Bowel Obstructions

Published on 10/02/2015 by admin

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40 Bowel Obstructions

Keith Boniface

image      Key Points

• Complete small bowel obstructions are at high risk for strangulation and typically require surgical treatment.

• Partial small bowel obstructions are at lower risk for strangulation and are generally managed nonoperatively.

• Closed-loop bowel obstructions have a higher likelihood of concomitant vascular compromise and strangulation.

• Strangulation may lead to bowel wall necrosis, perforation, peritonitis, sepsis, and death. Fever in a patient with bowel obstruction suggests strangulation and perforation.

• Plain radiographs can be used initially to evaluate cases of suspected bowel obstruction. Computed tomography may be necessary to confirm the diagnosis when plain radiographic findings are nondiagnostic. Computed tomography can exclude the diagnosis of closed-loop obstruction.

• Obstructions that complicate the first 30 days after laparotomy are often managed nonoperatively, whereas obstructions that occur after laparoscopy generally require surgery.

Perspective

Bowel obstructions develop from mechanical blockage of normal intestinal transit. Blockages may result from intraluminal matter (e.g., foreign bodies), intramural wall thickening (tumors, hernia, inflammation), or extraluminal compression (hernias, masses, adhesions). Bowel proximal to the obstruction progressively dilates, which leads to pain, obstipation (inability to pass flatus), and vomiting. Dehydration and electrolyte abnormalities ensue. As the bowel wall becomes edematous, increasing pressure causes collapse of the capillary bed and subsequent tissue ischemia, a condition known as strangulation. Strangulation may lead to bowel wall necrosis, perforation, peritonitis, sepsis, and death. Patients with small bowel obstruction have a 5% to 42% incidence of strangulation, which carries a mortality of 20% to 37%.1

Obstruction can be partial, with some intestinal contents allowed to progress through the area of narrowing, or complete. A patient with complete obstruction has a low likelihood of spontaneous resolution and a high likelihood of bowel strangulation, whereas a patient with partial obstruction can typically be managed nonoperatively because of the lower risk for bowel strangulation. The central dilemma in caring for patients with small bowel obstruction is to identify those at risk for strangulation to ensure timely surgical intervention.

Pathophysiology

Bowel obstructions can be divided into simple obstructions (bowel occlusion at one location with an intact blood supply) and closed-loop obstruction (occluded loop of bowel at two adjacent points, which may or may not involve a vascular pedicle). Closed-loop obstructions are most commonly caused by entrapment of bowel in a hernia (incarceration) or, less commonly, by volvulus. Closed-loop bowel obstructions have a higher likelihood of concomitant vascular compromise and strangulation, thus warranting a more aggressive management approach.

Large bowel obstructions are at increased risk for perforation if a closed-loop obstruction is present or if the ileocecal valve is functional (preventing proximal decompression into the small bowel). The most likely sites of perforation are at the cecum (especially if the diameter is larger than 10 cm) or at the site of a primary tumor.

Volvulus, or axial twisting of the bowel, represents a special category of closed-loop obstruction. Volvulus occurs most commonly at either the cecum or the sigmoid colon and much less frequently at the transverse colon, small bowel, or stomach. Sigmoid volvulus, which is responsible for 50% to 75% of all cases of colonic volvulus, is more common in elderly and institutionalized patients. Cecal volvulus is found in younger patients, who may have an anatomic predisposition because of abnormal fixation of the right colon.

Clinical Presentation

Bowel obstructions are manifested as colicky abdominal pain that precedes the onset of nausea and vomiting, abdominal distention, constipation, and obstipation. Proximal small bowel obstructions tend to have minimal distention and an early onset of intractable vomiting because the bowel proximal to the obstruction has minimal capacity to distend. Conversely, distal small bowel obstructions are characterized by abdominal distention, colicky abdominal pain, and obstipation before the onset of vomiting. Large bowel obstruction may be preceded by changes in stool caliber and progressive abdominal distention when it is caused by a slow-growing tumor, or it may be sudden in onset in the setting of volvulus.

Physical examination may detect signs of volume depletion, tachycardia, and hypotension. Fever suggests strangulation and perforation. The abdomen is variably distended and tympanitic, depending on the level of obstruction. Scars from previous surgery can provide valuable clues to the cause of the obstruction. Bowel sounds tend toward high-pitched rushes of “tinkling” borborygmi; a silent abdomen is an ominous sign of perforation and peritonitis. Tenderness may be present, but localized tenderness and peritoneal signs indicate perforation. The examination should include a search for hernias.

A digital rectal examination should be performed to exclude stool impaction in the elderly. Occult blood may be detected in cases of strangulated obstruction, intussusception, or an obstructing mass. A rectal mass may be identified as the cause of large bowel obstruction.

Differential Diagnosis

Many abdominal disorders may cause a functional ileus that can be mistaken clinically for bowel obstruction.

Potential causes of mechanical small or large bowel obstruction are summarized in Box 40.1.

Box 40.1 Causes of Mechanical Bowel Obstruction

Causes of Small Bowel Obstruction

Adhesions

Inflammatory bowel disease

Neoplasms

Hernias

Abscess

Intussusception

Foreign bodies

Causes of Large Bowel Obstruction

Neoplasms

Volvulus

Diverticulitis

Metastatic cancer (extrinsic compression)

Stricture

Hernia

Fecal impaction

Adhesions

Diagnostic Testing

Laboratory Testing

Laboratory abnormalities are not diagnostic of bowel obstruction but instead may indicate complications of obstruction. A complete blood count may demonstrate leukocytosis with a left shift in a patient with strangulation; serum chemistry evaluations may show dehydration, hypokalemia, and acid-base disturbances. The serum lactate concentration can be elevated in the setting of strangulation, but its measurement is neither sensitive nor specific.2

Radiographs

Plain supine and upright radiographs of the abdomen are the most commonly ordered initial diagnostic study for bowel obstruction because of their widespread availability and the low cost of radiographic evaluation (Fig. 40.1).

image

Fig. 40.1 Supine radiograph showing very distended small bowel identified by its central position, multiple loops, and valvulae conniventes.

(From Adam A, Dixon AK, Grainger & Allison’s diagnostic radiology. 5th ed. Philadelphia: Churchill Livingstone; 2008.)

Small bowel obstruction appears on radiographs as air-fluid levels and dilated loops of bowel. Air in the distal part of the colon and rectum implies early or partial small bowel obstruction. As the obstruction progresses, small bowel dilation and air-fluid levels become more prominent, and the distal end of the bowel decompresses and collapses.

Ileus is distinguished from mechanical obstruction by the presence of air-fluid levels at uniform height across an upright image of the abdomen; with obstruction, air-fluid levels are classically found at variable heights.

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