The anorectum has similar functions as the bladder and urethra: storage and emptying. To maintain continence, the anorectum must deal with solid, liquid, and gaseous contents. Continence is also dependent on proximal gut motility, the distensibility of the rectum, and the function of the anal sphincter complex (Figure 11-1). In addition, continence relies on intact sensory and motor including somatic and visceral systems, located in the anatomically correct position. Finally, the process of defecation not only depends on these systems to function independently, but requires appropriate interactions between them in order to provide socially appropriate, coordinated, and complete expulsion of fecal contents.

Once food that has been ingested is deposited through the ileocecal valve into the colon, transit through the colon is approximately 35 hours; stool consistency and colonic motility are correlated. Propagation of fecal matter into the distal rectum produces a sensation of rectal fullness once a certain volume has been achieved. An intact and tonically contracting levator ani muscle, particularly the puborectalis portion, angulates the rectum with respect to the anal canal. This anorectal angle helps transmit elevated intra-abdominal pressure (cough, sneeze, etc) across the rectum closing the lumen, rather than directly into the anal canal.

Rectal distention from a propagated stool bolus leads to a reflex, temporary relaxation of the anal canal to allow the high density of nerve endings and sensory cells in the anal epithelium to “sample” the stool contents. This information is transmitted via sensory nerves to the brain, and the decision to defecate or not is made. If it is not time to defecate, the individual can enhance the resisting pressure in the anal canal and pelvic floor by contracting the striated muscles of the external anal sphincter (EAS) and the puborectalis muscles (PRM).

If defecation is chosen, squatting helps to open the anorectal angle, thereby straightening the rectum-to-anus axis. As in the urethra/bladder system, stool will be expelled when the pressure in the rectum exceeds the pressure in the anal canal. A Valsalva maneuver, potentially accompanied by rectal contraction, increases rectal pressure. A coordinated inhibition of the tonic contraction of the striated EAS and smooth muscle internal anal sphincter (IAS), as well as relaxation and descent of the pelvic floor musculature, decreases the pressure at the outlet. Stool is then propelled from the body. Once defecation has been completed, a closing reflex increases the contractility of the muscles.

Investigations into the function or dysfunction of the anorectal system should consider all of the above aspects.

Direct Visualization

Endoscopy

Alterations in defecation and continence can be related to structural abnormalities of the large colon as well as the anorectum, including neoplasms. Therefore, direct visualization of the colonic mucosa may be required, and colonoscopy or flexible sigmoidoscopy with biopsies as needed should be performed when appropriate. The methodology for these procedures is beyond the scope of this textbook.

Anoscopy

The anorectum can be directly observed with an anoscope (Figure 11-2). An anoscope is a two-piece metal or plastic device used much like a vaginal speculum. Like vaginal speculums, they come in different sizes (typically 7–13 cm long with tapered apertures approximately 1.5–3 cm in diameter).

Clinical Utility

The presence of masses, fistulas, hemorrhoids, and even rectal prolapse can be diagnosed with this simple tool.

Technique

Anoscopy is easily performed as an extension of the physical examination. The two-piece assembly has a rounded obturator that fits inside the hollow portion. A generous amount of lubricant is used and the device is gently inserted until the base reaches the anoderm. The central obturator is then removed, allowing the mucosa to be visualized using an external light source.

Imaging

Multiple imaging modalities are used to assess pelvic floor structures, muscle function and integrity, and dynamic function of the pelvic floor. Some of these modalities are well-established part of clinical practice including endoanal ultrasound and fluoroscopy; some are newer methods of pelvic floor investigation and active area of research that are slowly finding their way into clinical practice including pelvic floor magnetic resonance imaging (MRI), dynamic MRI, MRI proctography, and pelvic floor two-dimensional (2D) and three-dimensional (3D) ultrasound. The following sections will discuss most commonly used anorectal static and dynamic imaging modalities.

Static Imaging

Colonic Motility (Sitzmark) Study

When evaluating patient with constipation or infrequent defecation, a useful assessment of colonic transit time is by marker ingestion studies. Plastic radio-opaque markers are ingested and x-ray follows their intestinal transit. Most healthy adults pass all markers in four to five days. This method is simple, inexpensive, repeatable, and reliable in assessing colonic transit. Radio-opaque marker studies are currently considered the gold standard for transit time studies.

