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PEDIATRIC IMAGING

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Anorectal Malformations: Finding the Pathway out of the Labyrinth1 ONLINE-ONLY SA-CME See www.rsna .org/education /search/RG

LEARNING OBJECTIVES After completing this journal-based SACME activity, participants will be able to: ■■Describe

the embryologic basis of ARMs and their classification. ■■Discuss

the role of imaging in diagnosis and evaluation of ARMs. ■■Identify

the most relevant syndromes and associations of congenital anomalies that include ARMs.

Leonor Alamo, MD • Blaise J. Meyrat, MD • Jean-Yves Meuwly, MD Reto A. Meuli, MD, PhD • François Gudinchet, MD Anorectal malformations (ARMs) are a complex group of congenital anomalies involving the distal anus and rectum, as well as the urinary and genital tracts in a significant number of cases. Most ARMs result from abnormal development of the urorectal septum in early fetal life. In most cases, the anus is not perforated and the distal enteric component ends blindly (atresia) or as a fistula into the urinary tract, genital tract, or perineum. ARMs are also present in a great number of syndromes and associations of congenital anomalies. The classification of ARMs is mainly based on the position of the rectal pouch relative to the puborectal sling, the presence or absence of fistulas, and the types and locations of the fistulas. All of this information is crucial in determining the most appropriate surgical approach for each case. Imaging studies play a key role in evaluation and classification of ARMs. In neonates, clinical and radiologic examinations in the first 3 days of life help determine the type of ARM and the need for early colostomy. In older children, preoperative pelvic magnetic resonance imaging is the most efficient diagnostic method for evaluating the size, morphology, and grade of development of the sphincteric musculature.

Introduction

Anorectal malformations (ARMs) are a complex group of congenital anomalies involving the distal anus and rectum, as well as the urinary and genital tracts in a significant percentage of cases. The prevalence is approximately one per 5000 live births, with a slight male preponderance (1,2). In most ARMs, the anus is not perforated and the distal enteric component may end blindly (ie, atresia) or as a fistula into the urinary tract, genital tract, or perineum. ARMs are associated with other congenital anomalies in up to 70% of cases (2–5) (Table 1). The final prognosis and quality of life for children with ARMs will depend, to a large extent, on the presence and gravity of these associated anomalies.

Abbreviations: ARM = anorectal malformation; EAS = external anal sphincter; VACTERL = vertebral anomalies, anal atresia, cardiac malformations, tracheoesophageal fistula, renal and limb anomalies; VCUG = voiding cystourethrography RadioGraphics 2013; 33:491–512 • Published online 10.1148/rg.332125046 • Content Codes: From the Departments of Diagnostic and Interventional Radiology (L.A., J.Y.M., R.A.M., F.G.) and Pediatric Surgery (B.J.M.), Centre Hospitalier Universitaire Vaudois, Rue du Bugnon 46, 1011 Lausanne, Switzerland. Recipient of a Certificate of Merit award for an education exhibit at the 2011 RSNA Annual Meeting. Received March 28, 2012; revision requested May 2; final revision received September 3; accepted September 14. For this journal-based SA-CME activity, the authors, editor, and reviewers have no relevant relationships to disclose. Address correspondence to L.A. (e-mail: [email protected]). 1

©

RSNA, 2013 • radiographics.rsna.org

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Table 1 Most Common Associated Anomalies in Patients with ARMs Affected System

Anomalies

Cardiovascular

Tetralogy of Fallot, atrial septal defect, ventricular septal defect, dextrocardia, coarctation of the aorta Esophageal atresia; duodenal, jejunal, or ileal atresia; absent colon; intestinal malrotation; volvulus; Meckel diverticulum Hip dislocation or dysplasia, fusion of iliac bones, Madelung deformity, arthrogryposis, clubfoot, polydactyly, syndactyly, limb deficiency Sacral agenesis, vertebral dysplasia, spina bifida, tethered cord, myelomeningocele

Gastrointestinal Musculoskeletal Spinal cord and spine Urogenital

Vesicoureteral reflux, hydronephrosis, bilateral or unilateral renal agenesis, renal dysplasia, renal ectopia, horseshoe kidney, polycystic kidney, renal duplication, megaureter, exstrophy of the bladder, micropenis, hypospadias, double uterus or double vagina, vulvovaginal atresia, ambiguous genitalia

Table 2 Most Common Syndromes or Multisystemic Conditions Associated with ARMs Type of Associated Entity

