Chapter 3-22: Small Bowel Obstruction (SBO)

TEXTBOOK PRESENTATION

The presentation is similar to that for LBO with the exception that patients are more likely to have a history of prior abdominal surgery.

DISEASE HIGHLIGHTS

1. Bowel obstruction accounts for 4% of patients with abdominal pain.

2. SBO accounts for 76% of all bowel obstructions.

3. Etiology

   1. Postsurgical adhesions, 70%

   2. Malignant tumor, 10–20%

      1. Usually metastatic

      2. However, 39% of SBOs in patients with a prior malignancy are due to adhesions or benign causes.

   3. Hernia (ventral, inguinal, or internal), 10%

   4. IBD (with stricture), 5%

   5. Radiation

   6. Less common causes of SBO include gallstones, bezoars, and intussusception.

4. SBOs may be partial or complete.

   1. Complete SBO

      1. 20–40% progress to strangulation and infarction. Strangulation may occur secondary to mesenteric twisting cutting off the blood supply or due to increasing intraluminal pressure directly compromising perfusion.

      2. Clinical signs do not allow for identification of strangulation prior to infarction: Fever, leukocytosis, and metabolic acidosis are late signs of strangulation and suggest infarction.

      3. 50–75% of patients admitted for SBO require surgery.

   2. Partial SBO

      1. Rarely progresses to strangulation or infarction

      2. Characterized by continuing ability to pass stool or flatus (> 6–12 hours after symptom onset) or passage of contrast into cecum

      3. Resolves spontaneously (without surgery) in 65–80% of patients

EVIDENCE-BASED DIAGNOSIS

1. Ideally, tests for SBO should identify obstruction and ischemia or infarction, if present (since ischemia and infarction are indications for emergent surgery rather than further observation). Unfortunately, even tests that successfully differentiate complete from partial SBO do not reliably determine whether there is ischemia and infarction.

2. See test characteristics of history and physical exam under LBO.

3. Physical exam findings are insensitive at predicting infarction. However, localized tenderness, rebound, or guarding would all suggest infarction is present.

4. WBC may be normal even in presence of ischemia.

5. Plain radiographs may show ≥ 2 air-fluid levels or dilated loops of bowel proximal to obstruction (> 2.5 cm diameter of small bowel).

   1. Sensitivity for obstruction 75%; specificity, 66%; LR+, 2.2; LR–, 0.37

   2. Rarely determines etiology

   3. Complete obstruction is unlikely in patients with air in the colon or rectum.

6. Ultrasound

   1. Can show dilated bowel (≥ 25 mm) proximal to normal or collapsed distal bowel.

   2. Formal ultrasound

      1. Sensitivity, 90%; specificity, 96%

      2. LR+, 14.1; LR–, 0.13

   3. Bedside ultrasound

      1. Has excellent accuracy

      2. Sensitivity, 97%; specificity, 90%

      3. LR+, 9.5; LR–, 0.04

7. CT scanning

   1. Sensitivity for determining obstruction is 87%; Specificity, 81%; LR+, 3.6; LR–, 0.18

      1. Obstruction is suggested by a transition point between bowel proximal to the obstruction, which is dilated, and bowel distal to the obstruction, which is collapsed.

      2. CT scanning should be performed prior to nasogastric suction, which may decompress the proximal small bowel and thereby decreases the sensitivity of the CT scan for SBO.

   2. May delineate etiology of obstruction

   3. Test of choice to diagnose SBO (not ischemia)

   4. Not reliably sensitive at determining the presence of ischemia and infarction (and the need for immediate surgery). Different studies have reported sensitivities ranging from 15% to 100% ...