(Radiology. 2000;217:392-394.)
© RSNA, 2000
US Identification of the Anal Sphincter Complex and Levator Ani Muscle in Neonates: Infracoccygeal Approach1
Tae Il Han, MD,
In-One Kim, MD,
Woo Sun Kim, MD,
Jin Young Chung, MD,
Kyuchul Choeh, MD,
Mun Kab Song, MD and
Yong Gyu Yoon, MD
1 From the Departments of Radiology (T.I.H., J.Y.C., M.K.S., Y.G.Y.) and Pediatrics (K.C.), Eulji University School of Medicine, 24-14 Mok-Dong, Jung-Gu, Taejon 301-726, South Korea, and the Department of Radiology, Seoul National University College of Medicine, South Korea (I.O.K., W.S.K.). From the 1999 RSNA scientific assembly. Received December 23, 1999; revision requested January 30, 2000; revision received April 3; accepted April 20. Address correspondence to T.I.H. (e-mail: tihan@emc.eulji.ac.kr).
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ABSTRACT
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PURPOSE: To identify the anal sphincter complex and levator ani muscle at transperineal ultrasonography (US) with the infracoccygeal approach.
MATERIALS AND METHODS: Infracoccygeal US was performed with a 7-MHz linear-array transducer in 40 healthy neonates. The babies were placed in the supine position, and both legs were drawn up to the chest. The buttocks were accordingly lifted up. The approach site was just inferior to the coccyx and posterior to the anus. Scanning was performed to obtain transverse images of the anorectal area. The thickness of the anal sphincter complex and the puborectalis muscle were measured.
RESULTS: Infracoccygeal US revealed the internal anal sphincter (IAS), the external anal sphincter (EAS), and the puborectalis muscle in all babies. The IAS and EAS were depicted as inner and outer hypoechoic ringlike structures, respectively. A round, hyperechoic space (intersphincteral space) was present between the hypoechoic IAS and EAS. The puborectalis muscle was identified as a U-shaped hypoechoic structure. The bulbocavernosus and ischiocavernous muscles were also identified. Mean thicknesses were as follows: IAS, 1.3 mm ± 0.3 (SD) (range, 0.8–1.9 mm); EAS, 1.6 mm ± 0.3 (range, 1.2–2.3 mm); and puborectalis muscle, 1.1 mm ± 0.3 (range, 0.6–1.9 mm).
CONCLUSION: Infracoccygeal transperineal US is an excellent diagnostic modality for demonstrating the anal sphincter complex and levator ani muscle in neonates.
Index terms: Anus, US, 757.1298, 757.92 • Infants, newborn, gastrointestinal tract, 757.92 • Ultrasound (US), in infants and children, 757.1298
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INTRODUCTION
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The imaging modalities currently used to examine the anal sphincter complex and levator ani muscle are endoanal ultrasonography (US) and endoanal magnetic resonance (MR) imaging (1–3). The disadvantages of these imaging modalities include patient discomfort, the necessity of using an endoanal probe or coil, and the inability to access an imperforate anus or stenotic anus.
Transperineal US in children has been used for the evaluation of distal anorectal anomalies (4). Transperineal US is also useful in diagnosing posterior urethral valves and in identifying a distal rectal pouch and an internal fistula in an imperforate anus. However, it does not allow adequate identification of the anal sphincter complex and levator ani muscle in neonates and infants. Conventional transperineal US has been mainly performed in the sagittal plane, and it cannot be performed in the transverse plane because of interference by the pubic bone.
To overcome this problem, we developed an infracoccygeal approach with transperineal US to identify the anal sphincter complex and levator ani muscle. This study was conducted to determine the normal appearance of the anal sphincter complex and levator ani muscle on US images obtained by using an infracoccygeal approach.
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MATERIALS AND METHODS
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From January 1999 through March 1999, 40 consecutive neonates (20 male and 20 female; age range, 1–21 days; mean age, 6 days) with normal meconium passage were examined with infracoccygeal US to identify the anal sphincter complex, levator ani muscle, and adjacent pelvic floor muscles. Our institutional review board approved the study protocol, and informed consent was obtained from the parents of the neonates. Gestational ages ranged from 33 to 40 weeks, with a mean of 38 weeks. Body weights ranged from 2,520 to 4,200 g (mean, 3,220 g). We excluded premature neonates (<37 weeks gestational age) with a low birth weight (<2,500 g).
