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Published online before print May 8, 2003, 10.1148/radiol.2281011900
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(Radiology 2003;228:226-229.)
© RSNA, 2003

Pediatric Imaging

Imperforate Anus: US Determination of the Type with Infracoccygeal Approach1

Tae Il Han, MD, In-One Kim, MD and Woo Sun Kim, MD

1 From the Department of Radiology, Eulji University School of Medicine, 24-14 Mok-Dong, Jung-Gu, Taejon 301-726, South Korea (T.I.H.); and Department of Radiology, Seoul National University College of Medicine, South Korea (I.O.K., W.S.K.). From the 2000 RSNA scientific assembly. Received November 26, 2001; revision requested February 5, 2002; revision received September 23; accepted November 18. T.I.H. supported by Korea Research Foundation grant KRF-2000-003-F00215. Address correspondence to T.I.H. (e-mail: tihan31@hanmail.net).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
PURPOSE: To assess the usefulness of infracoccygeal transperineal ultrasonography (US) in differentiation between high- and low-type imperforate anus.

MATERIALS AND METHODS: Infracoccygeal US was prospectively performed with a 7–10-MHz linear-array transducer prior to corrective surgery in 14 neonates with imperforate anus. The approach site was just inferior to the coccyx and posterior to the anus. Transverse images of the anorectal area were obtained. The puborectalis muscle was identified, and the relationship between the puborectalis muscle and the distal rectal pouch was evaluated. US findings were compared with surgical findings.

RESULTS: In 10 neonates, a low-type imperforate anus was correctly diagnosed at infracoccygeal US. In those with low-type imperforate anus, the puborectalis muscle was seen as a hypoechoic U-shaped band (n = 10), and the distal rectal pouch passed through the puborectalis muscle (n = 10). In four neonates with high-type imperforate anus, the puborectalis muscle was not identified (n = 4).

CONCLUSION: Infracoccygeal transperineal US enables the determination of the type of imperforate anus.

© RSNA, 2003

Index terms: Anus, abnormalities, 757.1433 • Anus, US, 757.1298, 757.92 • Infants, newborn, gastrointestinal tract, 757.92 • Ultrasound (US), in infants and children, 757.1298


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
In the diagnostic evaluation of neonates with imperforate anus, the primary goal is to distinguish between high-type (supralevator) and low-type (infralevator) imperforate anus to determine the correct type of surgery. The relationship between the distal rectal pouch and the puborectalis muscle is a critical factor in the estimation of whether an imperforate anus is high or low type.

The distinction between high- and low-type imperforate anus can usually be made on the basis of clinical data regarding the presence or absence of a visible perineal opening or passage of meconium through the vagina or urethra (1). Many reports are available in the literature about the level of the distal rectal pouch in patients with imperforate anus. However, preoperative imaging modalities have been limited in the ability to reveal the relationship between the puborectalis muscle and the distal rectal pouch (2–4).

Infracoccygeal ultrasonography (US) is an excellent diagnostic modality for demonstration of the puborectalis muscle and anal sphincter complex in normal neonates (5). To our knowledge, US has not been previously used to identify the puborectalis muscle preoperatively in neonates with imperforate anus. The purpose of our study was to assess the usefulness of infracoccygeal transperineal US in differentiation of low-type imperforate anus from high-type imperforate anus.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Patients
Fourteen consecutive neonates (10 male and four female; age range, 1–24 days; mean age, 6 days) with imperforate anus identified from two participating institutions were prospectively examined preoperatively with infracoccygeal US to distinguish between high- and low-type imperforate anus (Table). Our institutional review board approved the study protocol, and informed consent was obtained from the parents of the neonates. Gestational ages ranged from 36 to 41 weeks, with a mean age of 39 weeks. Body weights ranged from 2,670 to 3,500 g (mean, 3,080 g). On the basis of surgical results, 10 cases were classified as low- and four as high-type imperforate anus. The type of lesion was determined on the basis of the international classification, which is determined by the relationship of the level of the distal rectal pouch to the puborectalis portion of the levator ani muscle (6,7).


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  		Patient Characteristics

 
Imaging
The study was performed by one of two radiologists (T.I.H. or I.O.K.) with expertise in infracoccygeal US, and the radiographic findings were evaluated by both of the radiologists with consensus without knowledge of the type of lesion or clinical symptoms. Infracoccygeal US was performed with a 7-MHz linear-array transducer (model 128 XP10; Acuson, Mountain View, Calif) or a 5–10-MHz multifrequency linear-array transducer (HDI 3000; Advanced Technology Laboratory, Bothell, Wash). The neonates were placed in the supine position, and both legs were drawn up to the chest. The approach site for infracoccygeal US was just inferior to the coccyx and posterior to the anus (5).

Scanning was performed to obtain transverse images of the anorectal area. Standard gray-scale settings for the evaluation of small anatomic parts were used. Infracoccygeal US was easily performed in all neonates without sedation, and each examination required 3–5 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 (CT) or magnetic resonance (MR) images.

We identified the puborectalis muscle and evaluated the relationship between the distal rectal pouch and the puborectalis muscle on the transverse image obtained with an infracoccygeal approach (Fig 1). Additionally, midline sagittal scanning was performed to measure the distance from the end of the distal rectal pouch to the perineum. US findings were compared with surgical findings.



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  		Figure 1. Lateral schematic of the pelvis shows the transverse scanning level to evaluate the relationship between the puborectalis muscle and the distal rectal pouch. Scanning site (arrow) is just inferior to the coccyx.