Techniques. Multiple techniques for radio-opaque plastic marker studies have been described. In all techniques the patients are instructed on strict avoidance of any agent that may affect colonic motility including laxatives and enemas. With all techniques, the markers are swallowed (either all at once or over a course of few days) and then marker distribution and location are identified in the colon with either x-ray series or a single x-ray several days later. The simplest method is that 20 markers are swallowed on day 0 with the follow-up x-ray taken on day five. In patients with normal colonic transit, 80% of markers should be expelled by day five. Other methods have patients taking differently shaped markers on days 0, 1, and 2, with serial radiographs taken on days three and five (Figure 11-3).

Interpretation and Clinical Utility. Findings of residual markers in proximal portions of the ascending, transverse, and descending colon are consistent with slow transit constipation. The findings of markers trapped in the rectum after normally transiting through the proximal colon are consistent with outlet obstruction constipation. Frequently, patients with chronic constipation will have normal colonic transit marker studies and other pathophysiologic explanation should be sought.

Ultrasound

Endoanal Ultrasound. Anal sphincter complex structures can be visualized with multiple imaging modalities. Endoanal ultrasound has become the gold standard to evaluate the anal sphincter complex and can assist in the evaluation of fecal incontinence.

Clinical Utility. The most common clinical use of endoanal ultrasound is in assessment of the anal sphincter integrity in patients with fecal incontinence. Endoanal ultrasound is useful in surgical planning in patients with suspected anal sphincter defects. Furthermore, endoanal ultrasound can be an adjunct study in evaluating anorectal fistulas, abscess, and carcinoma.

Technique. No special preparation is required for the study; however, an empty rectum is usually preferred, and thus an enema prior to the study can be beneficial. An ultrasound machine equipped with an anorectal transducer (as in Brüel and Kjær, BK Medical) is required. The anorectal transducer is of high frequency (7 mHz or higher) with 360° panoramic view. The ultrasound is usually performed with patient in either supine in the lithotomy or left lateral position, which can facilitate the complete imaging of the anterior part of the sphincter complex as well as the perineum. The standard orientation for the ultrasound images is with 12-o’clock positioned at the anterior anal canal, which is used for cross-sectional imaging of the entire anal sphincter. Ultrasound imaging is done with either live 2D technique or 3D ultrasound volumes that can be examined offline.

Normal Structure Assessment. Using the endoanal technique four anatomic layers are identified (Figure 11-4). The IAS thickness is approximately 1.4 to 2.7 mm, whereas the EAS is approximately 5.4 to 7.4 mm thick. The IAS is not seen on the most distal images of the anal canal where only anal mucosa and the EAS are seen.

In the cephalad (proximal) region of the anal canal, the deep portion of the external anal sphincter (EAS) is continuous with the PRM. The PRM is a sling of mixed echogenicity that loops posteriorly around the anal canal. The anterior portion of the PRM that originates at the pubic rami cannot be fully visualized by endoanal ultrasound imaging.

Interpretation. Anal endosonography can detect anal sphincter defects with greater than 90% sensitivity and specificity. Anal sphincter defects are seen as thickening and changes of echogenicity or asymmetry along the sphincter, described based on a clock face where 12 o’clock is at the anterior position. Defects in the EAS appear hypoechoic, whereas defects in the IAS appear hyperechoic (Figure 11-4). Studies have shown very good intraobserver and interobserver agreement on recognition of sphincter defects using endoanal ultrasound.²

Transperineal/Translabial Ultrasound. Transperineal or translabial ultrasound assessment of the anorectum may offer a more detailed evaluation of the entire anal sphincter complex including anal mucosa, IAS, and EAS,^3,4 and the entire puborectalis muscle.

Clinical Utility. Transperineal ultrasound is a promising tool in assessment of anorectal dysfunction; however, this technique is used only in few centers and lacks evidence-based data that limit its widespread clinical use. Recent data suggest that injury in the puborectalis or pubovisceral muscles can result from vaginal delivery and lead to pelvic floor dysfunction. Transperineal and translabial pelvic floor ultrasound is an active area of research with promising future clinical utility in imaging the pelvic floor musculature.

Technique. A high-resolution transabdominal or endovaginal transducer is placed on labia majora or perineum and is used to evaluate anal sphincter complex with either 2D or 3D US volumes. The use of transperineal ultrasound allows for detailed assessment of the PRM and the pelvic floor hiatus (Figure 11-5).

