eMedicine Specialties > Gastroenterology > Colon

Colonic Obstruction: Differential Diagnoses & Workup

Author: Marc D Basson, MD, PhD, MBA, Professor, Chair, Department of Surgery, Michigan State University
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Updated: Jul 30, 2008

Differential Diagnoses

Workup

Laboratory Studies

• Studies are directed at evaluating the dehydration and electrolyte imbalance that may occur as a consequence of large bowel obstruction and at ruling out ileus as a diagnosis.
• Routine serum chemistries and urine specific gravity should be evaluated.
• Suggestion of an abnormal anion gap also should prompt an arterial blood gas measurement and/or a serum lactate level measurement.
• A decreased hematocrit level, particularly with evidence of chronic iron-deficiency anemia, may suggest chronic lower gastrointestinal bleeding, particularly due to colon cancer.
• A stool guaiac test also should be performed, for similar reasons.
• Although bowel obstruction, or even constipation, may mildly elevate the 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.

Imaging Studies

• Flat and upright abdominal roentgenography demonstrates dilation of the small and/or large bowel and air fluid levels.
• An upright chest x-ray generally is ordered simultaneously to determine whether free air is present, which would suggest perforation of a hollow viscus and ileus rather than organic obstruction.
• 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.
• This finding can be misleading, particularly if the patient has undergone rectal examinations or enemas.
• The characteristic bird's beak of volvulus may be seen.
• 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, imaging with contrast is indicated.
  • If localization is required for surgical intervention, imaging with contrast is indicated.
• Water-soluble Gastrografin has important advantages over barium as a contrast agent and generally should be used first.
  • It usually does not cause chemical peritonitis if the patient has colonic perforation.
  • 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 CT scanning is useful to help rule out intra-abdominal abscess or other causes of ileus, it generally is not used initially in patients with large bowel obstruction unless a diagnosis has been made.
  • CT scan, 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.¹

Other Tests

• 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 roentgenogram 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.
• Although flexible endoscopy is relatively comfortable for the patient and provides a better view than rigid sigmoidoscopy, the latter also may 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.

Procedures

• Endoscopic reduction of volvulus
  • This procedure is indicated for sigmoid volvulus when peritoneal signs are absent, which would imply dead bowel or perforation. It also is indicated when evidence of mucosal ischemia is not present upon endoscopy.
  • This procedure is not indicated for the less common cecal or transverse colon volvulus.
  • An experienced person should perform the procedure.
  • A rigid sigmoidoscope may be used if a flexible instrument is not available. The endoscopist must have sufficient experience with this technique.
  • Reduction of a volvulus does not imply cure. The sigmoid usually revolvulizes if definitive treatment is not carried out.
  • These patients generally are admitted, subjected to mechanical bowel preparation, and managed surgically by sigmoid resection, unless contraindications are present.
• Barium enema for reduction of intussusception
  • This is useful and often successful in children in whom a pathological leading point for the intussusception is unlikely.
  • It should be performed by an experienced radiologist because the risk of perforation is significant.
  • In adults, typically a pathologic leading point for the intussusception is present. Success is far less likely, and patients still require surgery to deal with their pathology.
• Cleansing enemas
  • Perform these if obstipation is suspected rather than true large bowel obstruction.
  • Also perform them to prepare the distal colon for endoscopic evaluation.
• Endoscopic dilation and stenting of colonic obstruction
  • This procedure is indicated for colonic near total obstruction through which some small amount of lumen remains.
  • The procedure may be palliative in a high-risk patient with an unresectable malignancy, accepting a risk of reobstruction of the stent, or preparatory to surgical resection.
  • In cases in which the stent is deployed prior to surgery, it permits relief of the acute obstruction, resuscitation of the patient, and mechanical bowel preparation prior to a one-stage colonic resection and reanastomosis, thus avoiding temporary or permanent colostomy.
  • The procedure should be performed only by an endoscopist experienced in such procedures.
  • Surgical consultation and backup should be available, as the risk of perforation is increased during attempts at such procedures, with a potentially catastrophic result.
  • Although some experience with stenting has been positive,² with some retrospective preference for the Ultraflex stent over the Wallstent because of ease of placement, a recent multicenter trial of endoscopic stenting using the Wallstent versus surgery for stage IV left-sided colorectal cancer was terminated early because of an unacceptably high incidence of perforation.³ Whether this reflects the technical aspects of the procedure in that study, the particular stent used, or a truly unacceptable incidence of this dangerous complication awaits further study.

Histologic Findings

Histology is of minimal relevance to the acute management of a large bowel obstruction because the obstruction must be relieved regardless of the pathology. However, the histological distinction between malignant and benign causes of obstruction obviously is important for subsequent patient management.

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