The small intestine and the large are very different, both in structure and in function. The small intestine absorbs small nutrient molecules from the liquid stool, whereas the large absorbs water and stores the stool until evacuated. The small is long and narrow, the large is short and wide. Stool passes quickly through the small - 6 meters in 2 to 3 hours. Stool passes slowly through the large, taking about 26 hours for 2 meters. These differences in structure and function are reflected in the incidence of cancer in the small and large intestine. Small bowel cancer is very rare as the carcinogens in the stool don't have the opportunity to cause any changes in the cells lining the bowel. In the colon and the rectum, however, stool sits there and carcinogens have plenty of time to live and have effect. Colon and rectal cancer is the 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 Ileoanal 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 the pouch gets older, researchers have found that they tend to lose their villi. The colon has a lot of mucous 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 the 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 the small bowel will mean there is a risk of cancer or colitis developing in the pouch. Polyposis patients are certainly prone to getting polyps in their pouches, but nobody has reported a cancer yet. As far as colitis is concerned, we already know about the syndrome of pouchitis which mimics colitis and can occur very early after pouch construction. This is different from a true return of colitis in older pouches.
Because of the theoretical risk of colon cancer developing in the Ileoanal pouch, we recommend that pouch patients come in 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.