 

   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
In 10 neonates, a low-type imperforate anus was correctly diagnosed with infracoccygeal US. In those with low-type imperforate anus, infracoccygeal US revealed the puborectalis muscle (n = 10). On the transverse image obtained through the upper part of the anal canal, the puborectalis muscle was seen as a hypoechoic U-shaped band (n = 10), and the distal rectal pouch passed through the puborectalis muscle (n = 10) (Figs 2, 3).



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  		Figure 2. Patient 6. Transverse infracoccygeal sonogram shows the distal rectal pouch (R), which passes through the puborectalis muscle (arrows), indicating low-type imperforate anus. U = urethra.

 


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  		Figure 3. Patient 10. Transverse infracoccygeal sonogram shows distal rectal pouch (R), which passes through the puborectalis muscle (arrows), indicating low-type imperforate anus. U = urethra.

 
In four neonates with high-type imperforate anus, the puborectalis muscle and anal sphincters were not identified on the transverse image obtained through the anal canal (Fig 4).



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  		Figure 4. Patient 2. Transverse infracoccygeal sonogram shows the distal rectal pouch (R). The puborectalis muscle cannot be identified.

 
On the midline sagittal scan, the distance between the end of the distal rectal pouch and the perineum was 6.3–13.0 mm (mean, 11.4 mm) in low-type imperforate anus and 11.5–14.0 mm (mean, 12.5 mm) in high-type imperforate anus. A distance of 10–15 mm between the pouch and the perineum was present in 12 of the 14 neonates, in eight of 10 neonates with low-type imperforate anus, and in all neonates with high-type imperforate anus.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
REFERENCES
 
The radiologic modalities used in imaging of imperforate anus include inverted lateral radiography (invertography) (8,9), distal colostography (loopography), US (2–4), CT (10,11), and MR imaging (12–14). They have been used to determine the level of the distal rectal pouch (2–4), to identify the presence of fistulas (15,16), and to diagnose any associated anomalies (17,18). To distinguish between high- and low-type imperforate anus is critical in the determination of the surgical approach. A low-type imperforate anus is treated with a perineal anoplasty or dilatation of an ectopic perineal orifice, whereas all cases of high-type imperforate anus require an initial colostomy followed by a definitive repair pull-through operation (19–21).

The puborectalis muscle is a landmark used to distinguish low- and high-type imperforate anus. However, invertography, loopograhy, and US are limited because of the inability to display directly the puborectalis muscle. Although CT and MR imaging can demonstrate the presence of the puborectalis muscle and external anal sphincter prior to surgery (10–14), they are limited in the ability to reveal the relationship of the distal rectal pouch to the puborectalis muscle.

Infracoccygeal US can directly demonstrate the puborectalis muscle in neonates with imperforate anus (5). The puborectalis muscle was identified as a hypoechoic U-shaped band at the level of the anorectal flexure. US finding of the distal rectal pouch passing through the puborectalis muscle suggests a low-type imperforate anus. The puborectalis muscle is the innermost portion of the levator ani muscle and is considered to have an important role in the control of bowel function. Sato et al (13) reported that all patients with low-type imperforate anus showed good development of the puborectalis muscle, whereas 12 of 15 patients with high-type imperforate anus showed poor development of the puborectalis muscle. In high-type imperforate anus, the puborectalis muscle is small and tightly applied to the urethra or vagina (22,23). In our study, the puborectalis muscle could not be identified on an infracoccygeal US image of high-type imperforate anus.

In contrast, conventional transperineal US cannot depict the puborectalis muscle. The differentiation of low- from high-type imperforate anus has been indirectly performed with the measurement of the distance from the distal rectal pouch to the perineum, which is now used routinely (2–4). A distance of 1.0 cm or less between the pouch and the perineum suggests a low-type imperforate anus, a distance of 1.0–1.5 cm indicates an intermediate-type imperforate anus, and a distance of 1.5 cm or greater implies a high-type imperforate anus (4). The limitation of this method is the overlap in the measurements between high- and low-type imperforate anus. A high-type imperforate anus can be mistaken for a low-type imperforate anus if US is performed when the infant is crying, because this displaces the pouch caudally. In our study, there is some overlapping in the measurements between high- and low-type imperforate anus.

In pediatric surgery, the physical examination is the most helpful method in the estimation of whether an imperforate anus is of the high or the low type (24). In male patients, if meconium appears anywhere in the perineum, either through an anocutaneous fistula or in the median raphe of the scrotum, the lesion is a low type. However, if meconium is passed in urine but is not visible elsewhere in the perineum, the lesion is almost certainly a high type. In female patients, an anocutaneous or anovestibular fistula to the posterior fourchette of the vagina is almost always visible in low-type lesions. If the opening of the bowel cannot be seen, it is likely that there is a high rectovaginal fistula, which requires a temporary diverting colostomy and formal reconstruction at a later date. However, the type of imperforate anus is not clear in a case without fistula, as observed at clinical examination. In some cases, external fistulas may not become apparent until the neonate is 12–24 hours of age, at which time the meconium moves distally into the rectum (25). Hence, physical examination does not provide enough information about the level of the distal rectal pouch; it must be defined radiographically.

In conclusion, infracoccygeal US in neonates with imperforate anus can help in the evaluation of the relationship between the puborectalis muscle and the distal rectal pouch and is an excellent diagnostic modality for determination of the type of imperforate anus.


   FOOTNOTES
 
Author contributions: Guarantor of integrity of entire study, T.I.H.; study concepts and design, T.I.H.; literature research, T.I.H.; clinical studies, T.I.H., I.O.K.; data acquisition, T.I.H.; data analysis/interpretation, T.I.H., I.O.K.; manuscript preparation, definition of intellectual content, and editing, T.I.H., I.O.K.; manuscript revision/review, T.I.H., I.O.K., W.S.K.; manuscript final version approval, T.I.H.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 

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