Small Bowel Obstruction

Lisa M. Kodadek, MD and Martin A. Makary, MD, MPH

Overview

Although the incidence of mechanical small bowel obstruction (SBO) is decreasing with the advent of laparoscopic surgery, it remains a common surgical problem. Postoperative adhesions are the most common cause and account for nearly 75% of all cases. More than 300,000 patients are hospitalized each year in the United States for an adhesiolysis operation, and SBO represents nearly 20% of surgical admissions for acute abdominal conditions. After some open surgeries, 4% to 30% of patients may develop adhesion-related SBO. Approximately 10% to 30% of patients who undergo adhesiolysis for SBO will develop recurrent SBO and will require an additional operation.

Hernia and neoplasm are other common causes of SBO. Metastatic disease, such as melanoma or ovarian cancer, is more common than primary neoplasm of the small bowel. Less common causes of SBO include Crohn's disease, abscess, intussusception, Meckel's diverticulum, bezoar, gallstone ileus, volvulus, radiation enteritis, traumatic intramural hematoma, congenital abnormalities, and superior mesenteric artery syndrome.

Approximately 25% of all patients with SBO need an operation during the index admission, and those who undergo operative management have fewer recurrent episodes of SBO when compared with those managed without surgery (Foster, 2006). Patients who have not had prior abdominal surgery are far more likely to need surgery than those with a history of abdominal surgery.

Diagnosis of SBO is most often made after a focused history and physical examination. Imaging studies are important adjuncts, and laboratory studies are important to ascertain the degree of metabolic derangement and fluid status. The white blood cell count can help indicate the timing and level of urgency to operate. The etiology of obstruction, the type and location of obstruction, and patient factors including surgical history and comorbidities are important considerations in determining the need for operative intervention. Some patients with regular patterns of recurrent bowel obstruction know themselves best. They often describe a predictable frequency and duration of episodes. In fact, in the absence of leukocytosis or other concerning signs, patients can play an important role in the decision to have surgery versus continue a pattern of nonoperative management, especially in patients at high surgical risk.

Much of the management of SBO surrounds recognition of signs and symptoms concerning for bowel strangulation. For most patients with SBO, bowel strangulation is not present. In these cases, early diagnosis, gastrointestinal decompression, bowel rest, and fluid resuscitation remain the most important strategies. When strangulation is a concern, timely operative intervention is critical to prevent further complications of bowel strangulation, such as perforation, septic shock, and death.

Evaluation and Diagnosis

Classic symptoms of SBO include colicky abdominal pain, nausea, emesis, distension, and obstipation. Diarrhea may also be present. Mechanical SBO must be distinguished from nonobstructive motility disorders such as paralytic ileus. Ileus may be associated with abdominal trauma, mesenteric ischemia, or electrolyte disturbances, such as hypokalemia, or most commonly, with open abdominal operations involving bowel manipulation,. The distinction can be subtle and often requires a careful history taking.

The clinical presentation of SBO may vary based on three factors: (1) severity of the obstruction; (2) anatomic site of obstruction; and (3) elapsed time between onset and presentation. Early recognition of SBO severity is critical for proper management. A SBO may be complete, meaning the bowel lumen is completely obstructed with no distal passage of stool or air, or only partial, meaning the bowel lumen is narrowed and there is some distal passage of bowel contents. An open-loop obstruction occurs when proximal decompression is possible via emesis or nasogastric tube (NGT). Closed-loop obstruction occurs when both proximal and distal bowel are obstructed; common causes include bowel incarceration in a hernia sac or intestinal torsion. Simple obstruction does not compromise blood flow to the bowel; strangulation involves compromise of blood flow with inevitable bowel necrosis. Anatomic site of obstruction may be classified as proximal (pylorus to proximal jejunum), intermediate (midjejunum to midileum), or distal (distal ileum to ileocecal valve). The anatomic site of obstruction may also be classified in relation to the intestinal wall as intraluminal (intussusception or bezoar), intramural (neoplasm or Crohn's disease), or extrinsic (adhesions or hernia).

A shorter period of elapsed time is typical between onset of symptoms and presentation for closed-loop obstruction as compared with open-loop obstruction because of the rapid evolution of symptoms in the former. Similarly, a complete obstruction often presents earlier and with more acute findings than partial obstruction. The character of vomitus should be ascertained; feculent vomitus is typically associated with a later presentation and a more established obstruction.