One examiner (T.I.H.) performed all US examinations in this prospective study by using a 7-MHz linear-array transducer (model 128; Acuson, Mountain View, Calif). We modified the transperineal US technique to use an infracoccygeal approach to obtain transverse images. The babies were placed in the supine position, both legs were drawn up to the chest, and the buttocks were accordingly lifted up (Fig 1a). The approach site for infracoccygeal US was just inferior to the coccyx and posterior to the anus.
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Figure 1a. (a) Photograph demonstrates placement of the linear transducer (T [7-MHz; Acuson, Mountain View]) between the anus and coccyx at infracoccygeal US. (b) Lateral schematic of the pelvis shows the transverse scanning levels: I, lower, to evaluate the anal sphincter; II, upper, to evaluate the puborectalis muscle; and III, anterior, to evaluate the bulbocavernosus and ischiocavernous muscles. Scanning sites (*) are just inferior to the coccyx and posterior to the anus.
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Figure 1b. (a) Photograph demonstrates placement of the linear transducer (T [7-MHz; Acuson, Mountain View]) between the anus and coccyx at infracoccygeal US. (b) Lateral schematic of the pelvis shows the transverse scanning levels: I, lower, to evaluate the anal sphincter; II, upper, to evaluate the puborectalis muscle; and III, anterior, to evaluate the bulbocavernosus and ischiocavernous muscles. Scanning sites (*) are just inferior to the coccyx and posterior to the anus.
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Scanning was performed to obtain transverse images of the anorectal area. Standard gray-scale settings for evaluation of small anatomic parts were used. Infracoccygeal US was easily performed in all babies without sedation, and each examination required 2–5 minutes. A thick layer of gel was applied between the transducer and the perineum to prevent artifacts from intervening air. US images were rotated vertically to correspond to the usual orientation on computed tomographic or MR images.
The echogenicity of the structures was evaluated as high (high-level echoes similar to those of fat) or low (low-level echoes similar to those of muscle) on the basis of US descriptions provided by previous investigators (5,6). If the echogenicity was between high and low, the structure was described as having intermediate echogenicity.
The thickness of the internal anal sphincter (IAS), external anal sphincter (EAS), and puborectalis muscle was measured in the resting state.
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RESULTS
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Infracoccygeal US revealed the IAS and EAS in the lower part of the anal canal and the puborectalis muscle in the upper part of the anorectal region (Fig 1b). On the transverse image obtained through the lowest part of the anal canal, two hypoechoic ring-shaped structures were identified in all babies (Fig 2). The inner, hypoechoic, ringlike structure was the IAS, and the outer structure was the EAS. The IAS had low echogenicity in all babies, whereas the EAS had low (n = 30) or intermediate (n = 10) echogenicity. A round hyperechoic space between the hypoechoic IAS and EAS was the intersphincteral space (Fig 2), which contained longitudinal muscle. On the upper transverse scan, the puborectalis muscle was identified as a U-shaped hypoechoic structure in all babies (Fig 3). On the anterior transverse scan of the anus, the bulbocavernosus muscle was well delineated in all female babies (Fig 4a). On more cranial images, the ischiocavernous muscle was identified as an inverted V-shaped hypoechoic structure in all babies (Fig 4b).
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Figure 2. Transverse infracoccygeal sonogram of the anal sphincter complex obtained at level I (Fig 1b) shows the inner hypoechoic IAS (arrows), outer hypoechoic EAS (arrowheads), and hyperechoic intersphincteral space.
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Figure 4a. (a) Transverse sonogram obtained at level III (Fig 1b) shows a hypoechoic bulbocavernosus muscle (arrowheads). The inner mixed hyperechoic structure is the vagina (V). (b) More cranial transverse sonogram shows the ischiocavernous muscle (arrows) as an inverted V-shaped structure. The urethra (U) and the vagina (V) are seen between two slings of the ischiocavernous muscle.
