Approach Considerations
Given the potential morbidity and mortality associated with large-bowel obstruction (LBO), the recommended diagnostic approach focuses on rapid evaluation and prompt surgical consultation. Relief of pain, control of vomiting, and correction of fluid and electrolyte abnormalities should occur simultaneously with diagnostic evaluation.
Laboratory studies are used to assess the degree of dehydration and electrolyte imbalance and to evaluate for infection, anemia, and ischemia. Radiographic studies are used to confirm obstruction, to identify its cause if present, and to idenitify other pathology that may be causing the patient's symptoms.
Laboratory Studies
Laboratory studies are directed at evaluating the dehydration and electrolyte imbalance that may occur as a consequence of large-bowel obstruction (LBO), as well as at ruling out ileus as a diagnosis.
Routine complete blood cell (CBC) count, serum chemistries, and urinalysis should be evaluated. A serum lactate level should be ordered if bowel ischemia is a consideration. A decreased hematocrit level, particularly with evidence of chronic iron-deficiency anemia, may suggest chronic lower gastrointestinal (GI) bleeding, particularly due to colon cancer. A stool guaiac test should be performed.
Obtain a prothrombin time (PT) as well as a type and crossmatch.
Although bowel obstruction, or even constipation, may mildly elevate the white blood cell (WBC) count, substantial leukocytosis should prompt reconsideration of the diagnosis or increase the suspicion for perforation. Ileus secondary to an intra-abdominal or extra-abdominal infection should also be considered.
Imaging Studies
Plain radiography
Although at times helpful in the evaluation of suspected bowel obstruction, plain radiography has largely been supplanted by computed tomography (CT) scanning owing to the latter's abilily to provide far more accurate and detailed images of the relevant pathology. [15]
An upright chest radiograph is useful to screen for free air (see the image below), which would suggest perforation and ileus rather than obstruction. Flat and upright abdominal radiographs can help distinguish severe constipation from bowel obstruction. Plain films may also help localize the site of obstruction (large vs small bowel).
This chest radiograph demonstrates free air under the diaphragm, indicating bowel perforation in a patient with large-bowel obstruction. Radiograph courtesy of Charles J McCabe, MD†.
Sigmoid or cecal volvulus may have a kidney-bean appearance on the abdominal films (see the images below). Intramural air is an ominous sign that suggests colonic ischemia. The absence of free air does not exclude perforation (this finding may be absent in half of all perforations).
Massive dilatation of the colon due to a sigmoid volvulus. Radiograph courtesy of Charles J McCabe, MD†.
![Abdominal (kidney-ureter-bladder [KUB]) radiograph](http://img.medscapestatic.com/pi/meds/ckb/65/40365tn.jpg)
Abdominal (kidney-ureter-bladder [KUB]) radiograph depicting massive dilatation of the colon due to a cecal volvulus. Radiograph courtesy of Charles J McCabe, MD†.
Contrast studies include an enema with water-soluble contrast (eg, Gastrografin) (see the images below). Contrast studies that reveal a column of contrast ending in a "bird's beak" are suggestive of colonic volvulus.
Gastrografin study in a patient with obstipation reveals colonic obstruction at the rectosigmoid level. Radiograph courtesy of Charles J McCabe, MD†.
Contrast study of patient with cecal volvulus. The column of contrast ends in a "bird's beak" at the level of the volvulus. Radiograph courtesy of Charles J McCabe, MD†.
Contrast study demonstrates colonic obstruction at the level of the splenic flexure, in this case due to carcinoma. Radiograph courtesy of Charles J McCabe, MD†.
Computed tomography
CT scanning is the imaging modality of choice if a colonic obstruction is clinically suspected; this imaging modality can confirm the diagnosis and identify the cause of large-bowel obstruction. [6, 15] Contrast-enhanced CT (oral and intravenous) can help to delineate between partial and complete obstruction, ileus, and small-bowel obstruction, as well as exclude large-bowel obstruction. Gastrografin (water-soluble contrast) should be used preferentially if bowel perforation is suspected. Multidetector CT (MDCT) scanning is particularly useful in identifying the site and cause of obstruction as well as any related complications. [16]
Magnetic resonance imaging
Dynamic magnetic resonance imaging (MRI) of gastrointestinal motility has evolved and shows promise for clinical assessment of gastric, small intestinal, and colonic motility in patients with inflammatory bowel disease, pseudo-obstruction, and functional bowel disorders. [17] Its advantages include it being a quick, noninvasive imaging modality; the absence of ionizing radiation; and its capability for visualizing the entire gastrointestinal tract. [17] More investigation is needed.
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This chest radiograph demonstrates free air under the diaphragm, indicating bowel perforation in a patient with large-bowel obstruction. Radiograph courtesy of Charles J McCabe, MD†.
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Abdominal (kidney-ureter-bladder [KUB]) film of a patient with obstipation. Dilatation of the colon is associated with large-bowel obstruction. Radiograph courtesy of Charles J McCabe, MD†.
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Gastrografin study in a patient with obstipation reveals colonic obstruction at the rectosigmoid level. Radiograph courtesy of Charles J McCabe, MD†.
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Contrast study demonstrates colonic obstruction at the level of the splenic flexure, in this case due to carcinoma. Radiograph courtesy of Charles J McCabe, MD†.
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Abdominal (kidney-ureter-bladder [KUB]) radiograph depicting massive dilatation of the colon due to a cecal volvulus. Radiograph courtesy of Charles J McCabe, MD†.
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Contrast study of patient with cecal volvulus. The column of contrast ends in a "bird's beak" at the level of the volvulus. Radiograph courtesy of Charles J McCabe, MD†.
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Massive dilatation of the colon due to a sigmoid volvulus. Radiograph courtesy of Charles J McCabe, MD†.
