Relationship between site of disease and familial occurrence in Crohn's disease.

Concordance in the extent of disease among the family members of patients with Crohn's disease has not been widely investigated. Furthermore, the relationship between the site of the disease and familial occurrence has never been studied. Our aim was to evaluate the familial occurrence of Crohn's disease in the various sites. Nine hundred thirty-four patients with Crohn's disease, observed consecutively in two gastrointestinal departments, were investigated to determine first-degree familial incidence (in both Crohn's disease and ulcerative colitis). Whenever two or more members were attending the same clinic, only one was regarded as a propositus. The analysis, therefore, was carried out on 882 patients. The exact site of the disease was determined in all patients either at diagnosis or during the follow-up by colonoscopy and by small bowel enema. The rate of concordance in the extent of disease and familial occurrence in the various sites was evaluated and the difference was calculated by chi-square test. Sixty-one propositi were identified among all the patients. Forty-nine had familial occurrence for the same disease (concordant patients), whereas 12 had at least one relative with ulcerative colitis (discordant patients). In 44 propositi with only one relative affected, the rates of concordance in the extent of the disease were 84, 68, 18, and 0% respectively, for the ileum, the ileum-right colon, the ileum-total colon, and the colon. The number of propositi in the various sites was as follows: 4 of 162 (2.4%) patients with the disease located in the colon, 1 of 9 (11%) with the jejunum site, 24 of 380 (6.3%) with the ileum site, 16 of 165 (9.7%) with the ileum and right colon site, and 16 of 164 (9.7%) with the ileum and total colon site. The chi-square values of propositi distribution among other sites and the colon was, respectively, as follows: jejunum, 2.2 (N.S.); ileum, 3.4 (P = 0.06); ileum and right colon, 7.4 (P = 0.006); and ileum and total colon, 7.4 (P = 0.006). This study shows a pronounced concordance in the site of the disease for family members with Crohn's disease and suggests that familial occurrence in Crohn's disease is less frequent when the disease is located in the colon rather than elsewhere.