Syndromes or Multisystemic Conditions

Associations of congenital anomalies

VACTERL (Vertebral anomalies, Anal atresia, Cardiac malformations, TracheoEsophageal fistula, Renal and Limb anomalies), OEIS (Omphalocele, Exstrophy, Imperforate anus, Spinal defects), MURCS (MÜllerian duct aplasia, Renal aplasia, Cervicothoracic Somite dysplasia) Trisomy 13, 18, and 21; parental unidisomy 16; deletion of 22q11.2 and 13q; heterotaxia Baller-Gerold, cat-eye, caudal regression, Christian, Currarino triad, Down, facio-auriculo-vertebral, Feingold, fetal alcohol, FG, Fraser, Ivemark, JohansonBlizzard, kabuki, Klippel-Feil, Lowe, MIDAS, Okihiro, Opitz, Pallister-Hall, Pallister-Killian, Rieger, Townes-Brock, ulnar-mammary, Walker-Warburg

Chromosomopathies Syndromes

Source.—Reference 2.

Urogenital abnormalities are the most frequently observed and appear in up to 60% of patients, with vesicoureteral reflux and hydronephrosis the most common findings (1). The spine and spinal cord are also often involved, with agenesis and dysplasia of the sacrum, vertebral dysplasia, and tethered cord syndrome the most frequently detected problems (6). ARMs are also present in a great number of syndromes and associations of multisystemic congenital anomalies (2,5), as shown in Table 2. The classification of ARMs is mainly based on the position of the rectal pouch relative to the puborectal sling and the presence or absence of fistulas (7,8). Imaging studies play a key role in the initial evaluation of ARMs. In neonates, an

accurate diagnosis of the type of ARM, the presence or absence of fistulas and their locations, and identification of associated anomalies are essential in deciding about immediate therapy. However, the real utility and the most appropriate sequence of performance of the different imaging diagnostic methods in the first days of life remain subject to discussion (4,9). For patients with ARMs, the ultimate objective of therapy is to achieve a good quality of life, with acceptable levels of bowel control and normal sexual and reproductive capacities. In infants and older children, pelvic magnetic resonance (MR) imaging is the most effective imaging technique for defining the anatomy and determining the grade of development of the sphincteric muscles (10–14), which play a significant role in continence. This information will help the medical team make deci-

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Figure 1.  Embryologic basis of ARMs. (a) Evolution of the cloaca at day 32. (b, c) Subdivision of the cloaca and the beginning of formation of the perineum at day 36 (b) and day 56 (c). 1 = allantois, 2 = cloacal membrane, 3 = cloaca, 4 = urorectal septum, 5 = Rathke fold, 6 = Tourneux fold, 7 = primitive urogenital sinus, 8 = anus, 9 = rectum, 10 = perineal area.

sions about the definitive surgical approach and provide orientation about the possible postoperative prognosis and remaining sequelae (2). In this article, we review the embryologic basis of ARMs and discuss their classification and its implications for therapy. Moreover, we propose an algorithm based on clinical and imaging findings for classification of ARMs during the neonatal period. We also describe the normal anatomy of the sphincteric muscles at pelvic MR imaging and present the most typical imaging findings for a wide spectrum of ARMs, including the most frequently observed associated conditions and syndromes.

Embryology

An appropriate knowledge of embryology is crucial for understanding ARMs. The early embryologic development of the anorectum, the primitive urogenital sinus, and the caudal neural tube is closely related (15,16), which helps explain the associated malformations of these three systems. In early embryonic life, the terminal portion of the hindgut—the primitive cloaca—is divided into dorsal and ventral parts by a coronal sheet of mesenchyme—the urorectal septum—and separated from the amniotic cavity by the cloacal membrane (17,18). Most ARMs result from abnormal development of the urorectal septum (Fig 1). Between weeks 4 and 6 of gestation, both the yolk sac or primitive hindgut and the allantois or primitive urogenital sinus enter into the cloaca (Fig 1a). The urorectal septum then develops forklike infoldings (Tourneux and Rathke folds) of the lateral cloacal walls; at the same time, the

