A colostomy operation is the surgical procedure in which the interior of the colon, at some point along its length, is brought to the outside through the abdominal wall. The contents of the bowel are allowed to drain into a bag.

The large intestine or large bowel consists of the colon and the section before the anus, the rectum. The colon is about 1.7m (5ft) long compared with the 6-7m (20 ft) of the small intestine and is roughly twice as wide as the small intestine. It connects the small intestine to the rectum.

The main function of the colon is to conserve water within the body by extracting it from the bowel contents. Of the volume of liquid content entering the colon, about 90% is reabsorbed into the blood, mainly as water.

The rectum is the short but stretchable length of bowel immediately above the anal canal. The anal canal is about 5cm long and is the short final portion of the intestinal canal. The anus has two muscle rings (sphincters), which keep the contents of the rectum inside until they can conveniently be discharged.

A common reason for removing a length of the colon or rectum is colorectal cancer. Treatment involves removing the length of the bowel containing the tumour. Once this is done, there are two possibilities. Firstly, the two free ends of bowel are joined together edge-to-edge in a procedure called an anastomosis, so that the bowel contents can continue to move along until they reach the rectum.

However, in some cases, especially if the cancer is close to the anus, the colon is brought to the surface of the skin on the abdomen with an opening (a stoma). A bag is worn over the stoma to collect the contents of the bowel. This is known as a colostomy.

In many cases colostomies are temporary, and when closed leave only a minor scar. Some colostomies have to be permanent because joining up the cut ends of the bowel is impracticable. Cancer of the rectum may involve removing the whole rectum and anus. This requires a permanent colostomy.

Why is it necessary

A colostomy may be performed for many reasons. A colostomy may be needed because of the presence of colorectal cancer. There may be a severe inflammation or infection of the colon. Sometimes colostomy is necessary to relieve a bowel obstruction. There may be inherited bowel problems, or bowel defects present at birth.

How is it performed

A cut is made through the wall of the abdomen to one side of the main incision. The upper cut end of the bowel is brought out through this opening and the edges are stitched to the margins of the opening. The lower cut end may be closed internally or may also be brought out.

The cut wall of the bowel heals to the edges of the surgical incision, which is thus kept open. Bowel contents pass out through the colostomy and are collected in a waterproof bag, which is sealed around the margins with special adhesive. A plug is sometimes also used.

Recent advances in surgical instrumentation have made closing up a colostomy easier. Until recently it was technically impossible to rejoin the bowel if a length had been removed that involved the upper rectum. However recent advances in surgical instrumentation has made a colostomy easier to join edges in places that are difficult to reach. Linear and circular staplers are now used routinely in colostomy surgery. They allow a low cut through the rectum, because the stapler can reach down and close off the upper cut edge of the rectum. A circular stapler can even be inserted via the anus to join the cut edge of the descending colon to the stump of the rectum.