INTRODUCTION — Fecal incontinence is one of the most devastating of all physical disabilities since it affects self-confidence and personal image and can lead to social isolation. Previously, the evaluation of fecal incontinence was largely limited to anal manometry, electromyography (EMG), and studies of pudendal nerve latency. Although helpful, these tests provide no direct evidence of structural sphincter injury. Imaging the anal sphincter is helpful in patients with fecal incontinence to assess the structure and integrity of the internal and external anal sphincter complex. Imaging modalities, such as endorectal ultrasound (which can be performed with a transrectal ultrasound probe or with endoscopic ultrasonography) and magnetic resonance imaging (MRI), have added to the diagnostic tools available to clinicians for the evaluation of fecal incontinence. Ultrasound imaging of the anal sphincter provides complementary structural information to the functional information that can be obtained with manometry and should be performed in combination with this test. This topic will focus on the use of endorectal endoscopic ultrasound (EUS) in the evaluation of fecal incontinence. An overview of fecal incontinence is presented separately. (See "Fecal incontinence in adults: Etiology and evaluation".)
GENERAL PRINCIPLES
Normal ultrasound appearance — The ultrasound appearance of the anorectum has been well described in individuals without anorectal disease [1-8]. Two discrete rings of tissue can be seen when using a radial scanning echoendoscope to examine the anorectum (image 1). The inner hypoechoic ring of tissue represents the internal anal sphincter (IAS), which is formed by the thickened continuation of the circular smooth muscle of the rectum. The outer hyperechoic ring of tissue represents the longitudinal muscle and the external anal sphincter (EAS), which is formed by the downward extension of the skeletal muscle of the puborectalis. The normal IAS is between 2 to 3 mm thick [2,5] and the normal EAS is between 7 to 9 mm thick [2,6,9]. The IAS becomes thicker and more hyperechoic with age, probably reflecting collagen replacement of the IAS [5]. Conversely, the EAS tends to become thinner with age [6]. The perineal body is not clearly defined with ultrasound [9,10]. The length of the anal canal varies from 25 mm for females to 33 mm for males [11].
There is a different configuration of the anterior of the EAS in males and females [12]. The anterior part of the sphincter seems to be shorter and slopes downward in females. This can make demonstrating a complete 360-degree ring of the EAS in one plane difficult [9]. It is essential to recognize this variation so that one does not incorrectly diagnose an anterior sphincter defect where one does not exist. The anococcygeal ligament appears as a hypoechoic triangular structure posteriorly and can be confused as a sphincter defect in this location.
Test characteristics — Data suggest that endosonography is useful for identifying sphincter injuries in patients with fecal incontinence (image 2) [13-25]. Several of these reports have compared ultrasound findings to operative findings. Sensitivity exceeded 90 percent in most reports; specificity is hard to determine from these studies since patients in whom a tear was not strongly suspected may not have undergone surgery. However, false-positive results have been described. The following illustrate the range of findings:
●One study compared ultrasound and manometric findings in 20 asymptomatic nulliparous patients, 20 asymptomatic parous patients, and 31 female patients with incontinence who underwent sphincteroplasty [26]. Endosonography correctly identified all sphincter injuries in patients with incontinence.
●In a similar report, endorectal EUS was performed prior to surgery in 28 patients with fecal incontinence [27]. EUS correctly identified all 25 of the IAS defects and all 10 EAS defects. However, it incorrectly diagnosed an EAS defect in three patients who had IAS defects that extended into the EAS.
●In another study, the ultrasound appearance was prospectively compared with the operative findings in 44 patients who underwent pelvic floor repair [28]. Endorectal ultrasound was 100 percent sensitive in detecting either IAS or EAS defects. One IAS tear seen on endosonography was not confirmed at surgery.
●The group from St. Mark's Hospital published a study prospectively comparing the results of endosonography with manometry and electromyography (EMG) in 12 patients who were scheduled to undergo sphincter repair [29]. Nine of the 12 patients were found to have an EAS defect at surgery. Histology was used as the gold standard. Ultrasound was superior to the other modalities with an accuracy of 100 percent for detecting a sphincter injury compared with an accuracy of 75 percent for manometry, 75 percent for EMG, and 50 percent for clinical assessment. In addition, there was an association between an EAS defect and a decreased maximum squeeze pressure on manometry.