embryo starts to curve as a result of the longitudinal growth of the developing neural tube and the mesodermal compartment. With these morphologic changes, the distance between the cloacal membrane and the tip of the urorectal septum is progressively reduced (Fig 1b). At the end of week 7, the urorectal septum and the cloacal membrane are located at the same level. The cloaca is thus divided into a ventral part (the urogenital sinus) and a dorsal part (the rectum and proximal anal canal). Between them, the tip of the urorectal septum becomes the perineal area (Fig 1c). At this time, the cloacal membrane ruptures by apoptosis, thus opening two orifices in the perineum: one ventral or urogenital and one dorsal or anal. Also at the end of week 7, a secondary occlusion of the anorectal canal takes place, initially by adhesion of the walls and later by formation of an epithelial “plug” at the anal level. This secondary closed anal orifice will rupture and recanalize by apoptosis at the end of week 8 (14,18). Embryologically, ARMs can thus be subdivided into two main groups according to when the disturbances occur: Those manifesting as an ectopic anal orifice or fistula are due to early abnormal development of the dorsal part of the cloaca and the cloacal membrane (at weeks 4–7), whereas those manifesting as an abnormal anus in a normal position are due to later defective recanalization of the secondary occluded anal orifice (at weeks 7 and 8) (14,18 ).

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Table 3 Comparison of Wingspread and Krickenbeck Classifications of ARMs in Male Patients Type of ARM

Wingspread Classification (1984)

Krickenbeck Classification (2005)

Low

Anal stenosis, anocutaneous fistula

Intermediate

Anal agenesis without fistula or with recto– urethral bulbar fistula Rectal atresia, anorectal agenesis without fistula or with recto–urethral prostatic fistula

High

Anal stenosis, imperforate anus without fistula, rectoperineal fistula Anal or anorectal agenesis without fistula or with recto–urethral bulbar, recto–urethral prostatic, or rectovesical fistula*

Source.—Reference 8. *These ARMs are considered both intermediate and high type.

Table 4 Comparison of Wingspread and Krickenbeck Classifications of ARMs in Female Patients Type of ARM

Wingspread Classification (1984)

Low

Anal stenosis, anal agenesis without fistula or with external fistula

Intermediate

Anal agenesis without fistula or with rectovestibular or rectovaginal fistula Rectal atresia, anorectal agenesis without fistula or with rectovaginal fistula, cloacal malformations

High

Krickenbeck Classification (2005) Anal stenosis, imperforate anus without fistula, anal agenesis with rectoperineal or rectovestibular fistula Anal or anorectal agenesis without fistula, rectal atresia, cloacal malformations with short (3 cm) common canal*

Source.—Reference 8. *These ARMs are considered both intermediate and high type.

Classification of ARMs

The best-known classification of ARMs is the Wingspread classification of 1984 (7). It divides ARMs into three types—low, intermediate, and high—depending on whether the rectal pouch is located below, at the level of, or above the puborectal sling, respectively, with special categories for cloacal and rare malformations. More recently, the Krickenbeck Conference of 2005 (8,19) established a new classification, which is based mainly on the presence or absence of fistulas and their type and location, as well as the position of the rectal pouch. The Krickenbeck classification distinguishes five types of fistulas: rectoperineal, rectovestibular, recto–urethral bulbar, recto–urethral prostatic, and rectovesical. Cloacal malformations, the absence of fistulas, anal stenosis, and rare regional variants complete this classification (8). The extremely rare rectovaginal fistula is considered a variant of cloacal anomaly. The Wingspread and Krickenbeck classifications for

males and for females are compared in Tables 3 and 4, respectively. The different types of fistulas in males and in females are shown in Figures 2 and 3, respectively. The most commonly used operative procedures for treatment of ARMs include perineal operations, posterior sagittal anorectoplasty according to deVries and Peña (20), and laparoscopic abdominoperineal rectoplasty techniques (21). Cloacal anomaly requires highly specialized reconstructive surgery. ARMs involving a rectal pouch located below the level of the puborectalis muscle, regardless of whether they are associated with a fistula— perineal or vestibular—are considered low-type ARMs. They may be managed early with a perineal approach involving opening of the rectal pouch and ligature of the fistula, if present. A rectal pouch lying at or above the level of the puborectal sling is considered an intermediate or high type of ARM; it is treated with colostomy in the first days of life and with posterior sagittal anorectoplasty alone or combined with laparoscopic abdominoperineal rectoplasty in a second

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Figure 2.  Possible locations of fistulas in males with ARMs according to the Krickenbeck classification. (a) Low-type ARMs have an external anocutaneous opening in the scrotum (1) or perineum (2, 3). (b) Intermediate- and high-type ARMs extend anteriorly to the base of the penis (1), the bulbar (2) or prostatic (3) urethra, or the urinary bladder (4).

