Anatomy of the Perineal Body, Distal Vagina, Rectum, and Anus

Perineal body defects and rectovaginal fistulas (RVFs) are part of a spectrum of conditions that impact the posterior pelvic floor. The surgical interventions used to address these defects must be based on a clear understanding of the anatomic structures involved. A postobstetric pinhole fistula may occur in isolation (i.e., the perineal body and anal sphincters are completely intact) ( Fig. 33.1 ); or maybe only the tip of an iceberg in which there is a significant defect of the entire perineal body and anal sphincters ( Fig. 33.2 ). Surgical management of these various defects requires a clear understanding of the anatomy of the distal portion of the posterior vaginal wall, the perineal body, the anal sphincters, the rectum, and anal canal.

Small rectovaginal fistula with intact perineum and anal sphincters.

Large rectovaginal fistula with complete perineal breakdown.

The surgical anatomy of the posterior pelvic floor remains an area of controversy among surgeons and anatomists. In recent years, there has been a clearer understanding of the anatomy of the posterior vaginal wall and perineum, however, surgical studies in this area continue to use terms that are not anatomically based. Historically, the tissue that lies between the posterior vaginal wall and anterior wall of the rectum has been termed rectovaginal or (Denonvilliers) fascia. Histologic studies have noted that what has previously been termed fascia is actually vaginal muscularis ( ). At the level of the mid vagina, histological assessment of the posterior vaginal wall from the lumen of the vagina to the lumen of the rectum notes the following layers; vaginal epithelium, the lamina propria of the vagina, fibromuscular wall of the vagina (smooth muscle cells, elastin, and type II collagen) adventitia, outer muscular wall of the rectum, inner muscular wall of the rectum, lamina propria of the rectum, and rectal mucosa. discussed posterior vaginal wall anatomy in a review article and likened it to an open container. The front wall of the container is formed of the posterior vaginal wall whereas the bottom of the container is made up of the perineal body and anal sphincters. The levator ani muscles form the lateral sides of the container and the levator plate, where the muscles decussate behind the rectum to create the iliococcygeal raphe, form the back wall of the container. The uppermost portion of the container would then be created by the attachment of the posterior vaginal wall to the uterosacral ligaments, which extend below the peritoneum. All of these boundaries are subject to defects that can give rise to different structural failures.

The perineal body has been described as the central tendon between urogenital and anal triangles ( Fig. 33.1 ). It contains interlacing muscle fibers from the bulbospongiosus and superficial transverse perineal muscles as well as the anterior portion of the external anal sphincter. There’s also contribution from the longitudinal rectal muscle and medial portions of the puborectalis muscles.

Normally, the anus appears externally as a closed anterior/posterior slip with its lateral walls closely opposed. Damage to the surrounding sphincter mechanism may result in what has been termed a patulous anus ( Fig. 33.2 ). The anus is anchored anteriorly by the perineal body and posteriorly by the anococcygeal ligament to the coccyx. The anal canal typifies the meeting of structures with an endodermal and ectodermal derivation. The dentate line demarcates the boundary between the structures of the inferior hind gut and those of the proctodeum. The columns of Morgagni denote longitudinal folds of rectal mucosa that end as anal valves at the level of the dentate line. There are anal crypts present between the valves most notably clustered in the posterior anus. Obstruction of these crypts can give rise to infection that may result in abscess or a fistula.

The anal sphincter apparatus involves the internal and external sphincters as well as the conjoint longitudinal muscle that encircles the anus. These structures play a crucial role in fecal continence. The internal anal sphincter is a smooth muscle that is described as a dense continuation of the inner circular layer of the rectum that extends approximately 2.5 to 4 cm beyond the rectum. It arises at the junction of the anus and the rectum (anorectal ring) and ends approximately 1 to 1.5 cm distal to the dentate line. In contrast, the larger external anal sphincter is an elliptical band of skeletal muscle circumscribing the entire length of the anal canal. This structure makes up the majority of the perineal body from its anterior portion. Superiorly, the external anal sphincter continues as the puborectalis muscle. Many anatomists and surgeons believe it should be considered a component of the puborectalis muscle, even though the two muscles have separate embryologic derivations and their muscle fibers do not intermingle ( Figs 33.3 ).

The perineal body is the central point between the urogenital and anal triangles. It consists mainly of interlacing fibers from the bulbospongiosus muscle, the superficial transverse perineal muscle, and the external anal sphincter. There are also contributions from the longitudinal rectal muscle and the medial fibers of the puborectalis muscle.

Widened vaginal hiatus, patulous anal opening, and complete loss of perineal body with significant retraction of the ends of the anal sphincter.

Classification and Presentation of Perineal Breakdown and Rectovaginal Fistula

Perineal tears are most commonly classified in the following fashion: first-degree is a laceration of the vaginal-epithelium or perineal skin only; second-degree is first-degree with involvement of the perineal muscles and fascia, but not the anal sphincters; third-degree is disruption of the skin, mucous membrane, perineal body, and anal sphincter muscles; and fourth-degree is a third-degree tear with disruption of the anal mucosa. The biggest problem with this classification is that it does not incorporate the depth of the external sphincter rupture or involvement of the internal sphincter. If a third-degree tear is incorrectly classified as a second-degree tear, inappropriate repair could result in suboptimal outcomes. has thus proposed that third-degree tears be subclassified into less than 50% thickness of the external sphincter torn, greater than 50% thickness of the external thickness torn, or internal sphincter also torn.

