Adhesions form as a result of previous surgery and can form from days to years after the surgery. They occasionally occur in the “virgin” abdomen (with no previous history of surgery) because of peritonitis. Adhesions cause obstruction in the following manner. Two external walls of bowel adhere to each other, and a loop of bowel wraps around or becomes lodged between the adhesion(s). There is a compromise in blood supply to that segment of intestine, with resultant ischemia and necrosis.

External hernias and internal hernias are the second most common causes of small bowel obstruction and less commonly colonic obstruction. Common examples of an external hernia are inguinal, paraumbilical, and ventral hernias. Internal herniation is rare by comparison. These occur within the abdominal cavity and are often symptomatically intermittent, obstructing and reducing spontaneously. Between episodes, when patients are often asymptomatic, it is difficult to make the proper diagnosis (Turley, 1979). Incarceration of a hernia is caused when the bowel becomes trapped in a hole in the muscle wall or in a foramen. Delay in diagnosing the obstruction leads to ischemic changes in the bowel wall (strangulation). Necrosis (death) of the small bowel, secondary to either external or internal hernias, will result if the ischemic changes are not reversed. The mortality rate associated with necrosis of the bowel, small or large, is very high and increases in patients with multiple medical problems (Sachs et al, 1981; Marston, 1989).

Volvulus occurs most often in the large bowel and is associated with a redundant portion of bowel, most often in the sigmoid colon and much less often in the cecum (Ballantyne, 1981; Frizelle & Wolff, 1996). Chronic constipation and laxative abuse have been suggested as causes of the redundant sigmoid colon. A volvulus is a twist in the colon, much like a twist in a pretzel, resulting in a compromise in the blood flow and ischemia. Patients usually present with abdominal distention and inability to pass flatus or have a bowel movement; they may or may not have abdominal pain.

Inflammatory conditions such as diverticulitis commonly cause bowel obstruction, usually in the sigmoid colon (Sleisenger, 1993). Occlusion of a diverticulum from fecoliths, peanuts, or any object small enough to enter and lodge within the diverticulum can result in diverticulitis. Microperforation of the diverticulum can occur as a complication of diverticulitis and can result in a peridiverticular abscess. The intense inflammatory response to this microperforation results in edematous changes and subsequent obstruction. This form of obstruction is usually not difficult to diagnose because it usually occurs in the left lower quadrant and is accompanied by fever, leukocytosis, and focal rebound tenderness.

The inflammatory bowel diseases, Crohn’s disease and ulcerative colitis, can also cause bowel obstruction. The most common site of obstruction in Crohn’s disease is the ileocecal region because it is the narrowest segment in the entire gastrointestinal tract. The mechanism for obstruction with inflammatory bowel disease initially involves inflammation and then stricture formation. In ulcerative colitis, which involves only the large bowel, chronic inflammation can lead to stricture formation with resultant obstruction (see Chap. 27).

Malignant causes of intestinal obstruction are due to mechanical obstruction secondary to the bulkiness of the tumor within the lumen of the intestine. Larger tumors are needed to cause obstruction in the cecum and other areas with a larger diameter. The descending colon has a narrower lumen, and malignancy in this part of the colon may present as an obstruction (see Chap. 24).

Occasionally a tumor can cause an obstruction by initiating an intussusception. This is most commonly seen in the small bowel and can be caused by both malignant and benign lesions. At the site of intussusception, the tumor acts as an initiator, and one portion of bowel “telescopes” over the adjacent piece of intestine. This causes edema and obstruction as well as a decrease in blood flow to the loops of intestine involved, with resultant ischemia.

Paralytic ileus must always be considered when a patient presents with evidence of an obstruction. With paralytic ileus, the peristaltic function of the small and large bowel is disrupted, and the intestine dilates. Patients who present with histories suggestive of sepsis more than likely have a paralytic ileus rather than an obstructive process. Other causes of a paralytic ileus include perforation, peritonitis, and electrolyte abnormalities. Peptic ulcer disease, blunt trauma to the abdomen, and cancers with resultant perforations all can result in the development of
a paralytic ileus. Another common cause for a paralytic ileus is abdominal surgery. All patients undergoing a laparotomy will have a postoperative ileus for a short period of time. This is expected and should not cause alarm. The bowel function usually returns to normal within a few days.

The most common benign causes of small bowel obstruction include adhesions and external and internal hernias. Adhesions can occur in anyone who has ever had abdominopelvic surgery. Volvulus is a less common cause of obstruction, occurring mostly in the elderly or institutionalized patients. Diverticular disease with obstruction also occurs most commonly in the elderly (Painter & Burkitt, 1975). Approximately 10% to 20% of patients with a history of diverticulosis will develop diverticulitis during their lifetime (Almy & Howell, 1980). Intussusception is an uncommon cause of obstruction in adults, accounting for approximately 5% of all obstructions (Azar & Berger, 1997). In adults, a causative factor can be found in up to 90% of cases of intussusception (Agha, 1986).