Figure 3.  Possible locations of fistulas in females with ARMs according to the Krickenbeck classification. (a) Low-type ARMs have an external opening in the perineum (1) or vestibular area (2). (b) Cloacal anomaly is a complex anatomic disorder that manifests as a unique external perineal opening with a short (1) or long (2) common canal for the genital, urinary, and digestive systems. Isolated rectovaginal fistulas are extremely rare and are considered a variant of cloacal anomaly.

intervention (1,10,20,21). During posterior sagittal anorectoplasty, the patient is placed in a prone position with the pelvis elevated. A strictly midline incision is then made from the tip of the coccyx to the perineum. Throughout the procedure, muscles are identified with the help of a muscle stimulator. All muscle groups are separated and opened as if paging through a book, without cutting them, until the rectal pouch is located. The levator ani muscle must then be divided to reach the rectal pouch. Very high fistulas, mainly recto–urethral prostatic or rectovesical fistulas in boys, are sometimes impossible to visualize exclusively through a perineal sagittal approach, and a laparotomy or laparoscopy (ie, abdominoperineal rectoplasty) is also required. If a laparotomy is needed, the patient is then positioned face up, allowing the surgeon to work simultaneously from the abdo-

men and the perineum. The rectum is then mobilized until a sufficient length is obtained for anal reconstruction. After that, the levator ani muscle is repaired, followed by repair of the muscle complex and external anal sphincter (EAS) (20,21). The Wingspread and Krickenbeck classifications are very similar. The Wingspread classification allows location of the blind rectal pouch. The Krickenbeck classification is more descriptive; its most important advantage is the preoperative identification and anatomic evaluation of not only the rectal pouch but also any fistulas. This information allows the surgeon to anticipate the extent of mobilization of the atretic rectal segment required during surgery and helps determine the most appropriate surgical approach for each case.

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Figure 4.  Cloacal anomaly in a female fetus. (a) Coronal T2-weighted half-Fourier acquisition single-shot turbo spin-echo (HASTE) (Siemens Medical Solutions, Erlangen, Germany) MR image at week 35 of pregnancy shows two symmetric pelvic cystic structures (V), which correspond to fluid-filled hemivaginas. Cloacal anomaly and genital duplication were confirmed postnatally. (b, c) Sagittal T2-weighted HASTE MR image (b) and T1-weighted volumetric interpolated breath-hold examination (VIBE) MR image (c) show one of the distended hemivaginas (V) and liquid retention in one of the hemiuteri (U in b). The bladder is anteriorly displaced (white arrow in c), and the rectum is not identifiable in its normal position. Note the associated oligohydramnios and the normal hyperintensity of meconium in the colon (black arrow in c). (Case courtesy of Marianne Alison, MD, and Guy Sebag, MD, Hôpital Robert-Debré, Paris, France.)

Role of Imaging in Initial Evaluation of ARMs

Imaging studies play a key role in initial evaluation of ARMs. They not only allow classification of the ARM but also facilitate identification and determination of the severity of associated anomalies.

Prenatal diagnosis of ARMs remains rare and occurs in only up to 16% of cases (22). Oligohydramnios is present in approximately 26% of ARMs. Other common prenatal imaging findings include an abdominal or pelvic cystic mass (52%), fetal hydronephrosis (49%), fetal ascites (22%), and less frequently intestinal distention (18%) (23). The diagnosis of an ARM should be

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Figure 5.  Algorithm for classifying ARMs on the basis of clinical and imaging findings in the first days of life according to the Krickenbeck classification. The proposed diagnostic imaging methods are in red. h = hours, INT = intermediate, Rx = radiography, VCU = voiding cystourethrography.

considered in the presence of a distended bladder associated with oligohydramnios. Cloacal anomaly should be suspected if multiple adjacent pelvic cystic structures are detected in a female fetus; these may correspond to a distended bladder and a unique or septate fluid-filled vagina (22). The differential diagnosis includes hydrometrocolpos secondary to imperforate hymen (24). Fetal MR imaging in the third trimester of gestation can help confirm ARMs suspected at ultrasonography (US) (Fig 4). In a normal fetus, urine has signal intensity similar to that of fluid, with homogeneous hyperintensity on T2-weighted images and hypointensity on T1weighted images, whereas normal meconium in the distal colon and rectum appears hyperintense on T1-weighted images and hypointense on T2weighted images. In some fetuses with ARMs, mainly long common-channel cloacal anomalies (Fig 4) in females and some rectourinary fistulas in males, increased signal intensity in the rectum and decreased signal intensity in the bladder can be observed on T2-weighted images as a result of the mixing of urine and meconium (23). However, the majority of ARMs are first detected at birth. At this point, one of the most relevant concerns is to establish the indication for an early colostomy. In infants with external fistulas

