Constipation Workup

Updated: Mar 30, 2020
  • Author: Marc D Basson, MD, PhD, MBA, FACS; Chief Editor: BS Anand, MD  more...
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Approach Considerations

An extensive workup of the constipated patient is performed on an outpatient basis and usually occurs after approximately 3-6 months of failed medical management. Rectal and perineal examination should already have been performed but should be repeated at this point. Anorectal tests should be used to evaluate for defecatory disorders if over-the-counter agents do not relieve the constipation. [3]

Laboratory evaluation does not play a large role in the initial assessment of the patient.

Imaging studies are used to rule out acute processes that may be causing colonic ileus or to evaluate causes of chronic constipation. In patients with acute abdominal pain, fever, leukocytosis, or other symptoms suggesting possible systemic or intra-abdominal processes, imaging studies are used to rule out sources of sepsis or intra-abdominal problems.

Lower gastrointestinal (GI) endoscopy, colonic transit study, defecography, anorectal manometry, surface anal electromyography (EMG), and balloon expulsion study may be used in the evaluation of constipation.

In the acute situation with a patient at low risk who usually is not constipated, no further evaluation is necessary. Consider sigmoidoscopy, colonoscopy, or barium enema for colorectal cancer screening in patients older than 50 years. Colonoscopy represents the current criterion standard. The role of computed tomography (CT) colography awaits further definition.

In children with refractory constipation, colonic and anorectal manometry may help in identifying a disorder of rectal evacuation or colonic motility. [20]  Those with rectal evacuation disorder may potentially benefit from biofeedback therapy or sphincter botulinum toxin. [20]

Regional colonic transit pattern does not conclusively identify evacuation disorders in constipated patients with delayed colonic transit. [21]


Laboratory Studies

A complete blood cell (CBC) count may reveal anemia that might be associated with rectal bleeding (gross or occult). Fecal occult blood should be tested in chronically constipated middle-aged or elderly adults to assess an obstructing neoplasm of the colon.

The leukocyte count is useful in patients presenting with abdominal pain or fever or providing an indication that the constipation is secondary to an ileus. This may lead to further, more aggressive evaluation.

Thyroid function tests may be helpful. Check thyroid-stimulating hormone (TSH) levels to rule out hypothyroidism in patients whose constipation is refractory to dietary management.

Serum chemistry may exclude a metabolic cause of constipation, such as hypokalemia and hypercalcemia. Determine the serum electrolyte profile, including measurements of potassium, calcium, glucose, and creatinine, in patients with recent-onset constipation to assess an acute electrolyte imbalance and in chronically constipated patients in whom the initial medical treatment has failed.



In patients with acute abdominal pain, fever, leukocytosis, or other symptoms suggesting possible systemic or intra-abdominal processes, imaging studies are used to rule out sources of sepsis or intra-abdominal problems. Order an upright chest roentgenogram and a flat and upright abdominal film. The abdominal film may reveal a colon full of stool (see the images below), confirming the diagnosis of obstipation.

Constipation. Note the large amount of stool throu Constipation. Note the large amount of stool throughout the colon on this radiograph.
Constipation. A large stool mass is visible in the Constipation. A large stool mass is visible in the hepatic flexure of the colon.

Differentiation of fecal impaction (see the images below), bowel obstruction, and fecalith (hard mass of stools) is possible. Diagnosis of fecaliths is important because of the dreaded complication of stercoral ulcers, which can lead to colonic perforation. Stercoral perforation is a rare but life-threatening surgical emergency of perforation due to pressure necrosis that can lead to peritonitis.

Constipation. This radiograph reveals colon disten Constipation. This radiograph reveals colon distention secondary to fecal impaction.
Constipation. Pseudo-obstruction secondary to feca Constipation. Pseudo-obstruction secondary to fecal impaction is shown on this radiograph.

Diabetic gastropathy, as well as fecal impaction, may be seen in patients with diabetic neuropathy. Residual barium (from barium enemas) can be visualized. Scleroderma and other connective-tissue diseases may be complicated by motor disturbances that mimic colonic obstruction on plain films. Myxedema ileus is a consequence of hypothyroidism.


