The anal canal is divided into three compartments. The upper anal canal (Figure 3-5.1) is defined as the level where the puborectalis muscle sling is clearly seen. At this level, there is a normal separation of the muscle fibers anteriorly. It is important to recognize this anatomic level, or this anterior muscle separation can easily be misinterpreted as a sphincter defect.
The mid anal canal (Figure 3-5.2) represents the location where the IAS and EAS wrap around the anal canal cir-cumferentially. We have arbitrarily chosen the site where the IAS muscle is at its maximum width as the mid anal canal; this is the level where sphincter defects are most often seen. The IAS is seen as a hypoechoic ring, which is surrounded by the mixed echogenic EAS.
The distal anal canal (Figure 3-5.3) represents the level where the IAS has tapered and is no longer visible, and the predominant muscle is the subcutaneous portion of the EAS. This mixed hyperechogenic band should be circum-ferentially intact.
Defects in the external or internal sphincters are defined as breaks in the continuity of the sphincter ring. The radial extents of the sphincter defects are measurable by the 360-degree view provided by the probe. Defects in the sphincters can be caused by obstetric trauma or prior operative
procedure including hemorrhoidectomy, sphincterotomy, and fistula surgery. Endoanal sonography is reliable in assessing sphincter injury and is often used in the evaluation of patients with fecal incontinence.
Defects of the IAS (Figure 3-5.4) are easily recognized given the prominent appearance of the IAS in the mid anal canal. Typically, patients will present with a history of anorectal surgery or dilatation. In this scenario, there is a defect of the IAS in the left lateral quadrant with a thickening of the remaining IAS, suggesting a retraction phenomenon.
Figure 3-5.4. Defect of the internal anal sphincter.
Defects of the EAS occur for a variety of reasons such as birthing injuries, prior anal fistula surgery, and trauma. The appearance of an EAS defect is, by definition, a break in the circumferential integrity of the mixed hyperechoic density pattern. A defect can have either a hypoechoic or a hyperechoic density pattern. It can be difficult to differentiate between scarring from prior surgery and a true defect. In these difficult cases, there may be a role for electromyo-graphy single-fiber density studies. A good example of an anterior EAS defect is demonstrated on Figure 3-5.5, where the EAS is discontinuous anteriorly with a defect measuring 142 degrees.
To easily measure the perineal body thickness, a finger is inserted into the vagina, held gently against the posterior vaginal wall, and then the distance between the anal mucosa and the ultrasonographic reflection of the finger is measured (Figure 3-5.6). Zetterstrom et al.5 determined that more than 90% of their patients with incontinence had perineal body thickness measurement less than 10 mm. Moreover, this maneuver improved the visualization of sphincter lesions in the majority of patients.
In patients with sphincter defects shown on endoso-nography, mean resting and squeeze pressures were significantly lower than in patients without anal sphincter defects in 46 subjects.6 However, when the group was divided by clinical history (symptomatic incontinence vs. asymptomatic), the association of results between manom-etry and endosonography was not significant. There was a correlation between anal ultrasound findings and fecal incontinence.
The ability to delineate clinically occult injuries of the anal sphincter musculature remains one of the strongest arguments in support of the anal ultrasound for the investigation of the patient with incontinence. Some investigators have advocated that ultrasonography combined with pudendal nerve terminal motor latencies be recognized as
the procedures of choice in the work-up of the patient with incontinence.7-9 Each of the individual physiologic tests offers valuable information relevant to the continence mechanism, whereas ultrasonography yields results that are complementary to other tests. However, in this age of cost containment, ultrasound offers the advantage of providing a great deal of anatomic data in a single test. The patients clearly prefer this examination to other more painful procedures, such as needle mapping with elec-tromyography. This fact also allows ultrasonography to serve as a surveillance tool to monitor the results after sphincteroplasty.10
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