Small bowel obstruction is a common surgical emergency due to a mechanical blockage of the bowel. Small bowel obstruction can be caused by many pathologic processes, but the leading cause in the developed world is intra-abdominal adhesions. Small bowel obstructions can be partial or complete and can be non-strangulated or strangulated.
Post-surgical adhesions most commonly cause small bowel obstruction. Incarcerated hernias are the second most common etiology. Other common etiologies include malignancy, inflammatory bowel disease (Crohn), stool impaction, foreign bodies, and volvulus. In the pediatric population, common causes include congenital atresia, pyloric stenosis, other congenital anomalies, and intussusception.
It is estimated that more than 300,000 laparotomies are performed each year in the United States for small bowel obstruction. The small bowel comprises 80% of bowel obstructions. There is a similar incidence of males and females. There is higher incidence with age and number of intra-abdominal procedures.
Twisting of the bowel leads to proximal bowel distention and distal bowel decompression. Initially, peristalsis may increase, leading to frequency bowel movements. Distention of the proximal bowel may lead to vomiting. The twisted bowel will first cut off venous blood flow and lead to bowel wall edema and inflammation. The third spacing of fluid often occurs as well. The thickened and inflamed bowel wall is at risk for ischemia and bacterial translocation. Bacterial translocation can cause peritonitis and bacteremia, most commonly from Escherichia coli. As the bowel further twists, the arterial flow will be cut off, leading to bowel ischemia and eventually perforation, peritonitis, and death if untreated.
History of previous abdominal surgeries, inflammatory bowel disease, malignancy, or a hernia is a critical point to ascertain. Patients often present with complaints of abdominal pain, distention, nausea, and vomiting. Abdominal pain may be progressive or intermittent. Patients may have constipation or obstipation but could also have flatus and even loose bowel movements.
Bowel sounds may be reduced and high pitched. Abdominal tenderness on exam may be diffuse or focal. Distention may be present. Signs of peritonitis such as rebound, guarding, and rigidity are late findings which may be present depending on the time of presentation. Evaluation for hernias, surgical scars, masses including in the rectum and fecal impactions may demonstrate the possible etiology. Additionally, patients can present with symptoms and signs of dehydration and sepsis.
Small bowel obstruction may be diagnosed with physical exam alone, but often further diagnostics are required for surgical evaluation and management. While traditionally a physical exam was used to diagnose small bowel obstruction, the invention of computed tomography has greatly improved the accuracy and characterization of this disease. X-ray is often used as a supplementary imaging modality; however, ultrasound is more sensitive and specific than an x-ray. Additionally, ultrasound does not result in radiation exposure and has the benefit of rapid and serial examinations.
Plain radiography has poor sensitivity, ranging from 50% to 80%, and may be an initial screening test for obvious air-fluid levels and free intra-abdominal air but cannot be relied upon to rule out small bowel obstruction. Small bowel diameter of greater than 6 centimeters, large bowel greater than 12 centimeters, and cecum greater than 15 centimeters is worrisome for obstruction.
CT is the gold standard imaging modality in many centers of care. Intravenous contrast should be used if the patient has normal renal function and does not have a contraindication. If the patient has subnormal renal function, a non-contrast study may be obtained, but consultation with the radiologist to determine the optimal study is best. Oral contrast is most often unnecessary in the evaluation of small bowel obstruction as it can lead to delayed diagnosis and complications. Magnetic resonance imaging may be appropriate in specific circumstances such as young patients who have had multiple computed tomography evaluations previously.
Ultrasound is not a replacement for CT and should not delay surgical consultation. However, it is useful in the instances when it can facilitate diagnosis, surgical consultation, and rule out other diagnoses.
Typical laboratory studies also need to be sent to evaluate for bowel ischemia, inflammation, extent of dehydration, pre-operative care, and to rule out possible confounding diagnoses. These may include a complete blood count, lactic acid, complete metabolic profile, urine studies, and coagulation studies.
Surgery consultation should be utilized without delay as many small bowel obstruction patients require surgical management. Initial treatment of small bowel obstruction involves fluid resuscitation, pain control, antibiotics, and, often, nasogastric decompression. Antibiotics of choice for small bowel obstruction should target gut flora and cover both gram-negative and anaerobic bacteria.
Ileus and partial small bowel obstructions often can be treated conservatively with nasogastric decompression. Surgical consultation should still be sought, but surgical intervention may not be required.
A dilated, non-compressible bowel is the hallmark of small bowel obstruction on ultrasound. A small bowel more than three centimeters is dilated. The small bowel wall is thick when it is more than three millimeters. Back and forth peristalsis and visualizing a transition point are specific findings for obstruction on ultrasound. CT scan remains an accurate method to diagnose and characterize a small bowel obstruction.