Mechanical bowel obstruction

Mechanical bowel obstruction occurs when there is an occlusion of the lumen of the intestine that causes a blockage of the normal flow of intraluminal contents through the gastrointestinal tract. As a common surgical emergency, 3.2 million cases of bowel obstruction occurred throughout the world in 2015 resulting in 264,000 deaths. ^, The small bowel is more much frequently affected compared with the large bowel and is involved in 80% of cases. In the United States, small bowel obstruction (SBO) accounts for 15% of hospital admissions for patients presenting with abdominal pain and more than 300,000 annual hospitalizations. Approximately 20%–30% of these patients will require operative management and contend with significant healthcare costs. ^,

Etiology

In developed countries, intraperitoneal adhesions that develop from prior abdominal surgery are the most common cause of mechanical SBO and account for 55%–80% of cases. Patients who have previously undergone pelvic surgery, specifically colorectal resection, appendectomy, and gynecologic surgery; required prior adhesiolysis for bowel obstruction; or had resection for a malignancy are at increased risk for developing adhesive SBO. ^, Malignant neoplasms are responsible for 20% of cases and are most commonly intraperitoneal metastases from primary gastric, pancreatic, colonic, or ovarian malignancies. Hernias account for approximately 10% of cases and result from innate weak points in the abdominal wall or arise from previous surgical incisions.

Pathophysiology

An obstruction is classified as complete when the intestinal lumen is completely occluded and no amount of gas or fluid is able to pass through the site of obstruction. Intestinal obstruction can lead to severe life-threatening complications if bowel ischemia were to occur. The progression to ischemia begins with bowel dilatation proximal to the site of obstruction, as intraluminal contents are unable to pass. As the bowel dilatation worsens, its absorptive function is lost and fluid and electrolytes accumulate in the intestinal lumen. These events result in an increase in the intraluminal pressure of the bowel that can cause a decrease in blood flow to the intestinal wall leading to ischemia. If malperfusion is severe enough, necrosis of the bowel wall and perforation can occur. The risk for bowel ischemia is greater in the setting of a complete bowel obstruction.

A closed loop bowel obstruction occurs when a segment of intestine is occluded at two different points along its course so that gas and fluid are trapped within this loop of bowel ( Fig. 91.1 ). Because intraluminal pressures are much higher in a closed loop obstruction, there is increased risk for the development of bowel ischemia. This is most commonly caused by an abdominal wall hernia, internal hernia, or volvulus. Strangulation is another serious complication of bowel obstruction that can also lead to bowel ischemia and is more commonly seen with a closed loop obstruction. This occurs when the blood supply from the mesentery to the intestine is occluded, with rapid progression to bowel necrosis and perforation if left untreated.

Fig. 91.1, A CT scan showing a closed loop obstruction involving the sigmoid colon caused by an incarcerated inguinal hernia.

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