Computed Tomography Scanning

Abdominal computed tomography (CT) may be indicated to further evaluate the possibility of an intra-abdominal abscess. Acute constipation in the setting of an empty rectal vault and a proximal colon that is dilated with air or stool suggests large bowel obstruction, which should be further evaluated via Gastrografin (diatrizoate meglumine–diatrizoate sodium solution) enema or lower GI endoscopy (see below). Gastrografin enema has the advantage of acting as an osmotic laxative, which may aid in the evacuation of the colonic contents.


Barium or Gastrografin Study

An air-contrast barium enema is useful for assessing the possibility of an obstructing colon cancer, intermittent volvulus, or colonic stricture in the setting of chronic constipation. A barium study is preferable to a Gastrografin study for patients who do not present with an acute process. On the other hand, a Gastrografin study is preferable for patients with an acute abdomen because it prevents the risk of extravasation of barium into the peritoneal cavity through a perforated diverticulum or colon cancer.

In patients with suspected colonic obstruction, the author prefers to use colonoscopy rather than barium enema, but either modality may suffice.



Defecography should be performed if an obstruction is suspected at the level of the anal canal. Fill the rectosigmoid with barium paste and fluoroscopically observe the act of defecation. This test may demonstrate alterations in the anorectal angle during defecation, presence of pelvic floor weakness, [17] or transient rectal prolapse or intussusception.


Colonic Transit Study

Colonic transit time should be determined in patients suspected of having a colonic motility disorder. Accomplish this by observing the passage of orally administered radiopaque markers via daily abdominal roentgenograms. Record the time taken for the passage of the markers and the site where they appear to be retained. A patient with outlet obstruction tends to retain the markers in the left colon and sigmoid, whereas a patient with colonic dysmotility may retain the markers throughout the colon.


Lower Gastrointestinal Endoscopy

Urgent lower GI endoscopy is useful in patients who are acutely constipated if large bowel obstruction is suspected based on an empty rectal vault and a distended proximal colon. Colonoscopy should not be performed if perforation or acute diverticulitis or other infectious processes are suspected because of the risk of worsening intra-abdominal contamination caused by colonic distension during the procedure.

In the acute setting, either a bowel preparation is not used or, at the most, 1-2 gentle enemas are employed.

Flexible endoscopy is generally preferred to rigid endoscopy because the former is more comfortable for the patient, provides a better view for the endoscopist, and permits access to more of the colon. Rigid endoscopy may be used in an urgent situation when flexible endoscopy is not available.

Advance the flexible endoscope into the rectosigmoid until the site of the obstruction is reached or until the splenic flexure is identified, which suggests the absence of a rectosigmoid obstruction. If the initial sigmoidoscopy reveals no abnormal findings or if the constipation is more chronic, the patient should subsequently undergo a standard oral bowel preparation and either colonoscopy (the author’s preference) or air-contrast barium enema for a fuller evaluation of the remainder of the colon.


Anorectal Manometry

Anorectal manometry documents several parameters, including the following:

  • External anal sphincter and puborectalis function

  • Reflex relaxation of the internal sphincter when the rectum is distended

  • Coordination of these muscles during the bear-down phase of defecation

  • Anorectal pressures during these events

  • The threshold at which rectal distention is perceived

Interpreting the results of anorectal manometry is complex and varies with the center performing the test. Consult a specialist familiar with the local testing facilities.


Other Studies

Controlled pressure-based rectal distention with fluoroscopic rectal imaging to measure the rectal diameter at the minimal distention pressure may be useful in identifying idiopathic megabowel in the absence of an organic cause or other problems. [22]

EMG documents paradoxical external sphincter or puborectalis spasm during defecation, consistent with the diagnosis of anismus. It is useful during subsequent biofeedback training because the patient is taught to relax these muscles.

In balloon expulsion test, a balloon filled with varying amounts of water is inserted rectally. The patient is asked to expel the balloon. Decreased ability to expel a balloon filled with 150 mL of water suggests decreased defecatory ability.

Anoscopy should be routinely performed on all constipated patients to look for anal fissures, ulcers, hemorrhoids, and local anorectal malignancy.

Deep rectal biopsy, sometimes with double or triple bite techniques, may be used to diagnose Hirschsprung disease.

Histologic findings include the histology of any obstructing colonic lesion (eg, neoplasms, strictures from Crohn disease, diverticulitis, or ischemia) and the agangliosis of Hirschsprung disease.