●Other studies have found a similar association between decreased maximal squeeze pressure and decreased EAS thickness or an EAS defect [10,30]. In contrast, a similar relationship between the resting pressure in the anal canal and IAS thickness has not been documented [8,30]. The IAS increases in size with age [6,31]. Thus, a thicker IAS may actually represent replacement of the smooth muscle of the IAS by connective tissue and not more muscle mass.
Comparison with other diagnostic tests
Ultrasound and MRI — A number of studies have compared the accuracy of transrectal ultrasound and MRI for detecting sphincter injuries. The results have been heterogeneous, in part reflecting the local expertise at the institution performing the study as well as different patient population and study designs. An illustrative report that included 22 female patients who underwent surgery for fecal incontinence found that MRI correlated better with the surgical findings [32]. In contrast, another study of 52 patients reached different conclusions [33]. The gold standard was a consensus opinion agreed upon by a gastroenterologist and surgeon after reviewing the history and all the imaging and physiologic studies. MRI and ultrasound were concordant in 32 patients; MRI was incorrect in 15 patients while ultrasound was incorrect in only six patients. Additional studies comparing the two modalities are needed to more clearly define each modality's strengths and weaknesses. However, ultrasound is less expensive and is more widely available.
Ultrasound and EMG — Endorectal ultrasound has replaced EMG as the primary method for identifying sphincter defects [34-36]. Historically, EMG was used to assess the integrity of the EAS. However, EMG studies were poorly tolerated since they required insertion of needles directly into the muscle. One of the earliest studies comparing EMG with ultrasound in patients with fecal incontinence showed that endosonography could accurately identify EAS defects [34]. The correlation between EMG and ultrasound was 0.96. In addition, ultrasound was better tolerated. A follow-up study of 45 patients found that ultrasound agreed with EMG assessment of the external sphincter [35]. In a separate study, the same group used endosonography to direct EMG needle placement into the sphincter defect seen on ultrasound [36]. In all of the patients where EMG was technically successful, EMG showed no evidence of electrical activity, thus confirming the accuracy of endosonography.
Three-dimensional ultrasound (3D-EUS) — Three-dimensional endoscopic ultrasound (3D-EUS) uses computer software to reconstruct standard EUS images into a 3D image. Although the data are somewhat mixed, a few studies have shown a benefit for 3D imaging in the evaluation of fecal incontinence. However, more studies are needed. When available, 3D-EUS allows the endoscopist to better understand the complex anatomy and associated defects identified with standard 2D-EUS images.
●In one study of 24 consecutive patients with sphincter injuries seen on regular EUS, 3D-EUS was able to demonstrate the radial angle of the IAS or EAS defect better than with standard EUS [11]. This study showed that the length of the defect was strongly related to the angle of the tear.
●In a study of 18 patients with fecal incontinence, 3D-EUS was similar to MRI in detecting EAS atrophy [37]. However, the two imaging techniques did differ in their assessment of the severity of the atrophy, with MRI labeling more patients with severe EAS atrophy.
CLINICAL APPLICATIONS
Evaluation prior to sphincteroplasty — Once the accuracy of endosonography had been established, a number of centers began using ultrasound for mapping the defects of the internal and/or external sphincter and for predicting therapeutic response after sphincteroplasty. Results of a number of small reports have been mixed [38-40]. Thus, the accuracy for predicting surgical outcomes remains uncertain. On the other hand, patients with improved continence after surgery also generally have improvement in the sphincter disruption visible on EUS [38,41,42]. (See 'Test characteristics' above.)
Incontinence related to childbirth — Vaginal delivery can result in fecal incontinence immediately or after many years. The most common injuries are anal sphincter tears or trauma to the pudendal nerve [43,44]. In one series, anal sphincter tears were detected by anal endosonography in 35 percent of primiparous and 40 percent of multiparous patients of whom symptoms of anal incontinence or fecal urgency were present in 13 and 23 percent, respectively [43]. Symptoms of urgency or incontinence usually developed within six weeks of delivery and were more likely in patients who had forceps delivery. (See "Fecal and anal incontinence associated with pregnancy and childbirth: Counseling, evaluation, and management".)
In another series, 13 of 59 (22 percent) previously nulliparous patients reported altered fecal continence after their first delivery [45]. Eight had persistent symptoms during their second pregnancy, of whom seven worsened following delivery. Similar findings have been described in other reports [46,47].