RVF is a congenital or acquired tract between the rectum and the vagina. The communication is lined with epithelium and may occur at any point along the vagina. Most fistulas actually arise in the anal canal distal to the pectinate line. RVFs are classified according to their location and size; careful attention to both features allows determination of the approach for surgical repair. In a low RVF, the rectal opening is located close to the dentate line, with the vaginal opening just inside the hymen. In a high RVF, the vaginal opening is near the cervix (or apex of the vagina in a post-hysterectomy patient); the communication into the intestinal tract may be located in either the sigmoid colon or rectum. These fistulas usually require a laparotomy for repair. Such fistulas may not be readily apparent on physical examination or endoscopy and may require contrast studies for diagnosis. A mid-RVF is found somewhere between the hymen and the cervix. RVFs range in size from tiny (<1 mm in diameter) to large where the rectovaginal defect encompasses the entire posterior vaginal wall. In using size as a criterion, fistulas <2.5 cm in diameter are considered small and those greater are described as large. Simple RVFs consist of small, low fistulas secondary to infection or trauma. These fistulas generally have healthy, well-vascularized surrounding tissue that can be repaired with local techniques. RVFs are considered complex if they are large (>2.5 cm) high, or caused by inflammatory bowel disease (IBD). Recurrent fistulas are also considered complex because of their association with tissue scarring and decreased blood supply. Because healthy well-vascularized tissue needs to be introduced after resection of diseased tissue, complex fistulas require more complicated surgical procedures for repair.

Another method of classification is based on the underlying cause of the fistula, which will be a better predictor of the ultimate success of the repair, as it takes into consideration the integrity of the local tissue and the health of the patient.

A patient with a perineal breakdown may be asymptomatic or present with an array of symptoms, including pain and dyspareunia, a gaping introitus, and a variety of defecatory symptoms, including fecal incontinence if the sphincter mechanism is involved. A patient with an RVF is usually symptomatic. She most often complains of passage of flatus or stool through the vagina. Occasionally, the presenting complaint is a recurrent vaginal or bladder infection, which is the result of fecal soilage. A small fistula may be symptomatic only when loose or liquid stool is passed. Determining the status of the anal sphincter mechanism is important when the patient’s complaints are consistent with fecal seepage.

Etiologies of Rectovaginal Fistula and Perineal Breakdown

Although the majority of cases of perineal breakdown are postobstetrical events, many different causes of RVFs have been identified ( Box 33.1 ); the cause varies with the location of the fistula. Congenital RVFs are rare and are not discussed here.

Diagnosis and Preoperative Evaluation

During the history-taking process, determining whether there is a previous history of anorectal surgery, complicated vaginal deliveries, radiation therapy, or IBD is important. Determining the patient’s degree of continence is also important. The perineum and anus should be inspected and palpated. A bidigital examination is performed to palpate the thickness of the perineal body; the majority of RVFs will be appreciated during this maneuver. If a fistula is expected and its location is not obvious with inspection and palpation, a careful vaginal speculum examination should be performed. Rigid proctoscopy may give information regarding the compliance of the rectum and health of the surrounding tissue. If necessary, the vagina can be filled with water, and the site of the fistula will show the escape of air bubbles. Also, a vaginal tampon can be placed after instilling methylene blue in the rectum. The tampon is withdrawn and inspected for blue staining after 15 to 20 min. If the previously described maneuvers still do not demonstrate a fistula, the fistula may be located in the upper rectum, and contrast studies are needed to establish the diagnosis. Vaginography with a water soluble contrast medium has a sensitivity of 79% to 100% ( ). A barium enema is not as sensitive in identifying the fistula but may provide general information as to the health of the colon. A computed tomography scan of the abdomen and pelvis, using gastrointestinal contrast, may be helpful because it may show contrast in the vagina.

Recently, magnetic resonance imaging (MRI) has been evaluated as a diagnostic tool for RVF. , in a retrospective review, noted MRI images correctly identified an RVF in 20 female patients with a known RVF. The MRI images also reclassified one patient as having a perianal fistula, a finding that was confirmed at the time of surgery. These authors concluded that MRI was superior to other radiologic tests in classifying RVF and identifying associated abscesses.

In most cases of perineal breakdown and or RVFs, especially postobstetric low fistulas, preoperative evaluation of the anal sphincter with transanal ultrasound should be considered. This allows clear visualization of the sphincter mechanism, which may be very useful in preoperative surgical planning. An assessment of a patient’s degree of fecal incontinence is critical, as one study noted that 48% of women with rectovaginal fistula have fecal incontinence ( ). Proctoscopy may also be necessary to evaluate for coexisting anorectal disease.