Mark Hess
EPIDEMIOLOGY
Small bowel obstruction (SBO) is more common than large bowel obstruction (LBO).
Intestinal obstruction is due to mechanical obstruction or functional (adynamic or paralytic ileus) obstruction, with ileus being more common. Mechanical obstruction may be due to either intrinsic or extrinsic mechanisms.
Adhesions following surgery are the most common cause of SBO. Incarcerated inguinal hernias are the second most common cause of SBO. Other causes of bowel obstruction are listed in Table 47-1.
LBO is most commonly due to neoplasm. Fecal impaction is common in elderly and debilitated patients. Sigmoid volvulus is common in the elderly, especially those taking anticholinergic medications. Cecal volvulus is more common in gravid patients.
Complications and mortality rise in those over 60 years of age. Mortality also increases dramatically if corrective surgery is delayed beyond 24 hours.
Ileus may be due to injury, infection, medications, or electrolyte abnormalities.
TABLE 47-1 Common Causes of Intestinal Obstruction
PATHOPHYSIOLOGY
Blockage prevents passage of luminal contents and results in dilatation due to accumulation of gastric, biliary, and pancreatic secretions.
With distention, intraluminal pressure rises, decreasing bowel wall blood flow. When pressure exceeds capillary pressure, absorption ceases and leakage of fluids (third-spacing) may occur. Microvascular changes may allow entry of gut flora into the circulation, resulting in bacteremia and sepsis. Necrosis and bowel perforation may follow.
With obstruction, oral fluid intake stops and vomiting occurs. This fluid loss, coupled with the third space losses mentioned above, leads to hypovolemia and shock.
Closed loop obstruction (LBO in presence of closed ileocecal valve) has a more rapid progression.
CLINICAL FEATURES
Classic history includes vomiting, abdominal distention, and pain, with a past history of abdominal surgery or hernia.
Abdominal pain is usually crampy and intermittent. SBO results in primarily periumbilical pain versus hypogastric pain for LBO. Pain with ileus may be constant.
Emesis is often bilious early and may be feculent with late SBO or with LBO.
Early in the disease course, bowel sounds have high-pitched rushes, but this finding diminishes with time.
The patient may have surgical scars, hernias, or intra-abdominal masses.
Peritoneal signs suggest perforation.
Clinical signs of dehydration and/or shock may be present (tachycardia, hypotension).
Rectal examination may reveal impaction, occult blood, or carcinoma. Passage of stool does not rule out obstruction.
Women may have palpable gynecologic neoplasms on pelvic examination.
DIAGNOSIS AND DIFFERENTIAL
Radiographs help localize SBO versus LBO. Plicae circulares are linear densities that traverse the small bowel lumen. Haustra in the large bowel do not extend fully across the lumen.
Dilated loops of bowel on supine film with stepladder air-fluid levels on upright film are diagnostic. Look on the upright or decubitus film for free air suggesting perforation, and for pneumonia or pleural effusions on the chest film.
Contrast-enhanced abdominal CT has been advocated to identify partial versus complete bowel obstruction. It also can differentiate between bowel obstruction and ileus as well as identify the site and cause of the obstruction.
Laboratory tests include complete blood count (CBC), blood urea nitrogen (BUN), serum electrolytes, and urinalysis. Liver function tests as well as crossmatch and coagulation studies may also be needed.
Leukocytosis with a left shift may suggest peritonitis, gangrene of the bowel, or an abscess. Serum lactate may be useful in assessing the presence of mesenteric vascular occlusion or severe dehydration.
As dehydration and shock develop, elevated urine specific gravity and metabolic acidosis may be seen along with hemoconcentration.
Sigmoidoscopy or barium enema may be useful in localizing the site of LBO.
Pseudo-obstruction (Ogilvie’s syndrome) is most commonly seen in the low colonic region. Intestinal motility is depressed (often due to tricyclic antide-pressants or anticholinergic agents), resulting in large volumes of retained gas. Air-fluid levels are rarely seen on radiographs. Pseudo-obstruction is treated by colonoscopy.
EMERGENCY DEPARTMENT CARE AND DISPOSITION
With mechanical bowel obstruction, prompt surgical consultation is required.
A nasogastric tube is used to decompress the bowel, especially if vomiting or distension is present.
Fluid resuscitation should be started using crystalloid. Monitor vital signs and urine output to measure response to fluids.
Appropriate antibiotic therapy (such as piperacillin-tazobactam 3.375 grams, or ampicillin-sulbactam 3.0 grams IV) should be started if perforation is suspected or surgery is anticipated.
For adynamic ileus, conservative treatments including nasogastric decompression, fluid replacement, and observation are usually effective.
For further reading in Emergency Medicine: A Comprehensive Study Guide, 7th ed., see Chapter 86, “Bowel Obstruction and Volvulus,” by Salvator J. Vicario and Timothy G. Price.