Physical examination findings and signs that portend risk of strangulation include findings consistent with an infection (fever, tachycardia, leukocytosis) and localized abdominal tenderness. Laboratory tests are often advocated for detection of strangulation, but keep in mind that there is no definitive laboratory test to diagnose imminent bowel necrosis and, similarly, there are no laboratory criteria to reliably exclude strangulation. However, increases in serum amylase, serum D-lactate, and intestinal fatty acid binding protein are associated with intestinal ischemia.

Radiographic evaluation often includes an abdominal x-ray series, which includes supine and upright plain radiographs of the abdomen and an upright chest radiograph. The most specific findings for SBO on plain film are dilated loops of small bowel (greater than 3 cm), air-fluid levels, and a paucity of gas in the colon (Figure 1). The chest film is critical to rule out pneumoperitoneum (free air) because this finding in the setting of bowel obstruction may warrant immediate operative exploration (Figure 2). Although an abdominal series may be a reasonable first study, the sensitivity of this test for SBO is poor.

FIGURE 1 Plain abdominal radiographs of a patient with complete small bowel obstruction. A, Supine film shows dilated loops of small bowel without evidence of colonic gas. B, Upright film shows multiple, short air-fluid levels arranged in a stepwise pattern. (Courtesy of Melvyn H. Schreiber, MD, The University of Texas Medical Branch, Galveston, Tx.)

FIGURE 2 Plain upright chest radiograph shows pneumoperitoneum in an individual with bowel perforation. (Courtesy of Lisa M. Kodadek, MD, The Johns Hopkins Hospital, Baltimore, Md.)

The computed tomographic (CT) scan with intravenous (IV) contrast has become the favored radiographic study for evaluation of SBO. Given the rapid study time, the CT scan has the advantage of more information than an x-ray and evaluation of the passage of contrast when an ileus is also suspected. CT scan also has the advantage of diagnosis of the cause of obstruction such as hernia, mass, inflammatory lesion, or intussusception (Figure 3). CT scan can also be used for assessment of the presence of strangulation with signs such as bowel wall thickening, poor contrast enhancement of the bowel wall, pneumatosis intestinalis, mesenteric vascular engorgement, and mesenteric haziness (Figure 4). CT scan is accurate in distinguishing malignant from benign obstruction in patients with a history of abdominal surgery for malignant disease (Figure 5). Studies have shown that CT scan has an 80% to 90% sensitivity and a 70% to 100% specificity for diagnosis of SBO. One study has shown that helical CT scan has an 84% sensitivity and 90% specificity for diagnosis of the cause of SBO and 100% sensitivity and specificity for diagnosing strangulation (Obuz et-al., 2003).

FIGURE 3 Strangulated small bowel obstruction from ileocecal tumor causing intussusception with characteristic ”bowel in bowel appearance.” (From Obuz F, Terzi C, Sokmen S, et al: The efficacy of helical CT in the diagnosis of small bowel obstruction, Eur J Radiol 48(3):299–304, 2003, Fig 5,A.)

FIGURE 4 Strangulated small bowel obstruction with computed tomographic scan shows poor contrast enhancement of the small bowel wall (arrow). (From Obuz F, Terzi C, Sokmen S, et al: The efficacy of helical CT in the diagnosis of small bowel obstruction, Eur J Radiol 48(3):299–304, 2003, Fig 4,A.)

FIGURE 5 Small bowel obstruction from peritoneal carcinomatosis. The patient had a history of total gastrectomy for gastric carcinoma. There is irregularly concentric wall thickness as a result of serosal implants and proximal bowel dilation. (From Obuz F, Terzi C, Sokmen S, et al: The efficacy of helical CT in the diagnosis of small bowel obstruction, Eur J Radiol 48(3):299–304, 2003, Fig 2.)

For evaluation of a SBO in the postoperative setting, a study with gastrointestinal contrast can evaluate obstruction. Contrast studies of the small intestine include CT enterography, enteroclysis, and small bowel series.

