Specialized Testing for Constipation
Specialized testing is reserved for patients with chronic constipation that is severe or difficult to treat. All such patients should undergo colonoscopy or barium enema to exclude an anatomic cause as well as the basic series of blood tests to exclude abnormalities that can cause constipation. If these studies are normal, a series of additional investigations should be undertaken.
Radiopaque marker study – The simplest and most readily accessible test is a radiopaque marker study to evaluate colonic transit. This involves ingesting 24 small markers treated so that they will be visible on an x-ray; these are commercially available prepackaged in a gelatin capsule. Methods vary, but in our practice an abdominal x-ray is taken on day 5. Retention of greater than 20% of the ingested markers is indicative of delayed transit. Prior to and during the test period, patients are instructed to avoid laxatives, enemas, and any medication that might interfere with normal bowel function.
A normal marker study, nonetheless, does not exclude a diagnosis of functional outlet obstruction. Similarly, a prolonged transit study result does not prove that the colon is the sole source of the delay; impaired emptying of the stomach or slowed small bowel transit can each contribute to an abnormal result.
Colonic scintigraphy – Colonic transit can also be measured using colonic scintigraphy. Patients ingest a meal containing a small amount of a radioactive substance (radioactive isotope) and images formed are recorded at specified time intervals using a special camera. During the next 24–48 hours, this test shows what proportion of the ingested meal has progressed through the colon. However, in contrast to the universally available marker study, colonic scintigraphy is generally available only in specialized centers.
Anorectal manometry – An anorectal manometry test is frequently used in the evaluation of patients with suspected pelvic floor evacuation difficulties.
Manometry can be used to measure resting and squeezing anal sphincter pressures, rectal sensation, and sphincter response. Anal canal resting and squeeze pressure can be normal, high, or low. The test can provide one indication of paradoxical sphincter contraction if anal pressure rises, rather than lowers, as the patient attempts to bear down.
Manometry is also used to assess rectal sensation; inflating a balloon in the rectum and recording the volumes associated with initial sensation can assess the urge to defecate and the feeling of uncomfortable fullness.
Additionally, manometry is used to confirm the presence of a normal rectoanal inhibitory reflex, which is relaxation of the internal anal sphincter in response to balloon distension in the rectum. This reflex is absent in patients with Hirschsprung’s disease, or with Chagas’ disease (a tropical disease caused by a parasite that destroys the rectal nerves).
Rectal emptying tests – The ability of the rectum to empty can be assessed in a number of ways. Failure of the sphincter to relax during attempted defecation is the most important cause of functional outlet obstruction.
Inappropriate contraction (or non-relaxation) of the anal sphincter mechanism can be diagnosed with manometry, defecography, and electromyography (EMG) – a test that uses electrodes to assess the health of muscles and the nerves controlling the muscles. Each of these techniques documents sphincter contraction during a straining effort. However, the results of these individual studies do not always agree with each other, so the results must be interpreted with caution. In addition, some individuals fail to understand the test instructions or are unable to relax in the laboratory setting, either of which situation can lead to a false positive result.
Rectal emptying can be accessed directly by the balloon expulsion test and by defecography.
The balloon expulsion test in its simplest form involves inflating a rectal balloon with 50 ml (about 2 ounces) of water or air and asking the patient to expel it into a toilet. In some centers balloon expulsion tests are performed,while the patient is lying down on their left side, with the addition of weights to the balloon to help quantify the severity of the defecation disorder.
Defecography is a dynamic x-ray test in which the patient’s rectum is filled with barium paste. The individual is seated on a commode and is asked to expel the rectal contents. X-ray images of the attempted defecation are recorded on videotape and evaluated. This test demonstrates both the ability of the sphincter mechanism to relax and the ability of the rectum to empty. In addition, the test is able to diagnose such associated conditions as internal intussusception (a condition in which the intestine or rectum folds into itself in a telescope fashion, causing obstruction), rectal prolapse, and rectocele.
At the end of the evaluation, the physician should have enough information to assign the patient’s condition to the appropriate diagnostic subgroup: normal transit constipation, slow-transit constipation, or defecation disorder (functional outlet obstruction). In addition, some patients may have an overlap syndrome, such as slow-transit constipation with functional outlet obstruction.