EM@3AM: Small Bowel Obstruction

Author: Brit Long, MD (@long_brit, EM Attending Physician, San Antonio, TX) // Edited by: Alex Koyfman, MD (@EMHighAK, EM Attending Physician, UTSW / Parkland Memorial Hospital)

Welcome to EM@3AM, an emDOCs series designed to foster your working knowledge by providing an expedited review of clinical basics. We’ll keep it short, while you keep that EM brain sharp.

A 63-year-old female presents with abdominal pain, decreased bowel movements, nausea, three episodes of vomiting, and abdominal distension. She has a history of prior hysterectomy, appendectomy, and cholecystectomy. She has been unable to keep food down. This has never happened before, and she denies fevers, changes in diet otherwise, night sweats, or other medical problems.

Triage vital signs (VS): BP 123/61, HR 107, T 98.9 Oral, RR 22, SpO2 97% on RA.

Physical exam reveals a patient who appears uncomfortable with abdominal distension. Her abdomen is diffusely tender, but she displays no peritoneal signs. The rest of her exam is normal.

What is the patient’s diagnosis? What’s the next step in your evaluation and treatment?

Answer: Small Bowel Obstruction


Background: Due to obstruction of the intestines and failure to pass contents through the intestinal cavity. Obstructions occur in two forms: Mechanical and Functional.

  • Mechanical: Physical barrier prevents passage through the intestines. Often due to adhesions (most common cause in developed countries), malignancy, inflammatory disease, hernia, intussusception, infection, foreign body.
    • Simple obstruction occurs at one point.
    • Closed-loop obstruction occurs at two points, which results in rapid increase in intraluminal pressure. Venous and arterial compromise may occur rapidly as well in the proximal and distal bowel segments.
  • Functional: Disruption of normal peristalsis with no true mechanical obstruction. These include post-operative states, hypokalemia, medications (opiates).
  • Pathophysiology: Obstruction causes dilation of the proximal bowel and accumulation of intestinal contents. This stimulates peristalsis resulting in bowel emptying distal to the obstruction and nausea/vomiting.
  • Bowel wall edema decreases further absorption, causing further dilation, and decreases venous return (which leads to decreased arterial flow).
  • Bacterial overgrowth due to stasis may occur, and increased permeability of the bowel wall can lead to translocation of bacteria and bacteremia.
  • Fluid loss may occur with transudative fluid movement into the bowel lumen.
  • Vomiting may cause metabolic alkalosis, electrolyte changes, and hypovolemia.


History and Exam:

  • Patients often present with crampy abdominal pain that may occur intermittently. Abdominal distension, nausea/vomiting (more common with proximal obstruction), early diarrhea (due to gut emptying), constipation (+LR 8.8, -LR 0.59), obstipation (late finding) may be present. Prior abdominal surgery is suggestive (+LR 3.86, -LR 0.19).
  • Exam may demonstrate generalized tenderness early. Decreased bowel sounds or high pitched abdominal sounds demonstrate +LR 6.3 if present and -LR 0.27 if absent. Later findings can include fever, abdominal distension (+LR 5.6-16.8, -LR 0.34-0.43). Absent bowel sounds and peritoneal signs can also occur late in obstruction.



  • CBC, LFTs, renal function, lipase, and lactate are recommended. Most tests are not sensitive or specific.
    • Lactate may elevate in the setting of bowel ischemia, but this is late in disease course.
    • WBC > 20,000 may suggest abscess, ischemia, or peritonitis.
    • Elevated BUN/Cr suggests volume depletion.
  • Abdominal X-ray
    • Dilated loops of bowel proximal to site of obstruction. In small bowel obstruction, valvulae conniventes are visisble. Air fluid levels and string of beads sign may also be present.
    • This test can be performed quickly, is non-invasive, and can be performed at the bedside. However, it has poor sensitivity (as low as 45%) and specificity (50%).
    • +LR 1.6 (1.1-2.5), -LR 0.43 (0.24-0.79).
  • Ultrasound
    • Dilated loops of bowel > 2.5 cm, decreased peristalsis, and retrograde peristalsis (or whirling/to and fro sign).
    • With infarction and ischemia, US may show bowel wall thickening, no peristalsis, and fluid filled bowel with extraluminal free air.
    • Test is noninvasive and rapid, but depends on physician skill. Obesity may limit ability to obtain images.
    • ED US: +LR 9.5 (2.1–42.2), -LR 0.04 (0.01–0.13); Formal US: +LR 14.1 (3.6–55.6), – LR 0.13 (0.08–0.20).
    • Please see this post from emDocs and 5 Min Sono for more on US.
  • CT
    • Dilated loops > 2.5 cm proximal to site of obstruction, with collapsed bowel past this site.
    • In the setting of ischemia, the bowel wall will be thickened, bowel wall will have increased attenuation, and pneumatosis intestinalis and/or portal venous gas may be present (both are not specific).
    • Oral contrast is NOT needed and may hinder evaluation for ischemia (per American College of Radiology, and see this great EP Monthly article).
    • This test takes more time and requires the patient to leave the ED, but it displays greater sensitivity and specificity compared to radiograph, identifies etiology, and identifies an alternative cause of symptoms if present.
    • +LR 3.6 (2.3-5.4), -LR 0.18 (0.09-0.35).
    • With thin slice CT (today’s generation), sensitivity and specificity range from 93-96% and 93-100%, respectively.



  • If patient is critically ill and/or presents with bowel ischemia/infarction, resuscitation is required. Airway intervention is uncommon but may be needed in septic obtunded patients. Intravascular depletion may require volume expansion, as well as electrolyte repletion.
  • Provide antiemetics and analgesia for symptom control.
  • NG placement is recommended for intractable vomiting, severe abdominal distension, and high risk of aspiration. Otherwise, NG tubes do not improve outcomes.
  • Antibiotics are not needed in all cases.  Patients with sepsis or hypoperfusion warrant broad-spectrum antimicrobials.
  • Consult general surgery, as patients with clinical deterioration, peritonitis, or bowel ischemia/infarction require surgical exploration. Other patients should be admitted for serial exams and symptom management.
  • Patients should be admitted to a surgical service, as literature suggests improved outcomes when patients are admitted to a surgical vs. medical service.
  • Patients with closed loop obstruction or bowel ischemia/infarction should be admitted to the ICU.










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