CT enterography involves administration of large volumes of oral contrast such as water-methylcellulose solution to achieve intraluminal distension. This study is most often used for patients with Crohn's disease–related strictures and allows excellent imaging of the bowel wall. However, this study is impractical in patients with gastrointestinal distress who are unable to tolerate contrast by mouth. The amount of oral contrast administered and the timing of the oral contrast should be determined based on the purpose of the CT scan.

Enteroclysis, a fluoroscopic study in which the proximal small bowel is instilled with air and contrast, can detect minimal adhesions and mucosal changes. However, this modality has many disadvantages, including the need for nasoenteric intubation and sedation; furthermore, enteroclysis is contraindicated in patients with complete obstruction.

If small bowel series is used, it is important to recognize that barium use may delay CT examination and surgery. Barium should not be used in cases of suspected perforation, strangulation, or a complete or closed loop obstruction. Barium impaction, although rare, is a known complication of this modality and may ultimately worsen a SBO.

An endoscopy procedure is contraindicated for a SBO. The exception to this rule is a gastric outlet obstruction in a patient who is not optimized for surgery (e.g., a malnourished patient). When a narrowing of the distal stomach or duodenum is the cause of a gastric outlet obstruction (e.g., a periampullary tumor, distal gastric mass, etc.) and endoscopic intervention can dilate or stent the obstruction, endoscopy may be warranted and can be ideal to bridge a patient to surgery at a time when the patient is less sick and better nourished. This interval strategy can lower the complication risk of definitive surgery.

Nonoperative Management

An initial trial of conservative therapy is appropriate for most cases of partial SBO, obstruction in the early postoperative period, obstruction from Crohn's disease, and those patients with history of SBO. Nonoperative management is appropriate provided there is no clinical deterioration and the patient shows some evidence of improvement over the first 12 to 24 hours. Heightened clinical awareness and repeat assessments are critical to ensure that no change in patient status necessitates operative intervention.

Nonoperative management begins with aggressive fluid resuscitation and correction of any electrolyte disorders. Adequate IV access and urinary output monitoring are important. Patients who present after protracted emesis classically have a hypochloremic, hypokalemic metabolic alkalosis with concomitant paradoxical aciduria.

Intraluminal distension can lead to mucosal ischemia. Thus, gastrointestinal decompression and bowel rest is paramount in management of a SBO. A standard NGT provides symptomatic relief, minimizes intraluminal distension with air and fluid, and allows for serial assessment of NGT output as a marker of antegrade small bowel movement. Care should be taken to ensure that nasoenteric tubes are properly functioning because they render the lower and upper esophageal sphincters incompetent and pose a risk for aspiration. A NGT may not be necessary in a patient who does not have gastric dilation.

The use of narcotic pain medication during nonoperative treatment of SBO has been debated because pain medication can mask worsening symptoms that may be diagnostic. Because CT scans are such a tremendous diagnostic tool in management of patients with a SBO, the authors advocate for adequate pain control with a patient-controlled analgesic device or equivalent regimen. Patients with escalating need for pain medication indicate the need for repeat CT scanning.

Many cases of SBO resolve with fluid resuscitation, bowel rest, and gastrointestinal decompression. Over the subsequent days, patients often begin to show improving symptoms, decreased distension, and improving imaging studies. Occasionally, small bowel series or enteroclysis may be helpful for those patients who do not achieve complete resolution, especially in the postoperative period when a prolonged ileus may be high on the differential diagnosis. For those with Crohn's disease, medical therapy (steroids and other immunosuppressive therapy) may help resolve partial SBO from inflammatory lesions. For those with SBO after blunt abdominal trauma, obstruction is typically the result of an intramural hematoma and usually resolves with nonoperative therapy after 2 to 4 weeks. Total parenteral nutrition may be considered for selected patients with SBO to prevent the sequelae of negative nitrogen balance. Patients with Crohn's disease and trauma are two common exceptions to the principle that SBO patients without prior surgery are more likely to need surgery intervention.

A recent systematic review and meta-analysis of 14 prospective studies has shown diagnostic and therapeutic use of water-soluble contrast in patients with adhesive SBO (Branco et-al., 2010). The appearance of contrast in the colon within 4 to 24 hours after administration had a sensitivity of 96% and specificity of 98% in predicting resolution of SBO. In cases in which contrast reached the colon, obstruction resolved in 99% without surgery. In cases in which contrast failed to reach the colon, obstruction failed to resolve without surgery in 90% of patients.