

Conclusions: Approximately one-fourth of individuals with mild UC within one year after diagnosis and half of those with mild CD progressed to moderate-severe disease over time. Young age at diagnosis increased the probability of progression, whereas increasing duration of mild disease decreased the probability.