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Figure 4b. (a) Transverse sonogram obtained at level III (Fig 1b) shows a hypoechoic bulbocavernosus muscle (arrowheads). The inner mixed hyperechoic structure is the vagina (V). (b) More cranial transverse sonogram shows the ischiocavernous muscle (arrows) as an inverted V-shaped structure. The urethra (U) and the vagina (V) are seen between two slings of the ischiocavernous muscle.
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The mean thickness of the IAS was 1.3 mm ± 0.3 (SD) (range, 0.8–1.9 mm). The mean thickness of the EAS was 1.6 mm ± 0.3 (range, 1.2–2.3 mm). The mean thickness of the puborectalis muscle was 1.1 mm ± 0.3 (range, 0.6–1.9 mm).
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DISCUSSION
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US evaluation of the anus and distal rectum has been performed with transperineal or endoanal US. Endoanal US in adults has evolved rapidly in recent years and has become an established procedure to evaluate the sphincteral defects in fecal incontinence or perianal abscesses including fistulous tracts (6–8). However, in children or neonates, endoanal US cannot be used in cases of anal stenosis or imperforate anus.
Infracoccygeal US provides a transverse image that enables us to identify the anal sphincter complex and levator ani muscle. On infracoccygeal US images, the IAS was seen as a clearly defined inner hypoechoic ring, which has a similar appearance on endoanal US images (5,9,10). The IAS is an intrinsic muscle surrounding the anal canal just beneath the mucosal layer. Anatomically, the circular smooth muscle layer of the rectum continues to the anal canal, where it thickens to become the IAS. The echogenicity of the IAS increases with age, with the replacement of smooth muscle by the connective tissues (7,11). In our study, the IAS in all neonates was hypoechoic.
The EAS was identified as an outer hypoechoic ring in our study. The EAS has three components: deep, superficial, and subcutaneous. The deep external sphincter blends imperceptibly into the inferior portion of the puborectalis muscle. The echogenicity of the EAS is variable with endoanal US in adults, and it was seen as a circular ring with low or intermediate echogenicity on infracoccygeal US images.
The measured thicknesses of the IAS (range, 0.8–1.9 mm) and the EAS (range, 1.2–2.3 mm) were less than the ranges reported (1,2,12) with endoanal US in adults (IAS range, 0.5–3.7 mm; EAS range, 5.8–9.8 mm).
The intersphincteral space was revealed as a round hyperechoic ringlike structure between the hypoechoic IAS and EAS. This space contains fat and longitudinal muscle, which is the terminal portion of longitudinal smooth muscle of the rectum (13). The longitudinal muscle is the least understood anorectal structure, but it is thought to play a role in binding and bracing the components of the anal canal together and in fixing the perineum to the pelvis (13,14).
Sultan et al (15,16) have suggested that the longitudinal muscle is hyperechoic partly because the direction of the fibers affects the reflectivity and also because the longitudinal muscle may be mixed with fibrous tissue. They described that while the longitudinal muscle could be identified in adult males, it was identified in less than half of the adult females; this finding can be explained by the differences in the echogenicity of the adjacent EAS (15,16). However, we could find the intersphincteral space as a hyperechoic structure regardless of sex. Identification of the IAS and EAS may be helpful in the postoperative evaluation of patients with imperforate anus and in the determination of the anatomic relationship of the pulled-through intestine with the anal sphincter complex.
The puborectalis muscle was also identified as a hypoechoic U-shaped band at the level of the anorectal flexure, which also merged with the hypoechoic EAS. The puborectalis muscle is the innermost portion of the levator ani muscle, and it is considered to have an important role in the control of bowel function after surgical repair of an anorectal anomaly. Both the EAS and the puborectalis muscle are composed of striated muscle fibers, and they are also in close contact with each other. The puborectalis muscle is a landmark used in distinguishing patients with low-type imperforate anus and those with high-type imperforate anus. Because infracoccygeal US can demonstrate the puborectalis muscle, it may be useful for the evaluation of the relationship between the puborectalis muscle and the distal rectal pouch; this evaluation is important in the determination of the type of imperforate anus.