(rectoperineal or rectovestibular), the diagnosis of a low type of ARM is evident, and a perineal surgical procedure should be performed early. If no external fistula is initially observed, the passage of meconium through the vagina or with the urine, a finding indicative of an intermediate or high type of ARM, may first become evident after 24–48 hours of life (25). Therefore, neonates should be reevaluated at day 2 of life. Opinions vary about the most useful imaging studies and their appropriate sequence of performance in neonates with ARMs. The formerly performed invertography has been shown to be inexact and should not be performed anymore (1,9,10). Imaging studies in the first 2 days of life should include radiography of the thorax, spine, and pelvis along with cardiac, perineal, abdominal, pelvic, and spine US to detect possible associated anomalies (Fig 5). 1. Radiographs of the thorax, spine, and pelvis in the anteroposterior and lateral views may allow identification of associated cardiac, costal, and vertebral anomalies (Fig 6), including alterations in the alignment of the spinal column and anomalies of the os sacrum. In cases of vertebral or sacral anomalies, spinal MR imaging should be performed.

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Figure 6.  Associated congenital anomalies in three neonates with ARMs. (a) Anteroposterior radiograph shows multiple thoracic vertebral anomalies (arrows), including hemivertebrae and congenital sagittal clefts (butterfly vertebrae). Note the distended gas-filled loops of small bowel and colon, in contrast to the absence of air in the rectum. (b) Lateral radiograph of the spine shows partial fusion of the sacral vertebrae (arrow). (c) Radiograph of the thorax shows dilatation of the proximal esophagus, dextrocardia, 13 pairs of ribs, and morphologically dysplastic twisted ribs (arrow) in a neonate with VACTERL syndrome who underwent surgery for esophageal atresia.

2. When performed by experienced hands, perineal US is an excellent method for evaluating the location of the distal rectal pouch and the anatomy and location of any rectourogenital fistulas (1,9). It is usually performed with a 10–12MHz high-resolution linear-array transducer by using the transperineal approach, with the child in a supine position and the pelvis and legs elevated. The bowel-skin distance between the rectal

pouch and the anus is then measured in the midline sagittal plane through the perineum. A distance of greater than 15 mm indicates a high type of ARM, whereas a distance of less than 15 mm suggests a low type of malformation (9,25–27). However, differentiation between a low lesion and a high lesion in a relatively low position may be difficult, if not impossible. Moreover,

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any straining or crying by the patient during the examination can increase the intraabdominal pressure, displacing the distal rectal pouch to the perineum and shortening this distance (12,26). Fistulas may be identified as linear tracts connecting the rectal pouch to the bladder, urethra, or posterior wall of the vagina. 3. Abdominal US and pelvic US are usually performed with an 8-MHz transducer, with the patient in both prone and supine positions. This technique facilitates optimal screening of the entire urinary tract, including the bladder, bladder neck, and posterior urethra in boys. Nevertheless, US evaluation of the urinary tract is limited in the first 24 hours after birth, as upper tract dilatation may be initially absent because of the physiologic dehydration and reduced urinary output of the newborn (28). Detection of any genitourinary anomalies will require complementary voiding cystourethrography (VCUG) (Fig 5). 4. Spinal US is currently accepted as a safe and inexpensive screening test for detection of spinal dysraphism (28–30). US images are obtained by using a 12-MHz high-resolution lineararray transducer in both midsagittal and transverse planes with the patient in a prone position. The method provides accurate information about the morphology and integrity of the os sacrum and distal vertebral column and allows identification of the level of the medullary cone and demonstration of a presacral mass. Tethered cord syndrome is found in a significant number of children with ARMs, regardless of the type of malformation. It consists of abnormal fixation of the distal spinal cord secondary to its attachment to a fixed structure, mainly a lipoma, dura mater, or skin, that interferes with and limits the motility of the cauda equina in a vertical direction (6,31,32). In cases of sacral or spinal cord anomalies, complementary spinal MR imaging is mandatory. 5. VCUG is mandatory in patients with pathologic renal or vesical findings at US or clinical suspicion of rectourinary fistulas (meconium in the urine) (Fig 5). Administration of a hydrosoluble contrast material through a bladder catheter enables identification of associated congenital urinary problems, mainly vesicoureteral reflux. This method may also demonstrate rectourinary fistulas (1,12). Indeed, most authors recommend VCUG for all patients, regardless of the type of ARM. Information obtained with these imaging studies will help physicians make the most ap-

