IBD is short for inflammatory bowel disease. It is a long-term (chronic) condition that involves inflammation of the intestines. As many as 80% of people with PSC also have IBD in Northern Europe 1,52. Current thinking suggests that if you have PSC but no marked symptoms of IBD, microscopic inflammation in the bowel may still exist 14.
This link between PSC and IBD has interested researchers (and patients) for a long time but, until now, the genetic relationship between the two diseases has not been fully understood. There have been clues that the IBD a PSC patient has (called PSC-IBD), is not the same as IBD in the general population 11,14,53,54.
In a landmark study 15, Professor Tom Karlsen and the International PSC Study Group (IPSCSG) studied DNA in people with PSC and in people with IBD. They found many regions of the genome associated with both PSC and IBD risk, but they also found others that were only associated with risk of PSC. This is critical, because it shows that there are unique genetic aspects to PSC, and the IBD seen in people with PSC cannot be explained by shared genetic risk. Simply put: PSC is not necessarily a complication of IBD; PSC is likely a distinct, separate condition.
More research is needed to understand the genetic and environmental factors involved in the development of PSC-IBD 14.
Generally speaking, IBD is still described as ulcerative colitis (UC) or Crohn's disease (CD) even though the IBD associated with PSC thought to be a different kind of IBD altogether (PSC-IBD) with its own distinct features. It is thought that:
- 85% of PSC patients with IBD have ulcerative colitis
- 2–14% have Crohn's disease
- the remainder have ‘colonic IBD unclassified/indeterminate colitis’ 55, which lends weight to the suggestion that it should be termed ‘PSC-IBD’.
Ulcerative colitis (UC)
UC in people with PSC appears to be distinct from those with UC alone 55. Characteristic features include a mild course of UC with:
- infrequent relapse (less flare ups) 55.
- colitis throughout the colon with no rectal bleeding (rectal sparing) (52% of PSC/UCvs. 6% of UC alone) 55.
- inflamed terminal ileum (backwash ileitis) (51% of PSC/UC vs. 7% of UC alone) 55
- an increased prevalence of pouchitis (inflammation of the pouch created during surgery) following colectomy and ileo-anal pouch formation 56.
- more inflammation on the right side of the colon 1.
Even though their UC is usually mild, people with PSC and UC are at a greater risk of developing colon cancer than those with UC alone 57. This is why people with PSC-IBD need an annual colonoscopy to look for pre-cancerous changes, allowing doctors to take action before a cancer develops.
Crohn’s disease (CD)
People with PSC and CD also seem to have a unique disease phenotype (presentation). Researchers have found more cases of small duct PSC in people with PSC and CD 55 than in PSC/UC. PSC with CD is associated with a lower risk of PSC progression and a lower risk of developing biliary tract cancer 7 and bowel cancer 55 than patients with ulcerative colitis.