Small Bowel Obstruction

Megan Fix, MD
Maine Medical Center

Edited By: David Gordon
Duke University, Durham, North Carolina

Small Bowel Obstruction Objectives

Upon completion of this self-study module, you should be able to:

  • List the common causes of a small bowel obstruction
  • Describe the classic presentation and physical examination findings of a small bowel obstruction
  • Discuss the diagnostic modalities available to diagnose a small bowel obstruction
  • List the classic findings of small bowel obstruction on plain films
  • Describe the treatment priorities for bowel obstruction
  • Identify which patients are in need of emergent surgical intervention or surgical consultation

Bowel obstruction should be considered as a potential surgical emergency when a patient presents with acute abdominal pain. It occurs when the normal flow of intestinal contents is interrupted. The most common causes are adhesions followed by tumors and hernias. Other causes include strictures, intussuseption, volvulus, Chrohn's disease, and gallstones. Obstruction is classified as small bowel obstruction (SBO) or large bowel obstruction (LBO) based on the level of obstruction. LBO is more commonly caused by malignancy and will not be discussed in detail in this module.

Small bowel obstruction begins when the normal luminal flow of intestinal contents is interrupted and the small intestine proximal to the obstruction dilates. Secretions are prevented from passing distally. As time progresses, the distension leads to nausea and vomiting and inability to take oral intake. Bacteria may ferment in the proximal intestine and cause feculent emesis. The bowel wall becomes more and more edematous as the process continues and leads to a transudative loss of fluid into the peritoneal cavity. This increases the degree of dehydration and electrolyte abnormalities that are present in these patients. Decreased urine output, tachycardia, azotemia and hypotension can also be seen.

Bowel obstructions can be defined as partial or complete and simple or strangulated. Partial obstruction is when gas or liquid stool can pass through the point of narrowing and complete obstruction is when no substance can pass. Partial obstruction is further characterized as high grade or low grade according to the severity of the narrowing. Complete bowel obstruction typically requires surgery, whereas partial bowel obstruction is often managed conservatively. Strangulation is the most severe complication of small bowel obstruction and is a surgical emergency. This occurs when bowel wall edema compromises perfusion to the intestine and necrosis ensues. This will eventually lead to perforation, peritonitis and death if not intervened upon.

Classic Presentation

History

The patient with a small bowel obstruction will usually present with abdominal pain, abdominal distension, vomiting, and inability to pass flatus. In a proximal obstruction nausea and vomiting are more prevalent. Pain is frequently described as crampy and intermittent with a simple obstruction. If the pain becomes more severe, it may indicate the development of strangulation or ischemia. Patients also may complain of diarrhea early in the course of bowel obstruction, with inability to pass flatus and obstipation occurring after the distal portion of the bowel has emptied (up to 12-24 hours).

A history of prior abdominal surgery is important to ascertain because adhesions are the most common cause of small bowel obstruction. Also important is a history gastrointestinal disorders such as Crohn's disease. A patient who presents with a small bowel obstruction with no history of prior surgery is called a "de novo small bowel obstruction". These are caused by tumor until proven otherwise and usually require a surgical intervention.

Physical Exam

Physical examination findings include: abdominal distension (more prevalent in distal obstructions), hyperactive bowel sounds (early), or hypoactive bowel sounds (late). Fever, tachycardia and peritoneal signs may be associated with strangulation. It is also important to look for possible causes of obstruction such as inguinal hernias so always include a genitourinary examination. Rectal examination is important as well, because gross blood or hemoccult positive stool suggests strangulation or malignancy.

Diagnostic Testing

Plain radiography

All patients suspected of having a small bowel obstruction should have plain radiographs obtained. It is essential is to obtain an upright chest film (looking for free air/perforation), an upright abdominal film (looking for air fluid levels), and a supine abdominal film (looking for distended loops of bowel). The diagnosis is made when multiple air-fluid levels are seen along with distended loops of small bowel. Absence of air in the colon or rectum suggests a complete obstruction while the presence of air in the colon suggests a partial obstruction. Plain radiographs can be misleading in difficult cases. Diagnostic failure rates of as much as 30% have been reported. In one small study, the sensitivity of plain radiographs was reported as 75%, and specificity was reported to be 53%. Free air on upright chest film mandates operative management. The presence of strangulation, however, is very difficult to ascertain on plain films alone.

