Endoluminal ultrasound, specifically of the rectum and anus, can be an extremely useful adjunct in the diagnosis of various conditions. As with any other ultrasound modality, this technique is very user-dependent, so specialized training is quite important for reproducible results. A practice that provides a consistent need for this test is also valuable.
Endoanal ultrasound is used most commonly in the evaluation of the anal sphincters in the workup of fecal incontinence. In women who have a history of vaginal delivery or any patient with a history of anal trauma, symptoms of incontinence may indicate scarring of the internal sphincters. Ultrasound, in conjunction with anal manometry, pudendal nerve testing, and defecography, can help determine the cause of the loss of fecal control and guide optimal surgical options.
Endorectal ultrasound is key to staging and evaluation of rectal cancers. In the hands of an experienced ultrasonographer, depth of invasion as well as nodal status can be measured with a fair degree of certainty. Advanced techniques include ultrasound-guided biopsy of lymph nodes for stage verification, which can guide decisions regarding the need for neoadjuvant therapy.
Standard ultrasound images are in 2-D, requiring frozen images to display normal and abnormal anatomy. Newer models include 3-D imaging, which capture the anatomy within a cube; this can be manipulated on the unit or on a computer at leisure to reveal anatomy through various cuts along multiple axes or through spinning the cube to allow views from different angles.
Normal Anatomy, Normal Ultrasound
The normal anatomy of the anus can be evaluated through ultrasound with great accuracy. The structures include the anal mucosa, the internal anal sphincter (IAS), the external anal sphincter (EAS), the puborectalis, the perineal body, the seminal vesicles in a man, and any bowel that may fall into a particularly deep cul-de-sac at the level of the upper anal canal.
The anal canal is divided into three general levels, each of which demonstrates different characteristics: upper, mid, and lower. The upper anal canal is marked by the visualization of the puborectalis muscle, which appears as a U-shaped sling surrounding the anus at the superior-most border of the sphincters. Here, the seminal vesicles and occasionally some bowel may also be seen anterior to the anus (Figure 12-1).
The mid-anal canal is best identified by a clear delineation of the IAS and the EAS in the shape of dark and white rings, as well as an inner layer of mucosa containing the hemorrhoidal bundles (Figure 12-2). Any disruption of these rings can be an indication of the abnormalities that would be the source of the patient’s complaint. A measurement of the perineal body can be done at this time of the examination by placing a finger against the posterior wall of the vagina to highlight that structure and measuring the distance between that and the anterior mucosa of the anus. Normal thickness of the perineal body is generally considered to be anything greater than 8 or 9 cm.
The distal anal canal is discernible by the loss of the internal anal sphincters, leaving the EAS alone as a distinct structure (Figure 12-3). In the surrounding soft tissues beyond the anal canal and rectum lie the various perianal and perirectal spaces that become significant in the setting of malignancy and infection, but are not usually of much moment in a normal patient.
Normal rectal anatomy is also represented by light and dark rings, but the rings now correlate to the different layers of muscle and mucosa (Figure 12-4).1 The perirectal fat is generally a heterogeneous shade of gray, in which enlarged lymph nodes can be identified by dark, circumscribed spots within the fat. These can sometimes be confused with perirectal vessels, but the vessels can be distinguished by moving the probe and following their linear courses, rather than limited area circumscribed by the outlines of a node.
Rectal ultrasound. First white line: interface of balloon and mucosa; first dark line: mucosa, muscularis mucosae; second white line: submucosa; second dark line: muscularis propria; third white line: interface of rectum and perirectal fat.
Scanning Technique for Obtaining These Images
The patient is brought into the office procedure room or endoscopy suite and placed in the left lateral decubitus position. Sedation is not generally necessary, although it is an option for an anxious patient or if a patient has a bulky tumor around which manipulation of an ultrasound probe might be too uncomfortable. The ultrasound unit is set up and the patient’s information is entered.
For an anal ultrasound, a 10-MHz transducer is generally used for close-up images; this is attached to the end of the ultrasound probe. A small rigid cap is fitted over the transducer and filled with saline. After doing a visual examination of the external anal canal and an initial digital examination, the probe is lubricated and placed into the anal canal. The operator will know when the upper anal canal is reached by identifying the puborectalis sling. Orientation is also achieved by turning the probe until the sling is shown to resemble the letter “U” with the lateral arms reaching upward. The probe is then withdrawn slowly to the mid- and then distal-anal canal, looking for abnormalities. The perineal body in a woman is measured in the manner previously described at the mid-anal canal (Figure 12-5). The 3-D function available on some scanners has the advantage of not needing to withdraw the probe and the ability to manipulate the image in any direction.
Measurement of the perineal body.
