Crohn’s Disease arises in genetically susceptible individuals following an interaction with environmental factors.
As of 2013, more than 160 IBD genes have been discovered, many shared with other inflammatory conditions including ulcerative colitis, but across populations they only account for a minority (≈30%) of disease burden.
The environmental factors that may predispose to disease are largely unknown. They include smoking, family history of IBD, infectious gastroenteritis, childhood use of antibiotics, and possibly NSAID use.
Fistulating perianal disease
Crohn’s disease fistulae are related to local inflammation and/or stenosis. Crohn’s is characterised clinically by an abnormal, exuberant inflammatory response, which means that small abscesses / areas of inflammation often extend, leading to the development of a fistula. This is more likely if there is obstruction to the flow of fecal matter as can occur with anal or rectal or colonic stenosis.
Micro-abscesses in perianal glands can be an early lesion, which explains why Crohn’s disease fistula often occur adjacent to the anus.