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Endorectal endoscopic ultrasound in the evaluation of fecal incontinence

Authors
David A Schwartz, MD
Maurits J Wiersema, MD
Section Editor
Douglas A Howell, MD, FASGE, FACG
Deputy Editor
Anne C Travis, MD, MSc, FACG, AGAF

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 structural evidence of sphincter injury. Imaging of the anal sphincter is helpful in all patients with fecal incontinence to assess the structure and integrity of the sphincters. Newer imaging modalities, such as endorectal ultrasound (which can be performed with a transrectal ultrasound probe or with endoscopic ultrasonography) and 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 review 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".)

NORMAL ULTRASOUND APPEARANCE

The normal ultrasound appearance of the anorectum has been well delineated in several studies performed on normal subjects [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 anal canal length varies from 25 mm for women to 33 mm for men [11].

There is a different configuration of the anterior part of the EAS in men and women [12]. The anterior part of this sphincter seems to be shorter and slopes downward in women. 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.

ENDORECTAL ULTRASOUND AND EMG

Before the development of endosonography, electromyography (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. The first reports on the use of ultrasound to study the anal sphincter came from St. Mark's Hospital in the late 1980s and early 1990s. A study comparing electromyography (EMG) with ultrasound in 15 patients with fecal incontinence showed that endosonography could accurately identify EAS defects [13]. 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 in all patients studied [14]. In a separate study, the same group used endosonography to direct EMG needle placement into the sphincter defect seen on ultrasound [15]. In all of the patients where EMG was technically successful, EMG showed no evidence of electrical activity, thus confirming the accuracy of endosonography. It is largely because of these studies that endosonography has replaced EMG as the investigation of choice to identify sphincter defects.

TEST CHARACTERISTICS

A number of studies have reported the utility of endosonography in identifying sphincter injuries in patients with fecal incontinence (image 2) [16-33]. Several of these reports have compared the ultrasound findings to the results of surgery (sphincteroplasty). 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 been operated on. However, false positive results have been described. The following illustrate the range of findings:

       

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Literature review current through: Nov 2016. | This topic last updated: Mon Aug 22 00:00:00 GMT+00:00 2016.
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