Rectal ultrasounds are performed in a slightly different manner. The patient is prepared with an enema prior to the procedure. The 10-MHz transducer is preferred for examination of shallower structures, while the 7-MHz transducer is chosen for deeper, more distant structures. Again, for most purposes, the 10-MHz transducer is generally sufficient. A latex balloon is attached to the probe and inflated with saline, taking care to remove all air bubbles. It is then desufflated prior to insertion. The external anus is visualized and a digital examination performed. A rigid proctoscope is passed transanally and the target lesion examined, taking care to mark distance, size, quadrant, and characteristics. The proctoscope is advanced to a level just proximal to the lesion in order to ensure that the entire lesion can be examined by the ultrasound. The probe is passed through the proctoscope; once the transducer is past the open end of the scope, the scope can be withdrawn slightly for better manipulation. The balloon is then inflated so that the rectal wall is fully distended for optimal contact with the rectal mucosa. The probe is again withdrawn slowly, marking the disruption of the light and dark lines, and looking for any prominent lymph nodes in the surrounding perirectal fat. As with the anal ultrasound, a 3-D module is available to help in creating a block of images that can be examined in an infinite variety of ways after the examination.
Possibly the most common reason for performing an anal ultrasound is in the workup of fecal incontinence. In conjunction with pudendal nerve latency testing, anal manometry, and defecography, anal ultrasound can confirm structural defects and scarring in the internal and external anal sphincters.2 The likeliest origin of anal incontinence is sphincter damage after vaginal delivery, often presenting decades after the inciting injury.3 Sphincter defects should be suspected in a peri- or postmenopausal woman with a history of vaginal delivery, many times complicated by high-birth-weight babies, prolonged labor, use of vacuum or forceps assistance, or episiotomy. Common findings in these patients include anterior scarring or a complete break of the IAS and/or the EAS (Figure 12-6),4 and thinning of the perineal body (less than 8 mm thickness on digital examination at the mid-anal canal).5 Some amount of anterior scarring, which appears as a break in the rings of tissue with mixed echogenicity, is likely present in many women in the general population; it is unclear what distinguishes those women who are symptomatic from those who are not.6 However, confirmation of scarring can help delineate surgical options for these patients. Specifically, an overlapping sphincteroplasty is an option for the appropriate patient. In cases of recurrence of symptoms after such an operation, or for follow-up evaluation after this procedure, an ultrasound may be performed. This shows a classic “6” or “reverse 6” sign in an intact repair (Figure 12-7).7
Anterior sphincter defect.
“6” sign of overlapping sphincteroplasty.
Other patients who have fecal incontinence and have no history of traumatic vaginal delivery may have other findings. Multiple or eccentric tears, or scarring, or no scarring at all can be evident from less common types of injuries, limiting the options for surgical intervention. Congenital anomalies can be delineated by ultrasonography and help to determine what structures are absent or aberrant. It can also be useful in identifying occult rectovaginal fistulas, which may mimic or add to the symptoms of fecal incontinence.
Another common use of ultrasound is the evaluation of an anal abscess or anal fistula.8 Abscess cavities and any fistulous tracts can be seen on ultrasound as dark cavities outlined in white (Figure 12-8).9 This mode of evaluation is especially useful in tracing complex or high fistula tracts that would be difficult to follow by other methods. The injection of hydrogen peroxide into the external opening of a fistula will make the tract hyperechoic from the bubbling effect of the fluid from oxygenation and help to distinguish it from other structures in the surrounding tissue.10
Anal tumors, both benign and malignant, can be clearly examined with anal ultrasound. Sonographic images can show the relationship of benign tumors to anal anatomy and define tissue planes. Most lesions will be hypoechoic, and ultrasound-guided biopsies may be performed by an advanced operator. Malignant tumors, most commonly squamous cell carcinomas, can be used in pre- and post-treatment staging. While some of the rarer cancers may warrant surgical intervention, the treatment of squamous cell malignancies of the anus is largely radiation. Incomplete response or recurrence can be followed by serial ultrasounds. Staging follows a standard TNM protocol (Tables 12-1 and 12-2) and can be measured either by diameter or by depth of invasion.11 Staging by ultrasound is labeled by a “u” preceding the standard nomenclature to distinguish it from the final pathologic designation.
