Normal bowel contains gas as well as gastric secretions and food. Intraluminal accumulation of gastric, biliary, and pancreatic secretions continues even if there is no oral intake. As obstruction develops, the bowel becomes congested and intestinal contents fail to be absorbed. Vomiting and decreased oral intake follow. The combination of decreased absorption, vomiting, and reduced intake leads to volume depletion with hemoconcentration and electrolyte imbalance, and may lead to renal failure or shock.
Bowel distention often accompanies mechanical obstruction. Distention is due to the accumulation of fluids in the bowel lumen, an increase in intraluminal pressure with enhanced peristaltic contractions, and air swallowing. When intraluminal pressure exceeds capillary and venous pressure in the bowel wall, absorption and lymphatic drainage decrease, the bowel becomes ischemic, and septicemia and bowel necrosis can develop. Shock ensues rapidly. Mortality approaches 70% if bowel obstruction has progressed to this degree. This sequence of events may occur more rapidly in a closed-loop obstruction with no proximal escape for bowel contents. Examples of closed-loop obstruction include an incarcerated hernia and complete colon obstruction in the presence of a closed ileocecal valve.
Small bowel obstruction is approximately four times more common than large bowel obstruction. The most common cause of small bowel obstruction is adhesions after abdominal surgery. Although in most cases several months to years have passed from the time of the previous surgery, small bowel obstruction may occur within the first few weeks after surgery. The second most common cause of small bowel obstruction is incarceration of a groin hernia (see chapter titled "Hernias in Adults"). Other sites that occasionally are responsible for small bowel obstruction secondary to hernia include the umbilicus, femoral canal, and, rarely, the obturator foramen. Umbilical hernias are more readily apparent and occur in any age group. Obturator or femoral hernias are much less common. Elderly females are particularly susceptible to these hernias, which may present with femoral or medial thigh pain. Finally, a defect in the mesentery itself may cause intestinal obstruction. In the elderly population, adhesions and hernias are still common causes of small bowel obstruction, whereas carcinoma is the most likely cause of large bowel obstruction because of the increased likelihood of cancer as people age. Patients >60 years old are more likely to succumb secondary to complications of bowel obstruction.
Bariatric surgery may be complicated by internal hernias after Roux-en-Y gastric bypass.2,3 Other causes of small bowel obstruction are much less common and generally are the result of intraluminal or intramural processes. Primary small bowel lesions include polyps, lymphoma, or adenocarcinoma. Hamartomatous polyps are common in Peutz-Jeghers syndrome; polyps occur in patients between the ages of 10 and 30 years and cause obstruction in about 40% of patients.4 An unusual cause of intraluminal obstruction is gallstone ileus. In this situation, a gallstone has eroded from the gallbladder through the bowel wall and can cause obstruction at the ileocecal valve. Signs of gallstone ileus include bowel obstruction and air in the biliary tree. Lymphomas may be the leading point of intussusception and present as small bowel obstruction. Bezoars are most commonly composed of vegetable matter or pulp from persimmons. Patients who have undergone GI pyloroplasty or pyloric resection are most susceptible to intraluminal obstruction by bezoars.
Inflammatory bowel disease and infectious processes, including abscesses, may obstruct the small bowel at various sites. Radiation enteritis should be considered as a possible cause of small bowel obstruction in patients who have undergone radiation therapy. Blunt abdominal trauma may cause a duodenal hematoma. This condition is seen in children as a result of lap belt use and may present as intra-abdominal pain and vomiting similar to other causes of small bowel obstruction.
Visualization of the entire small bowel can be accomplished by capsule endoscopy. An important complication is capsule retention, with literature-reported frequencies of 1% to 20%.5 Capsule retention can lead to obstruction and perforation, so patients with abdominal pain after capsule introduction should be carefully evaluated for these complications.5
Neoplasms are by far the most common cause of large bowel obstruction. Colonic obstruction is almost never caused by hernia or surgical adhesions and should prompt an evaluation for a neoplasm. Diverticulitis may create significant mesenteric edema and secondary obstruction. Stricture formation may occur with chronic inflammation and scarring. Fecal impaction is a common problem in the elderly or debilitated and may present with symptoms of colonic obstruction.
The next most frequent cause of large bowel obstruction after cancer and diverticulitis is sigmoid volvulus. Elderly, bedridden, or psychiatric patients who are taking anticholinergic medication are most at risk for volvulus. A history of constipation may precede the development of volvulus. Radiographic appearance is usually classic (Figure 1). Cecal volvulus may cause large bowel obstruction. There is a higher incidence of cecal volvulus in gravid patients.6
Sigmoid volvulus. Extends into the T10 area or higher. [Reproduced with permission from Wikiradiography.com.]