Does appendectomy reduce the risk of ulcerative colitis?

Persons who have had an appendectomy are less prone to develop ulcerative colitis than those who have not. The underlying mechanism and the clinical use of this finding are unknown.

Two competing hypotheses were examined: (1) the inverse association between appendectomy and ulcerative colitis is conferred by a mutual determinant such as genes, and (2) the appendix itself has biologic effects that promote the development of ulcerative colitis. We performed a population-based cohort study of all 234,559 persons who had an appendectomy performed in Denmark 1977 through 1999. We used a bidirectional cohort design, calculating the standardized incidence rate of ulcerative colitis both before and after the appendectomy. If the hypothesis of a constant, confounding factor were true, incidence rates of ulcerative colitis would be equal before and after appendectomy. If the incidence of ulcerative colitis were lower after appendectomy than before, it would support the second hypothesis and a true protective effect of the appendectomy. We controlled confounding by age and sex using direct standardization.

Of 234,559 persons who had an appendectomy, 559 developed ulcerative colitis during a mean follow up of 17.5 years. The standardized incidence rate of ulcerative colitis was lower in the postappendectomy period than in the preappendectomy period (incidence rate ratio = 0.74; 95% confidence interval = 0.62-0.88).

The inverse association between appendectomy and ulcerative colitis is not explained by time-stable factors such as genes. Appendectomy might have a genuine protective effect toward development of ulcerative colitis.