The rectal wall is made up of five layers (Fig. 18.5)

All the pelvic organs adjacent to rectum are clearly visualized, identifying its relation with the rectal wall layers in multiple anatomic planes.

The ultrasound clearly identifies the presence of sphincter defects, combined EAS and IAS or if the injury involves the EAS alone. Using a 3-D modality, additional measurements include the following: length of the anterior EAS and radial angle of the EAS defect, length of the anterior and posterior IAS, length of the posterior EAS plus the PR muscle (EAS + PR), and the gap length (distance from the proximal edge of the posterior PR to the proximal edge of the anterior EAS, corresponding to the area in the anterior quadrant without striated muscle). Those measurements can be correlated with incontinence score (Murad-Regadas et al. 2014a).

The endoanal ultrasound scanning identifies injured muscles and the extension of the injury in relation to the anal circumference and the length of the anal canal. The angle of the lesion is measured by drawing two lines tangentially to the injured muscle and making them converge to the center of the circumference. On ultrasound scans, muscle injuries appear as interruptions (or changes) in the echogenicity of the original musculature. Internal anal sphincter (IAS) injuries appear as lighter-colored single or multiple disruptions of the normal hypoechoic circumferential image, while external anal sphincter (EAS) injuries are characterized as areas of reduced hyperechogenicity, depending on the amount of fibrous tissue formed (Sultan et al. 1993; Felt-Bersman et al. 1995; Karoui et al. 1999; Fig. 18.6). The “septum maneuver,” used routinely in the measurement of the perineal body, is helpful in the identification of sphincter injuries of the anterior quadrant as it helps view the extremities of damaged muscles. It consists of measuring the distance between the finger of the examiner held against the posterior vaginal wall and the internal border of the IAS (normal > 10.0 mm) (Zetterstrom et al. 1994; Fig. 18.7). However, this technique cannot be used clinically with patients previously submitted to perineoplasty with sphincteroplasty since the size of the perineal body in these patients exceeds 10 mm, even before muscle repair.

Anal ultrasonography is also particularly useful in the evaluation of the results of surgical repair of the anterior and posterior anal sphincter, identifying adjacent or overlapping muscles or documenting persistent muscle injury (Nielsen et al. 1994; Savoye-Collet et al. 1999; Fig. 18.8).

Changes in muscle thickness may correlate with atrophy and symptoms of fecal incontinence. Atrophy is characterized by generalized sphincter thinning and fatty replacement. The similar results of MRI for the evaluation of atrophy have not been found with EUS. This could probably be explained by the fact that EUS is not able to distinguish fatty infiltration from normal muscle tissue. For that purpose, MRI is the best technique to be used for detecting EAS atrophy (Briel et al. 1999; Williams et al. 2001).

It can be also used the same probe (360° rotating anorectal transducer) in the endovaginal assessment to evaluate the Levator Ani muscle defect. This muscle has a recognized role in pelvic floor–supporting structures. The nomenclature regarding the Levator Ani varies. As proposed by DeLancey, it is used the term pubovisceral muscle (PVM) as synonymous with the term pubococcygeus/puborectalis since the two components can’t be distinguished on imaging (DeLancey 2001; Fig. 18.9). Anatomic alteration of the insertion of the PVM has been clearly demonstrated after vaginal childbirth. The most common form of major levator trauma appears to be an avulsion injury at the insertion of the muscle on the pubic branch, which has been demonstrated by MRI and ultrasonography (DeLancey et al. 2007; Dietz and Simpson 2008; Abdool et al. 2009; Murad-Regadas et al. 2013) in 15–55 % of parous women after vaginal delivery, and which is associated with pelvic organ prolapse and ballooning hiatal dimensions (DeLancey et al. 2007; Dietz and Simpson 2008; Murad-Regadas et al. 2014). The ultrasound examination identifies pubovisceral defects, defined as the detachment (discontinuity) of the PVM from its insertion on the pubic branch. Complete detachment of the PVM involves the entire muscle and partial detachment is either unilateral (involving just one side) or bilateral (both sides). The volume of the defect (sum of microsections at 2 mm) can be measured and the measurements also include the anteroposterior diameter, laterolateral diameter, and the full area. Murad-Regadas et al. demonstrated that severity of FI symptoms is significantly related to extent of the defect as determined by the novel 3-D ultrasound score, including the anal sphincters and pubovisceral muscle defects in women who had undergone to vaginal delivery, and suggested both evaluation of the anal sphincter and PVM to identify the defects and determine a strategy for treatment in women with FI after vaginal delivery (Fig. 18.10). They also evaluated the position of the anorectal junction, as measured from the anorectal junction to the lowest margin of the symphysis pubic (Fig. 18.11), and the position of the bladder neck, as measured from the bladder neck to the lowest margin of the symphysis pubic (Murad-Regadas et al. 2014).