In conclusion, infracoccygeal US in the neonate shows the anatomy of the anal sphincter complex and levator ani muscle and will be helpful in the detection of disease in this area.
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ACKNOWLEDGMENTS
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The authors thank Seo-Yoon Choi for drawing the schematic representation.
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FOOTNOTES
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Abbreviations: EAS = external anal sphincter,
IAS = internal anal sphincter
Author contributions: Guarantor of integrity of entire study, T.I.H.; study concepts, T.I.H., I.O.K.; study design, T.I.H.; definition of intellectual content, T.I.H.; literature research, T.I.H.; clinical studies, T.I.H.; data acquisition, T.I.H., I.O.K., W.S.K., J.Y.C.; data analysis, T.I.H., I.O.K.; statistical analysis, T.I.H.; manuscript preparation and editing, T.I.H., I.O.K.; manuscript review, all authors.
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REFERENCES
|
|---|
-
Nielsen MB, Pedersen JF, Hauge C, Rasmussen OO, Christiansen J. Endosonography of the anal sphincter: findings in healthy volunteers. AJR Am J Roentgenol 1991; 157:1199-1202.[Abstract/Free Full Text]
-
Hussain SM, Stoker J, Lameris JS. Anal sphincter complex: endoanal MR imaging of normal anatomy. Radiology 1995; 197:671-677.[Abstract/Free Full Text]
-
deSouza NM, Gilderdale DJ, MacIver DK, Ward HC. High-resolution MR imaging of the anal sphincter in children: a pilot study using endoanal receiver coils. AJR Am J Roentgenol 1997; 169:201-206.[Abstract/Free Full Text]
-
Teele RL, Share JC. Transperineal sonography in children. AJR Am J Roentgenol 1997; 168:1263-1267.[Free Full Text]
-
Alexander AA, Miller LS, Liu JB, Feld RI, Goldberg BB. High-resolution endoluminal sonography of the anal sphincter complex. J Ultrasound Med 1994; 13:281-284.[Abstract]
-
Alexander AA, Liu JB, Merton DA, Nagle DA. Fecal incontinence: transvaginal US evaluation of anatomic causes. Radiology 1996; 199:529-532.[Abstract/Free Full Text]
-
Bartram CI, Sultan AH. Anal endosonography in faecal incontinence. Gut 1995; 37:4-6.[Free Full Text]
-
Nielsen MB, Hauge C, Pedersen JF, Christiansen J. Endosonographic evaluation of patients with anal incontinence: findings and influence on surgical management. AJR Am J Roentgenol 1993; 160:771-775.[Abstract/Free Full Text]
-
Schafer A, Enck P, Furst G, Kahn T, Frieling T, Lubke HJ. Anatomy of the anal sphincters: comparison of anal endosonography to magnetic resonance imaging. Dis Colon Rectum 1994; 37:777-781.[Medline]
-
Law PJ, Bartram CI. Anal endosonography: technique and normal anatomy. Gastrointest Radiol 1989; 14:349-353.[Medline]
-
Nielsen MB, Pedersen JF. Changes in the anal sphincter with age: an endosonographic study. Acta Radiol 1996; 37:357-361.[Medline]
-
Sangwan YP, Solla JA. Internal anal sphincter: advances and insights. Dis Colon Rectum 1998; 41:1297-1311.[Medline]
-
Lunniss PJ, Phillips RK. Anatomy and function of the anal longitudinal muscle. Br J Surg 1992; 79:882-884.[Medline]
-
Halligan S. Imaging fistula-in-ano. Clin Radiol 1998; 53:85-95.[Medline]
-
Sultan AH, Kamm MA, Hudson CN, Nicholls JR, Bartram CI. Endosonography of the anal sphincters: normal anatomy and comparison with manometry. Clin Radiol 1994; 49:368-374.[Medline]
-
Sultan AH, Nicholls RJ, Kamm MA, Hudson CN, Beynon J, Bartram CI. Anal endosonography and correlation with in vitro and in vivo anatomy. Br J Surg 1993; 80:508-511.[Medline]
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ABSTRACT
INTRODUCTION
