Background & aims: In 1988, we reported the first twin study in inflammatory bowel disease. The aim of the current study was to follow up these twins regarding new cases of inflammatory bowel disease and Crohn's disease characteristics using the Vienna classification.
Methods: The official Swedish population register and the cause of death register were used to search for the twins. All living patients were interviewed.
Results: Three monozygotic twins earlier classified as healthy had been diagnosed with inflammatory bowel disease (ulcerative colitis, n = 2; Crohn's disease, n = 1). Retrospectively, all 3 were symptomatic at the original survey. This changed the pair concordance in monozygotic twins from 6.3% to 18.8% in ulcerative colitis and from 44.4% to 50.0% in Crohn's disease. A high degree of concordance regarding age at diagnosis, disease location at diagnosis and during the course, and disease behavior was found in concordant monozygotic twin pairs with Crohn's disease. Seven of 9 pairs were identical in 3 or more of these disease characteristics compared with an expected number of 1.5 (P = 0.000076).
Conclusions: This study confirms that the genetic influence is stronger in Crohn's disease than in ulcerative colitis. A remarkable phenotype similarity within concordant pairs with Crohn's disease was found using the Vienna classification.