• Morten Frisch, Bo V Pedersen, and Roland E Andersson.
• Department of Epidemiology Research, Division of Epidemiology, Statens Serum Institut, DK-2300 Copenhagen S, Denmark. mfr@ssi.dk
• BMJ. 2009 Jan 1; 338: b716.

ObjectiveTo determine whether the repeatedly observed low risk of ulcerative colitis after appendicectomy is related to the appendicectomy itself or the underlying morbidity, notably appendicitis or mesenteric lymphadenitis.DesignNationwide cohort studies.SettingSweden and Denmark.Participants709 353 Swedish (1964-2004) and Danish (1977-2004) patients who had undergone appendicectomy were followed up for subsequent ulcerative colitis. The impact of appendicectomy on risk was also studied in 224 483 people whose parents or siblings had inflammatory bowel disease.Main Outcome MeasuresStandardised incidence ratios and rate ratios as measures of relative risk.ResultsDuring 11.1 million years of follow-up in the appendicectomy cohort, 1192 patients developed ulcerative colitis (10.8 per 100 000 person years). Appendicectomy without underlying inflammation was not associated with reduced risk (standardised incidence ratio 1.04, 95% confidence interval 0.95 to 1.15). Before the age of 20, however, appendicectomy for appendicitis (0.45, 0.39 to 0.53) or mesenteric lymphadenitis (0.65, 0.46 to 0.90) was associated with significant risk reduction. A similar pattern was seen in those with affected relatives, whose overall risk of ulcerative colitis was clearly higher than the background risk (1404 observed v 446 expected; standardised incidence ratio 3.15, 2.99 to 3.32). In this cohort, appendicectomy without underlying appendicitis did not modify risk (rate ratio 1.04, 0.66 to 1.55, v no appendicectomy), while risk after appendicectomy for appendicitis was halved (0.49, 0.31 to 0.74).ConclusionsIn individuals with or without a familial predisposition to inflammatory bowel disease, appendicitis and mesenteric lymphadenitis during childhood or adolescence are linked to a significantly reduced risk of ulcerative colitis in adulthood. Appendicectomy itself does not protect against ulcerative colitis.

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