Table 12-1 TNM Staging of Anal Malignancies, by Size of Tumor |Favorite Table|Download (.pdf)
Table 12-1 TNM Staging of Anal Malignancies, by Size of Tumor
- Primary Tumor (T)
- TX: Primary tumor cannot be assessed
- T0: No evidence of primary tumor
- Tis: Carcinoma in situ
- T1: Tumor 2 cm or less in greatest dimension
- T2: Tumor more than 2 cm but not more than 5 cm in the greatest dimension
- T3: Tumor more than 5 cm in the greatest dimension
- T4: Tumor of any size that invades adjacent organ(s), eg, vagina, urethra, and bladder*
- Regional Lymph Nodes (N)
- NX: Regional lymph nodes cannot be assessed
- N0: No regional lymph node metastasis
- N1: Metastasis in perirectal lymph node(s)
- N2: Metastasis in unilateral internal iliac and/or inguinal lymph node(s)
- N3: Metastasis in perirectal and inguinal lymph nodes and/or bilateral internal iliac and/or inguinal lymph nodes
- Distant Metastasis (M)
- MX: Distant metastasis cannot be assessed
- M0: No distant metastasis
- M1: Distant metastasis
- AJCC Stage Groupings
- Stage 0
- Stage I
- Stage II
- Stage IIIA
- T1, N1, M0
- T2, N1, M0
- T3, N1, M0
- T4, N0, M0
- Stage IIIB
- T4, N1, M0
- Any T, N2, M0
- Any T, N3, M0
- Stage IV
Table 12-2 Alternate uT Classification by Depth of Invasion |Favorite Table|Download (.pdf)
Table 12-2 Alternate uT Classification by Depth of Invasion
|uT1||Confined to submucosa|
|uT2a||Confined to IAS|
|uT2b||Through IAS, confined to EAS|
|uT3||Invades EAS into perianal tissues|
|uT4||Invades adjacent structures|
Rectal ultrasound is used primarily in staging of rectal malignancies.12 The first area to evaluate is depth of invasion; staging is similar to the staging used for anal tumors (Table 12-3). Lymph nodes can also be visualized for more accurate preoperative staging.
Table 12-3 Staging Classification of Rectal Cancer by Ultrasound |Favorite Table|Download (.pdf)
Table 12-3 Staging Classification of Rectal Cancer by Ultrasound
|uT0||Confined to mucosa|
|uT1||Confined to submucosa|
|uT2||Invades into but not through muscularis propria|
|uT3||Invades to perirectal fat|
|uT4||Invades adjacent organs|
|uN0||No evidence of spread to lymph nodes|
|uN1||Evidence of spread to lymph nodes|
Lesions confined to the mucosa are generally considered to be benign, with little risk of local or distant spread. Fortunately, the more superficial the lesion, the more accurate the ultrasonography is in predicting final staging. Accuracy is from 81% to 96% for uT0 lesions when compared to pathologic staging.13 The white line that correlates to the submucosa should be intact beneath the lesion in an unbroken fashion. Local excision is acceptable for these tumors, either by transanal excision or by transanal endoscopic microsurgery (TEMS), as there is no need for lymph node excision.
The earliest stages of malignancies fall in the uT1 category. The lesion has invaded the mucosa and submucosa without entering the muscularis propria. On ultrasound, this will look like a mass outlined by an irregular white line and a normal outer black line (Figure 12-9). Any minor breaks in the line of the white muscularis layer indicate a higher T stage. Certain uT1 lesions may be treated locally; a clear lack of abnormal regional lymph nodes, smaller mass size less than 4 cm, less than 1/3 of the circumference, and more distal lesions that would be better amenable to transanal procedures are factors to consider when deciding on a treatment plan.
These lesions are important to distinguish from uT1 lesions as more invasive surgical and neoadjuvant therapies may be indicated for the patient. Invasion of the muscularis propria shows up on ultrasound as a disruption of the middle white line and the adjacent black line without a break of the outermost white line that represents perirectal fat (Figure 12-10). Some clinicians may make the distinction in the degree of invasion, deep or early, in order to decide on if neoadjuvant therapy is an appropriate option for the patient.
The observation of invasion into the last white line classifies a lesion to be a uT3 lesion. All the intervening layers are disrupted, and the outline of the mass is generally very irregular or unclear (Figure 12-11). These patients are not candidates for local excision, and usually have neoadjuvant chemoradiation therapy included in their treatment plan. It is very important in these patients to look closely for regional lymph nodes as the likelihood of nodal spread increases dramatically with this depth of invasion.
Invasion into adjacent structures, such as the vagina, uterus, or cervix in women, the prostate or seminal vesicles in men, or the bladder in either gender, qualifies a lesion this advanced to be a uT4 lesion. Typically, these lesions are nonmobile on examination prior to the ultrasound. There is a lack of distinction between the rectum and the surrounding structures. (An example of intact adjacent viscera is shown in Figure 12-12).
The detection of lymph nodes in the perirectal fat is an important clue in determining prognosis for a patient. Accurate preoperative staging helps to determine the need for neoadjuvant therapy, and therefore makes the long-term survival rate easier to determine.
Unfortunately, the accuracy of ultrasound in detecting nodal disease is poorer than that for measuring depth of invasion. Some studies show accuracy to range from as low as 50% to a high of 88%.14 Most other diagnostic tools are no better, however, with the possible exception of multiphase MRI or endorectal coil MRI.15 These have comparable results, but may not be available in all institutions; radiologists with a particular interest in reading these MRI studies are critical for accurate interpretation.
Normal lymph nodes in the mesorectum do not usually appear distinct on endorectal ultrasound. Malignant nodes tend to be large, hypoechoic, and round.16 Inflammatory nodes, on the other hand, look hyperechoic and irregular. Determining which lymph nodes are truly malignant can be difficult, as size is not a reliable indicator of metastasis, although larger size may indicate a higher likelihood of spread.17 Also indicative of malignancy is an area of mixed echogenicity, which can be suggestive of lymphovascular invasion.