In a bowel obstruction (intestinal obstruction), a blockage prevents the contents of the intestines from passing normally through the digestive tract. The problem causing the blockage can be inside or outside the intestine. Inside the intestine, a tumor or swelling can fill and block the inside passageway of the intestine. Outside the intestine, it is possible for an adjacent organ or area of tissue to pinch, compress or twist a segment of bowel.
A bowel obstruction can occur in the small bowel (small intestine) or large bowel (large intestine or colon). Also, a bowel obstruction can be total or partial, depending on whether any intestinal contents can pass through the obstructed area.
In the small intestine, the most common causes of bowel obstruction are:
Adhesions — Adhesions are areas of tough, fibrous connective tissue that are a type of scar. Adhesions develop on the outside of injured intestine or pelvic organs as they heal after surgery or infection. Gynecological surgeries and surgery involving the appendix or colon are particularly likely to result in adhesions. Adhesions do not always cause symptoms when they first form. Because of the motion of the intestine, it is common for these collections of scar tissue to be stretched into string-like or band-like tethers over time. An area of adhesions can cause obstruction of the small bowel if the adhesions are pulled into the shape of a constricting band, pinching a portion of the small intestine closed from the outside. Adhesions also can bind to neighboring loops of intestine, and later tighten, pulling the intestine into an abnormal configuration that limits the flow of intestinal contents. Adhesions are the most common cause of small-bowel obstruction in the United States, accounting for 50% to 70% of all cases.
Hernia — If there is a structural weakness in the muscles and fibers that are part of the wall of the abdomen, a portion of the small intestine may protrude through this weakened area, and appear as a lump under the skin. This protruding segment of intestine is called a hernia. The section of small intestine that becomes a hernia can become obstructed if it is trapped or tightly pinched at the point where it pokes through the abdominal wall. In extreme cases, the pinched intestine also may "strangulate," meaning the blood supply is cut off. Hernias are the second most common cause of small-bowel obstruction in the United States, accounting for about 25% of all cases. Typically, hernias appear as lumps near the navel (umbilical hernia), between the navel and breastbone (ventral hernia), at the site of a healed surgical incision (incisional hernia), near the groin (inguinal hernia), or at the front of the upper thigh (femoral hernia).
Tumors – Cancerous tumors can cause small-bowel obstruction either by pressing on the outside of the bowel and pinching it closed, or by growing within the wall of the intestine and slowly blocking its inner passageway. Cancers account for a small percentage of all small-bowel obstructions. In most cases, the tumor does not begin in the small intestine itself. More often, it is a cancer that has spread (metastasized) to the small bowel from another site in the colon, female reproductive tract, breast, lung or skin.
In the large intestine, the most common causes of bowel obstruction are:
Colorectal cancer — About half of all large-bowel obstructions are caused by colorectal cancer. Undiagnosed colon or rectal cancer may cause a gradual narrowing of the large intestine's inner passageway. Usually patients experience intermittent constipation for a while before the bowel finally becomes obstructed.
Volvulus – Volvulus is an abnormal twisting of a segment of bowel around itself. This twisting motion typically produces a closed loop of bowel with a pinched base, leading to intestinal obstruction. In Western countries, volvulus is most common among people over age 65, and these patients often have a history of chronic (long-lasting) constipation.
Diverticular disease – In the large bowel, diverticula are small, balloon-shaped pouches that protrude from the wall of the intestine. If diverticula become infected this is called diverticulitis. During healing from infection, scars may form in the wall of the colon as it. A scar that encircles the colon is called a colon stricture. As a stricture ages and tightens, it can narrow the intestine gradually, eventually causing a blocked colon.