Sagittal section of pelvic anatomy demonstrating pelvic musculature and puborectalis “sling” around the sphincters. (Used with permission of Mayo Foundation for Medical Education and Research. All rights reserved)
The anorectum is the terminal portion of the large intestine. The rectum is approximately 15 cm in length with the anal canal (2.3–3.5 cm) occupying the most distal segment and surrounded by internal and external sphincters.
The internal anal sphincter is an extension of the circular smooth muscle surrounding the rectum. It is an “involuntary” structure supplied by parasympathetic nerves. The external anal sphincter is a “voluntary” structure innervated primarily by S4 through the inferior rectal nerve. The external sphincter blends into the more proximal puborectalis muscle (Fig. 35.2).
Anatomic juxta position of internal anal sphincter (ani internus) and external anal sphincter (ani externus) surrounding the rectum (Sagittal view). (Used with permission of Mayo Foundation for Medical Education and Research. All rights reserved)
Rectal distention is perceived by the patient as a localized “fullness” associated with the urge to pass flatus or defecate. The anal canal, on the other hand, from the proximal dentate line (mucocutaneous line) is exquisitely sensitive to light touch, pain, and temperature.
Physiology of Defecation/Continence
During defecation stool is transferred into the rectum by a series of propagated colonic contractions. Distention of the rectum subsequently prompts relaxation of the internal anal sphincter and the sensation of fullness. There is an associated urge for relief with voluntary contraction of the puborectalis and external sphincter muscle.
Anorectal continence (stool retention) is a combination of competent anosphincter function, anal sealing caused by distal anovascular tissue and maintenance of the angle formed between the rectum and sigmoid by the levator ani and puborectalis and external sphincter muscle.
Perineal Inspection/Anorectal Exam
Valuable information concerning the patient’s defecatory problem can often be obtained by inspection and digital rectal examination of the patient. The appearance of scars, fistula tracks, asymmetry or “bulging” of perineal fold, rectal tissue eversion or prolapse, prominent hemorrhoids or perianal excoriations can provide evidence of defecatory problems. Visualization of perineum during a straining maneuver may demonstrate excessive pelvic descent (>3 cm) or bilateral gluteal infolding. This observation of a paradoxical “squeeze” while supposedly straining is tantamount to a diagnosis of dyssynergia !
Four quadrant testing of perineal sensation with a cotton Q-tip should elicit an anocutaneous reflex (“anal wink ”) attesting to perineal nerve function.
The digital rectal exam, performed with the gloved finger, adequate lubricant and with the patient in the left lateral posture can assess the sphincteric resting tone and squeeze pressure. The strength of the puborectalis sling pressure and sensation can be demonstrated during a “squeeze” maneuver. The presence of a rectocele or mass can be detected and evaluation of the prostate is essential in the male patient.
The majority of patients with defecatory problems do not require a battery of sophisticated tests to answer their difficulty. A detailed history, physical exam, and endoscopic inspection of the colon may be all that is needed in eliminating a structural cause for the patient’s problem and directing appropriate conservative treatment.
Patients with more severe difficulty with elimination may be candidates for clinical testing to help elucidate an answer. It should be noted at the outset that the specificity and sensitivity of any one test currently in use in clinical practice to detect the etiology of a defecation disorder is either unsubstantiated or questionable.
This test of anorectal function and sensation involves placement of a small catheter containing a distal balloon into the patient’s rectum. Two Fleets enemas prior to the test are usually sufficient to cleanse the rectum but the preliminary digital exam will verify this fact.
Currently a solid-state probe with high-resolution manometry microtransducer is the recording instrument of choice. One version contains 12 circumferentially placed sensors which permits radial pressure recording over a 2 cm length of the anal canal . An additional two sensors placed proximal record pressure within the rectum and within the small balloon. The resulting pressure averages can be displayed as both an isobaric contour plot as well as a basic pressure profile (Fig. 35.3).
High-resolution solid-state manometry (isobaric contour plot) display demonstrating external sphincter contraction during to ansient and sustained squeeze (top)
The anorectal manometry test assesses internal and external sphincter function, rectal sensation, anorectal reflex, and rectal compliance. The following data points are determined from the study.
Resting (basal) Sphincter Pressure .
This pressure measures predominantly (70%) internal (involuntary) sphincter tone.
The amplitude/duration of maximal external (voluntary) sphincter and puborectalis contraction is recorded during brief voluntary squeezing and during a sustained one-minute contraction. The influence of coughing on external sphincter contraction provides a maximal “provoked” measurement for comparison with the patient’s voluntary effort.
The patient response to incremental increases in rectal balloon volumes (up to 150 ml depending on patient tolerance) is recorded for an initial “sensation” and subsequent “urge” sensation felt by the patient.
Anorectal Inhibitory Reflex
The internal anal sphincter relaxes at a point during graded volume inflation of a rectal balloon. This demonstrates the integrity of the myenteric plexus communication between the rectum and anal canal. This reflex is classically absent in Hirschsprung’s disease .
Rectal Balloon Expulsion.
This test monitors simulation defecation by expulsion of a 50 ml rectal balloon over a period of 1 min. The test is best performed with the patient seated on a toilet facility. The ratio of the intraabdominal pressure vs. the relaxation pressure of the pelvic floor and external sphincter required to expel the balloon provides a value called the defecatory index.
Pressure–volume relationship between the rectal wall and graded balloon volumes approximates a measure of rectal compliance.
Clinical Usefulness of Anorectal Manometry Test
Anorectal manometry is most useful in determining a pattern of obstruction defecation as the cause for the patient’s distress .
Normally during simulated defecation there is an increase in intrarectal pressure associated with relaxation of the anosphincter. In many patients an obstructive defecation pattern is characterized by inadequate relaxation or contraction of the pelvic floor during simulated defecation. The term dyssynergia has been used to describe this apparent paradox. Several reproducible variations between the rectal and anorectal pressures have been recorded in patients with obstructive defecation disorders defined as dyssynergia .
A recent report in a large population of woman with defecatory disorders described three manometric patterns indicative of dyssynergia (hypertensive anosphincter (basal) pressure (Fig. 35.4); low rectal pressure or a hybrid) . These findings correlated with abnormal rectal balloon expulsion times.
Defecography image showing the anorectal angle at rest (76.2° angle)
No one test result is sufficiently predictive to diagnose dyssynergia however. For example, contraction of the anosphincter during simulated defecation has been observed in 20% of patients without apparent problems with defecation. While unsuccessful passage of a rectal balloon is important for the diagnosis of dyssynergia, successful expulsion does not obviate the diagnosis either. For this reason, the positive results from several tests are necessary to fulfill the diagnosis of dyssynergia. In our laboratory, we require three abnormal test results to define the criteria for dyssynergia, i.e., obstructive features on manometry, inability to expel a rectal balloon, and an abnormal defecatory index (<1.2%). Some authorities also require an abnormal defecography study to complete the criteria for dyssynergia.
Defecography is a radiologic examination observing the rectal expulsion of barium paste by the patient during simulated defection . The anorectal angle at rest and during straining can be measured (Figs. 35.5 and 35.6). The perineal descent is estimated and contrast emptying noted. In addition to evaluating rectal and pelvic floor dynamics structural abnormalities can be detected, e.g., rectocele, enterocele, rectal prolapse, and megarectum. Recently defecography has been performed during real-time magnetic resonance imaging. This technique provides more information concerning sphincteric and soft tissue visualization but is more costly. Defecography provides important information about these pelvic structural abnormalities which might benefit from surgical therapy. An adequate defecography study requires an interested and experienced radiographer and cooperative patient