The small intestine and the large intestine are very different, both in structure and in function. The small intestine absorbs small nutrient molecules from the liquid stool, whereas the function of the large bowel is to absorb water and to store the stool until evacuation occurs. The small bowel is long and narrow; the large bowel is short and wide. Stool passes very quickly through the small bowel, transiting its 6 meter length in 2 or 3 hours. Stool passes slowly through the large bowel, taking an average of 36 hours to pass through its 2 meters. These differences in structure and function are reflected in the different incidence of cancer in the small and large intestine. Small bowel cancer is very rare as cancer-causing chemicals (carcinogens) in the stool don't have the opportunity to cause any changes in the cells lining the bowel. In the colon and rectum, however, stool sits there and carcinogens have plenty of time to have an effect. Colon and rectal cancer is the third most common cancer in the United States.
When we make a pouch, we are changing the structure and function of the small intestine to make it work like a colon. It now stores stool and absorbs water. It comes as no surprise therefore to learn that as an ileal pouch gets older, it starts to look like a colon. Under the microscope, the lining of the colon is flat. The small intestine normally has finger-like projections called villi that help with nutrient absorption. There are no villi in the colon. As pouches get older, researchers have found that they tend to lose their villi. The colon has a lot of mucus-producing cells (goblet cells), much more than the small bowel. Older pouches have increasing numbers of goblet cells. Older pouches also lose the look of small bowel and start to look like rectums. These changes usually take several years to start becoming apparent. What does this mean for the patient's state of health?
Our concern as physicians who care for patients with pouches is whether this tendency of small bowel pouches to become like large bowel will mean that there is a risk of cancer or "colitis" developing in the pouch. Indeed some preliminary reports suggest that in a few patients, pre-cancerous changes can be found in the cells of a pouch. Polyposis patients are certainly prone to get polyps in their pouch but nobody has reported a cancer yet. As far as colitis is concerned, we already know about the syndrome of "pouchitis" that mimics colitis and can occur very early after pouch construction. This is different from a true return of colitis. There is as yet no sign of a return of true "colitis" in older pouches.
Because of the theoretical risk of "colon cancer" developing in an ileal pouch, we recommend that patients come for yearly pouch checks. At this time biopsies will be taken to look for "dysplasia," an appearance of the cells lining the pouch that suggests that cancer may possibly develop. As time goes by and the number of patients with maturing pouches increases, the natural history of the elderly pouch will become more obvious. Stay tuned for more information.
