A contribution to knowledge of the aetiology and indirect impact of inflammatory bowel diseases : (based upon analysis of routinely and semi-routinely available data)
The incidence of the idiopathic inflammatory bowel diseases ulcerative colitis and Crohn’s disease appears to have risen markedly during the 20th century. These diseases now account for a considerable proportion of the workload of gastroenterologists in the developed world, and may affect as much as 1% of the population at some point in their lives. The aetiology of these diseases has been subject of much research over a number of decades and it is clear that both genetic and environmental factors are involved. The certain knowledge of environmental risk factors however remains scant. Similarly although inflammatory bowel diseases cause considerable morbidity and a small amount of mortality for their sufferers directly there is little agreement as to their overall impact once indirect effects are accounted for. This thesis contains studies contributing to the knowledge of both these areas using routinely or semi-routinely collected data. It examines two hypotheses relating to the aetiology of IBD (that risk is related to the season of birth, and that it is related to antibiotic use), and two areas of the impact of the diseases (overall mortality and fracture risk). With regard to aetiology the studies described show no variation in the risk of IBD with season of birth. They do show an increase in risk associated with the use of antibiotics, but since this is not specific (it is seen to occur with other groups of drugs also) it is far from clear that the association is causal. With regard to the indirect impact of the diseases a significant excess in overall mortality is demonstrated which is greater in Crohn’s disease than in ulcerative colitis, and is greatest in relative terms in the young but in absolute terms in the elderly. An excess is also shown for hip fractures in those with inflammatory bowel diseases, which is only partially explained by the use of corticosteroids.