Large-Bowel Obstruction Workup

Laboratory studies are directed at evaluating the dehydration and electrolyte imbalance that may occur as a consequence of large-bowel obstruction (LBO) and at ruling out ileus as a diagnosis.

Routine complete blood cell count (CBC), serum chemistries, and urine specific gravity should be evaluated. 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 also should be performed, for similar reasons.

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. Ileus, secondary to an intra-abdominal or extra-abdominal infection or another process, is a possibility.

The suggestion of an abnormal anion gap also should prompt an arterial blood gas (ABG) measurement and/or a serum lactate level measurement.

Obtain an upright chest radiograph to determine whether free air is present, which would suggest perforation of a hollow viscus and ileus rather than organic obstruction, as well as flat and upright abdominal radiographs, which may demonstrate dilatation of the small and/or large bowel and air-fluid levels.

Chest radiographs will demonstrate free air if perforation has occurred (see the first image below); abdominal radiographs may be diagnostic of sigmoid or cecal volvulus (ie, kidney bean appearance on the radiograph) (see the second and third images below, respectively). 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).

Tracing colonic air around the colon, into the left gutter, and down into the rectum or demonstrating an abrupt cut-off in colonic air suggests the anatomic location of the obstruction.

A dilated colon without air in the rectum is more consistent with obstruction. The presence of air in the rectum is consistent with obstipation, ileus, or partial obstruction. However, this finding can be misleading, particularly if the patient has undergone rectal examinations or enemas.

Contrast studies include an enema with water-soluble contrast (ie, Gastrografin) (see the following images) or computed tomography (CT) scanning with intravenous (IV) and oral (PO) or rectal (PR) contrast. Contrast studies that reveal a column of contrast ending in a "bird's beak" are suggestive of colonic volvulus.

Indications for imaging with contrast

Radiopaque contrast material may be administered and imaging of the colon may be performed under the following circumstances:

• Perform it if the diagnosis of large bowel obstruction is suspected but not proven
• If differentiation between obstipation and obstruction is required
• If localization is required for surgical intervention

Water-soluble contrast vs barium

Water-soluble Gastrografin has important advantages over barium as a contrast agent and generally should be used first. Gastrografin usually does not cause chemical peritonitis if the patient has colonic perforation, and it has an osmotic laxative effect that may actually wash out an obstipated colon.

If large-bowel perforation is ruled out using a Gastrografin study but a more detailed anatomic definition is required (particularly of the right colon), a barium enema may be performed.

Although computed tomography (CT) scanning is useful to help rule out intra-abdominal abscess or other causes of ileus, this imaging modality is generally not used initially in patients with large-bowel obstruction (LBO), unless the diagnosis is still in question.

CT scanning, particularly with rectal contrast, may demonstrate a mass or evidence of metastatic disease. Generally, the findings do not alter management, because these patients will be explored and operatively decompressed, regardless of the CT scan findings.

CT colography may be useful in evaluating these patients, not only to delineate the source of the obstruction but also to rule out synchronous proximal lesions, which may occur in about 1% of patients and which might motivate a more extended resection if identified and if the patient's condition will tolerate the more extensive procedure.^[12]

Flexible endoscopy preceded by rectal enema may be useful in evaluating left-sided colonic obstruction, including the anatomic location and pathology of the lesion. Because the cecum is not reached in such cases, the endoscopist must be alert to the possibility of incorrectly identifying anatomic landmarks and the location of the obstruction.

An abdominal radiograph with the tip of the endoscope at the site of the obstruction may be extraordinarily helpful in identifying and documenting the location of the large-bowel obstruction (LBO).

Although flexible endoscopy is relatively comfortable for the patient and provides a better view than rigid sigmoidoscopy, rigid sigmoidoscopy may also be used, depending on the availability of resources and training of personnel.

Right-sided colonic obstruction is more difficult to evaluate without first administering an oral bowel preparation, which is contraindicated in the setting of bowel obstruction.