Small Bowel Series

The diagnosis and degree of small bowel obstruction can be confirmed by a small bowel follow-through or enteroclysis (the duodenum is instilled with air and contrast). These studies used to be considered the gold standard for determining whether an obstruction was partial or complete. More recently, CT has been replacing small bowel follow-through for definitive diagnosis.

Computerized Tomography

Computerized tomography has been replacing the small bowel series as the study of choice to differentiate partial versus complete obstruction as well as to identify strangulation early. One small series reported a sensitivity of 93%, specificity of 100%, and accuracy of 94% in detecting obstruction.

Obstruction is present if the small-bowel loop is greater than 2.5 cm in diameter dilated proximal to a distinct transition zone of collapsed bowel less than 1 cm in diameter. A smooth beak indicates simple obstruction without vascular compromise; a serrated beak may indicate strangulation. Bowel wall thickening, pneumatosis, and portal venous gas all suggest strangulation.

CT can also differentiate between the etiologies of SBO, that is, extrinsic causes such as adhesions and hernia from intrinsic causes such as neoplasms or Crohn's disease. Furthermore, it has the ability to identify a myriad of other causes of acute abdominal pain such as abscess, hernia, tumor, or inflammation. The CT should be obtained with both PO and IV contrast, unless the patient has renal failure or IV contrast allergy.

Laboratory studies

Laboratory studies are generally not helpful in making the diagnosis of SBO, but they can be helpful in patient management, especially in determining the degree of dehydration. The following labs should be ordered to identify dehydration as well as to prepare for possible operative management: serum chemistry panel, CBC, type and screen, and coagulation profile. Lactic acid, liver function tests, lipase and urinalysis are also of value in guiding diagnosis, resuscitation, and post-surgical care.

Serum BUN and creatinine may be elevated. A leukocytosis with a left shift may indicate strangulation. Metabolic alkalosis can be seen in patients with persistent vomiting. Hematocrit may be elevated in severe dehydration.

Any patient who has had previous abdominal surgery and presents with abdominal pain, distension, vomiting and decreased flatus should be considered to have a small bowel obstruction. Use of abdominal series plain films (upright CXR, upright and supine abdominal films) will reveal the diagnosis in up to 75% of cases. If there is free intraperitoneal air, an emergent surgical consultation for operative management is indicated. If the diagnosis is not revealed on plain radiographs, CT scan is indicated to identify bowel obstruction versus other acute abdominal pathology. Laboratory studies are adjunctive and will help in treatment and resuscitative measures and correcting electrolyte abnormalities.

How do I make the diagnosis?

Any patient who has had previous abdominal surgery and presents with abdominal pain, distension, vomiting and decreased flatus should be considered to have a small bowel obstruction. Use of abdominal series plain films (upright CXR, upright and supine abdominal films) will reveal the diagnosis in up to 75% of cases. If there is free intraperitoneal air, an emergent surgical consultation for operative management is indicated. If the diagnosis is not revealed on plain radiographs, CT scan is indicated to identify bowel obstruction versus other acute abdominal pathology. Laboratory studies are adjunctive and will help in treatment and resuscitative measures and correcting electrolyte abnormalities.

Treatment

Initial management of small bowel obstruction consists of the following goals:

If the patient is acutely ill and/or has peritoneal signs, an emergent surgical consult and aggressive resuscitation should ensue. Crystalloid replacement should start with 2 Liters wide open with standard oxygen and monitoring per protocol. If surgical intervention is acutely needed, prophylactic antibiotics may be given.

In the stable patient in whom the diagnosis of SBO is made, it is important to consult with surgery to determine if operative management is warranted. If the patient's exam or CT scan suggests strangulation (peritonitis, thickened bowel wall, etc) then operative intervention should ensue. If there are no signs of impending strangulation, then non-operative management may be appropriate.

Non-operative management consists of GI decompression with a nasogastric tube, intravenous fluid hydration, bowel rest, and symptomatic treatment. Frequent reassessment is important to make sure that the patient is not developing signs of strangulation. Intravenous medications for pain control (morphine, fentanyl) and nausea (zofran) should be given as needed. The patient should be admitted for observation and serial abdominal examinations. If no improvement is shown, operative management should ensue. Any patient with a "de novo obstruction" will also likely need surgical intervention.

Disposition

Pearls and Pitfalls

References

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