These data demonstrate that the damage that occurs with childbirth may remain clinically silent in the majority of patients. However, such patients are at risk for developing incontinence as they age or with future deliveries. Ultrasound may be helpful in screening patients who are at risk of developing incontinence with future vaginal deliveries. Patients who had previously had a forceps delivery, episiotomy, or a prolonged first and/or second stage of labor are more likely to have a sphincter defect [46]. Delivery via cesarean section could be considered in high-risk patients to preserve continence especially if there is any evidence of symptoms (ie, minor soiling, incontinence for a brief period after the previous childbirth, etc).
TECHNICAL ASPECTS — The typical work-up for patients who present with fecal incontinence includes anal manometry and an endoscopy to exclude inflammatory bowel disease or malignancy followed immediately by endorectal EUS, permitting assessment of the colonic mucosa and the anal sphincters during the same evaluation. For bowel preparation, patients are given enemas per rectum until clear. (See "Bowel preparation before colonoscopy in adults", section on 'Flexible sigmoidoscopy preparation'.)
Endoscopists can use a rigid, rotating ultrasound probe with a plastic cap to conduct these examinations or a 7.5/12 MHz radial scanning echoendoscope. The diameter of this scope is smaller than the traditional rigid rectal probe (12.7 versus 17 mm), which allows for less distortion of the anal canal. We grip the scope with one hand up against the anus to stabilize the scope within the anal canal. The examination is carried out with the patient in the left lateral position. For female patients, we occasionally perform the examination with the patient in the prone position in order to more clearly delineate the anterior aspect of the EAS. The St. Mark's group advocates this scanning position because they feel it provides more complete imaging of the anterior part of the EAS and improved visualization of the perineum [48]. A 10 mL water-filled syringe can be placed in the vagina to facilitate orientation in patients with more difficult imaging.
Imaging is begun in the upper anal canal. At this point, the puborectalis can be visualized posteriorly as a hyperechoic sling. The EAS is not present anteriorly at this level. As the probe is slowly withdrawn, the EAS becomes apparent anteriorly in the mid anal canal. This is usually the area of maximal IAS thickness. Finally, in the lower anal canal, only the subcutaneous aspect of the EAS is visualized.
The examination is performed with the balloon minimally inflated (ie, just enough water to establish contact of the balloon with the anal canal) to reduce distortion of the anal canal during imaging. Several authors have proposed a transvaginal approach to eliminate any distortion of the anal canal during scanning [49-51]. However, in a study of 47 patients conducted to determine the sensitivity of transvaginal ultrasound in detecting anal sphincter defects, the sensitivity of transvaginal ultrasound for IAS and EAS tears was only 44 and 48 percent, respectively [52]. The authors hypothesized that because the perineal body lies below the level of the vaginal introitus, only the proximal anal canal can be imaged in a plane parallel to its axis. In addition, the echoendoscope has a diameter of only 12.7 mm. This causes only minimal, if any, distortion of the anal canal. Thus, a transvaginal approach is not necessary.
IAS tears appear as hyperechoic breaks in the normally hypoechoic ring in contrast to EAS tears, which appear as relatively hypoechoic areas in the normally hyperechoic ring. This corresponds to replacement of the normal striated muscle with granulation tissue and fibrosis (image 2).
The results obtained with rectal EUS are somewhat dependent upon the endoscopist's experience. However, it is fairly easy to learn and intra and inter-observer variability is low [53].
INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.
Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)
●Basics topics (see "Patient education: Fecal incontinence (The Basics)")
●Beyond the Basics topics (see "Patient education: Fecal incontinence (Beyond the Basics)")
SUMMARY AND RECOMMENDATIONS
●General principles – Endorectal endoscopic ultrasound (EUS) is a well-tolerated and sensitive means of detecting anal sphincter defects in patients with fecal incontinence (image 2). As a result, it has largely replaced electromyography (EMG) for this purpose. (See 'General principles' above.)
Three-dimensional EUS may offer an advantage over standard rectal ultrasound, but additional studies are needed to confirm this benefit. (See 'Three-dimensional ultrasound (3D-EUS)' above.)
●Clinical applications – EUS can detect a sphincter injury in up to 35 percent of patients who delivered vaginally, potentially providing a means to identify patients at high-risk for developing fecal incontinence after additional deliveries. (See 'Incontinence related to childbirth' above.)
EUS can be used to predict the therapeutic response to sphincteroplasty. (See 'Clinical applications' above.)