Laboratory Tests and Ancillaries
Routine colon cancer screening tools (eg fecal occult blood test) are recommended in all patients ≥50 years of age. In patients presenting with constipation without alarm symptoms, there is not sufficient data to make recommendations for routine use of colonoscopy, flexible sigmoidoscopy, barium enema, thyroid function tests, serum calcium, etc.
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A complete blood count may be done to evaluate chronic constipation in the absence of other signs and symptoms. Specific metabolic diagnostic tests (eg thyroid and renal function tests, fasting blood sugar, calcium) may be performed in patients with additional signs or symptoms of an organic disorder.
Imaging
In a patient who presents with alarm symptoms or is ≥50 years of age, objective testing with blood biochemistry, imaging studies or colonoscopy is needed to verify the diagnosis, exclude organic disease, identify underlying pathophysiology in refractory cases, and determine corresponding treatment.
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An abdominal X-ray can identify complications related to constipation (eg fecal impaction, pneumoperitoneum from stercoral perforation, colonic pseudo-obstruction) and is useful in distinguishing colonic volvulus and intestinal obstruction. Flexible sigmoidoscopy and colonoscopy can identify lesions that narrow or occlude the bowel. Colonoscopy is preferred in patients with iron-deficiency anemia, a positive guaiac stool test, or a first-degree relative with colon cancer. Computed tomography (CT) colonography, a less invasive alternative to colonoscopy, has a sensitivity similar to or better than that of double-contrast barium enema in detecting polyps and colon cancer.
If extracolonic and mechanical causes of constipation are ruled out with lab tests and colorectal imaging, then a complete physiologic evaluation may be useful, although the interpretation should be guarded as patient cooperation is critical. Examples are anal manometry, balloon insertion, defecography and colonic transit studies. Two or more abnormal imaging and anorectal tests are required to diagnose a defecatory disorder.
Specialized Physiologic Tests
Specialized physiologic tests are recommended for patients with functional constipation who are unresponsive to laxative treatment for at least 12 weeks or are highly suspected of having a defecatory disorder.
Anorectal Manometry with Balloon Expulsion Test (BET)
Anorectal manometry with balloon expulsion test provides measurement of internal and external anal sphincter pressure, relaxation of the internal anal sphincter during rectal distension and straining, and rectal sensation to distension in patients with chronic constipation resistant to medical therapy. This may be the test of choice for outlet obstruction, pelvic floor dysfunction and for excluding Hirschsprung’s disease and psychogenic megacolon, may even be considered before a trial of laxatives in patients with a strong suspicion of pelvic floor dysfunction. This is recommended for patients unresponsive to a high-fiber diet and/or simple laxatives. Patients with normal findings may be treated with a secretagogue or prokinetic agent. The expulsion time of >1-3 minutes is generally considered an abnormal test result and suggests a defecatory disorder. Confirmation with other physiological tests is required to establish a firm diagnosis.
Colon Transit Studies
Radiopaque markers are used to measure GI transit. Delayed transit is confirmed with an abdominal radiograph showing retention of >20% of the markers after 4 days. This differentiates slow transit constipation from a defecatory disorder. This is considered when an outlet obstruction is not demonstrated by clinical and proctologic exams. This is performed on patients with symptoms unresponsive to laxatives or first-line pharmacological treatment or if an anorectal test does not demonstrate a defecatory disorder.
Segmental colon transit time (CTT) differentiates the subtypes of chronic constipation as follows:
- Delayed right CTT: Colonic inertia or slow transit constipation
- Delayed left CTT: Hindgut dysfunction
- Delayed rectosigmoid CTT: Pelvic outlet obstruction
Other approaches include scintigraphy and a wireless pH-pressure capsule, which can measure both gastric emptying time and small intestinal transit.
Defecography
Defecography may be done if results of anorectal manometry and rectal balloon expulsion are inconclusive or if patients have persistent symptoms after biofeedback therapy. This may be performed with anorectal manometry in patients with suggestive obstructed defecation. This test reveals structural posterior compartment abnormalities (eg rectocele, rectal prolapse, enterocele and intussusceptions) and evaluates mobility of the pelvic floor and degree of rectal emptying in patients suspected of having an evacuation disorder. Pelvic floor dysfunction is diagnosed by the observation of insufficient descent of the perineum and less-than-normal change in the anorectal angle. Barium defecography is considered in assessing structural rectal abnormalities and posterior compartment disorders.
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Magnetic resonance (MR) defecography can evaluate all pelvic compartments in patients in whom multicompartmental structural defects are suspected and can visualize anatomic landmarks for measurement of pelvic floor mobility. Echodefecography or MR defecography may be preferred over video defecography in order to avoid ionizing radiation. This test is operator dependent and has poor reliability.
Electromyography of Anal Sphincter
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Electromyography of the anal sphincter is used in diagnosing the presence of paradoxical contraction of the puborectalis muscle.
Hydrogen Breath Test
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The hydrogen breath test evaluates orocecal transit time in
patients with colonic inertia.