Magnetic Resonance Imaging (Static or Endoanal MRI). MRI offers a detailed assessment of pelvic floor structures and can be used as adjunct imaging of anal sphincter structures in patients with fecal incontinence. As a method of global pelvic floor imaging, MRI offers a detailed assessment of levator ani complex. Defects in the levator ani muscles have been found in women after vaginal delivery and reported at higher prevalence in women with fecal incontinence and prolapse. In addition, a recent MRI study reported higher prevalence of levator ani muscle defects in women with anal sphincter tears during delivery.⁵

Clinical Utility. Endoanal MRI is comparable to endoanal ultrasound in assessment of anal sphincter defects.⁶ In most studies, MRI is found either equivalent or slightly inferior to endoanal ultrasound in anal sphincter defects detection. Some investigators believe that MRI is better than 2D endoanal ultrasound in assessing EAS atrophy in patients with fecal incontinence; however, a recent study showed equivalent detection of atrophy between MRI and 3D endoanal ultrasound.⁷ Identifying defects in levator ani muscles is important in elucidating the etiology of pelvic floor dysfunction; however, the clinical utility of MRI findings are not well defined.

Technique. Static MRI or endoanal MRI is performed with the use of an endoanal coil to achieve high–resolution multiplanar analysis of anal sphincter structures. The anal coil is placed in the anal canal. The images are obtained in axial plane with T2-weighted fast spin and T1-weighted spin sequences. On T2-weighted axial images, the IAS is seen as a sharply defined hypersignal ring and EAS appears as relatively hyposignal ring (Figure 11-6). In addition, coronal images show the structures from the anal verge to where the EAS meets the levator ani. Images can also be taken in the puborectalis plane, which enables visualization of the IAS, EAS, as well as detailed assessment of the levator ani. Muscle defects can be seen as disruption of the anal sphincter ring or hyposignal scarring, whereas atrophy appears as thinning or fatty replacement.

Dynamic Imaging

Fluoroscopic Evacuation Proctography (Defecography)

Fluoroscopic evacuation proctography, also known as defecography, is a morphologic and functional examination of the anorectum and the pelvic floor, which allows for real–time physiologic assessment of defecation.⁹

Clinical Utility. Defecography allows for evaluation of dynamic changes of the perineum and the pelvic floor, coordination, and morphology of defecation. Defecography is indicated in patients with constipation when outlet obstruction is suspected. The outlet obstruction defecation could be caused by functional disorders including hypertonic pelvic floor with nonrelaxing PRMs (dyssynergia), or anatomic disorders including posterior vaginal wall prolapse (rectoceles, enteroceles), or intussusception.

Technique. No special preparation for the study is required; however, it is preferable that the rectum is empty. Most of the time use of glycerin or dulcolax suppositories prior to the study would suffice. Rectal opacification is required for the study and is achieved by rectal instillation of barium paste with consistency approximating stool. Small bowel opacification is achieved by ingestion of dilute barium suspension prior to study. Most investigators recommend use of vaginal opacification with mixture of contrast and ultrasound gel. If visualization of the bladder is clinically indicated to assess anterior vaginal wall prolapse, sterile water-soluble contrast is instilled into the bladder (cystodefecography). Prior to start of the defecation phase of the study an upright lateral view is obtained to localize bony landmarks and check for opacification of the vagina, small bowel, and rectum. The patient is seated on a radiopaque commode for filming the dynamic process of defecation. Dynamic images are obtained at rest, and again while the patient is instructed to squeeze, and then finally to strain and defecate.

Interpretation. Defecography is analyzed in three phases: rest, evacuation, and recovery. The assessment of anorectal angle at rest with contraction and strain and attempted defecation is measured; however, the normal range varies widely. Additional findings of perineal descent and dynamic relation of the opacified vagina, small bowel, and bladder can be characterized. In patients with dyssynergia, the anorectal angle does not widen and may in fact narrow with an attempted defecation (Figure 11-7). Defecography can identify rectal mucosal intussusception, rectocele, and rectal prolapse.

Ultrasound

Dynamic ultrasound with use of transperineal or translabial approach is a promising technique to assess posterior vaginal compartment or rectoceles. This technique can be used along with the static transperineal/translabial imaging of the anal sphincters and puborectalis/pubovisceralis muscles where patients are instructed to strain. With strain the descent of the posterior vaginal wall and the rectum can be seen. The assessment can be performed with 2D and live 3D assessment (4D). The clinical use of this technique is still limited to few centers.^10,11