The ultrasound is useful to show the location, extension of the abscess cavity, and relation to the sphincter muscles and to the rectal wall, making possible its classification. Abscesses appear as a non-homogenous hypoechoic area due to the inflammatory process associated with more hypoechogenic area in those cases with liquid content in the cavity. A hyperechogenic area inside the cavity suggests residual air.

The ultrasound image is also able to show early inflammatory processes or the late absorption stage. And these findings cannot be identified by proctological exam alone and are not able to determine if the therapy should be conservative or surgical. The US images are likewise useful to determine the location and extension of large abscesses in relation to the sphincter muscles as well as in the choice of the best treatment approach. It is also well tolerated because the scanning procedure is quick and images may be analyzed posteriorly.

The examination may be difficult or inconclusive in patients with Fournier’s syndrome due to the fibrosis and excessive damage muscle.

The anorectal ultrasonic scanning is able to identify all the fistula complex, facilitating the surgical planning and preventing recurrence and fecal incontinence. The 3-D scanning mode allows to accurately view and classify the entire extension of the fistulous tract and its relation to the sphincter muscles, the exact position of the internal opening in relation to the anal margin and any secondary tracts and / or cavities. If the external opening is pervious, hydrogen peroxide (H₂O₂) should be applied through a fine polyethylene catheter to confirm findings and identify any previously secondary tracts (Cheong et al. 1993).

The scan is performed in two steps, administering 0.1–3.0 ml of 10 % H₂O₂ at normal pressure (first step) and heightened pressure (second step) (Murad-Regadas et al. 2010). Fistulous tracts typically appear as hypoechoic areas and the echogenicity increases and then appears as hyperechoic, as a result of bubble formation due to the contact between H₂O₂ and the tissue. The internal fistulous opening appears on the image as a rupture in the IAS (in the absence of previous sphincterotomy) and subepithelium tissue. When the use of hydrogen peroxide is applied, a hyperechoic area is clearly observed in the subepithelium space, crossing the IAS towards the endoprobe. Based on the classification proposed by Parks, the anorectal fistulas are showed with the following ultrasound features (Parks et al. 1976):

The percentage of sphincter muscle to be sectioned during surgery must be previously determined. To calculate this percentage, the total length of the compromised sphincter is measured as well as the distance from the distal part of each muscle to the point where it is crossed by the fistulous tract (Figs. 18.15 and 18.16). These measures are used in surgical planning and help prevent fecal incontinence (Murad-Regadas et al. 2010).

The ultrasound scanning may show the fistulous tract and its relation to the anal canal or rectum and helps to evaluate the anatomic integrity of the sphincter muscles, improving consequently the surgical planning and preventing recurrence and fecal incontinence.

The identification of the opening in the vagina and the injection of hydrogen peroxide help to visualize the tract, especially at the point where it crosses the perineal body and in cases of associated fibrosis. The transducer may subsequently be introduced into the vagina to confirm or expand the findings.

The anorectal ultrasonography is particularly useful for staging rectal cancer as it provides accurate information on rectal wall infiltration, anal canal invasion and perirectal lymph node enlargement and select patients for surgery or to neoadjuvant radiochemotherapy (RCT) (Hildrebant and Fiefel 1985; Milsom and Graffner 1990; Katsura et al. 1992; Dattala et al. 2000; Garcia-Aguilar et al. 2002). Using the 3-D modality, it is easily measured tumor length and volume, the distance between the distal margin of the tumor to proximal margin of the anal sphincter muscles and determine the distal margin (Murad-Regadas et al. 2009; Fig. 18.17), and the closest predicted radial tumor-mesorectal margin (Fig. 18.18; Phang et al. 2012). The 3-D anorectal ultrasound (3-DAUS) is also useful to evaluate the response after RCT. Murad-Regadas et al. identified residual tumors in rectal wall and lymph nodes and complete response with a high level of accuracy (Murad-Regadas et al. 2009). In addition, 3-DAUS can help in the choice of surgical approach by providing important information on the distance between the tumor and the anal muscle and select patients for sphincter saving resection (Murad-Regadas et al. 2011b). In the follow up, this modality detect early local recurrence in the rectal wall or perirectal lymph nodes (Beynon et al. 1986)

Ultrasonographic tumor staging – In 1985, Hildebrand and Fielfel proposed using ultrasound scanning for the staging of rectal tumors, based on the TNM classification (Hildrebant and Fiefel 1985):

Perirectal lymph nodes – Observed in the perirectal fat proximally or distally to the lesion and measuring over 1.0 mm. They are easily distinguished from blood vessels, because the latter assume a longitudinal or branch-like form when the transducer is moved. The size, echogenicity, and shape help distinguish between inflammatory and metastatic forms. When observed in the perirectal fat in the form of rounded and hypoechoic (or tumor-like) (Fig. 18.19) areas with irregular borders, lymph node metastasis should be suspected. In contrast, oval structures with regular borders and a hyperechoic area in the center (corresponding to the hilum) suggest inflammatory lesions.