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propriate decisions about initial therapy (Fig 5). In patients with low ARMs, corrective surgery performed with a perineal approach is usually performed at day 2 or 3 of life. Colostomy is required only if there are severe associated malformations. In patients with clinical and radiologic evidence of intermediate or high types of ARMs, a descending colostomy is usually performed at day 3 to allow alimentation. In these patients, high-pressure distal colostography and spinal and pelvic MR imaging are performed after the newborn period, before definitive surgical therapy. 6. High-pressure distal colostography is the most effective imaging technique for demonstrating fistulas. It consists of manual injection of hydrosoluble contrast material through a Foley catheter inserted into the distal colostomy, which is sealed by means of gentle traction of the inflated balloon. To increase the possibility of detecting a rectourinary fistula, the injection of contrast material should be continued until the patient voids the bladder (10). In most cases, the technique also provides accurate information about the position of the rectal pouch. However, sometimes the pressure required to demonstrate the fistula causes a slight depression of the filled distal rectum; this depression may reduce the real distance between the rectal pouch and the anus (9). 7. Spinal MR imaging is the most sensitive imaging modality for detecting vertebral and spinal cord anomalies (6). It is mandatory in cases of vertebral anomalies, sacral dysplasia, or abnormal results at spinal US. Indeed, spinal cord anomalies may be present regardless of the type of ARM, associated vertebral anomalies, or neurologic symptoms (29,31,32). Consequently, it is recommended that routine MR imaging of the spine be performed in all ARM patients.

Pelvic Muscle Anatomy: MR Imaging Correlation

Pelvic MR imaging is used in children with ARMs to evaluate both the real position of the rectal pouch and the size, morphology, and grade of development of the sphincteric muscles before definitive surgical correction. The final prognosis of ARMs depends significantly on the grade of development of these muscles. Interpretation of images requires an accurate knowledge of the normal anatomy of the muscle groups involved in defecation. The striated

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musculature of the pelvic floor consists of the levator ani muscle group—formed by the levator prostate or sphincter vaginae, puborectalis, pubococcygeus, and iliococcygeus muscles—and the EAS, which is subdivided into three layers: subcutaneous, superficial, and deep. These muscles are anatomically continuous and work as a single coherent unit, playing a major role in continence (11). In healthy patients, the levator ani muscle group forms the main support of the pelvic floor, separating the pelvis from the perineum and contributing to the support of the pelvic viscera (33,34). The pubococcygeus muscle arises from the superior part of the pubic rami and is directly attached to the obturator internus fascia, whereas the iliococcygeus muscle arises from the tendinous arch of the pelvic fascia and the obturator internus fascia. Both muscles are triangular, extending from the ischia to the lateral part of the coccyx. The puborectalis is the most inferior part of the levator ani muscle group and the only muscle not attached to the coccyx. It arises anteriorly from both rami of the os pubis, forms a sling around the rectal wall of the perineal flexure, and crosses to the opposite side to form a muscular raphe posterior to the rectum (33,34). The three layers of the EAS show anatomic continuity and are difficult to differentiate. The deep portion is the only layer posteriorly attached to the coccyx. Pelvic MR imaging has become the preferred imaging study for evaluating the grade of development of the sphincteric muscle complex in infants. Its multiplanar capacity allows accurate determination of the level of the rectal pouch relative to the puborectal sling (35). This method may also help identify and characterize fistulas. Two important reference transverse planes are used with MR imaging for evaluation of ARMs. The first is the pubococcygeal plane, which extends from the upper border of the os pubis to the os coccyx (Fig 7). As described by Stephens et al (7), this plane corresponds to the attachment level of the levator ani muscle to the pelvic wall. The pubococcygeal plane includes the prostate in males, the cervix in females, and the rectum in both sexes (11). In this plane, the puborectalis muscle appears as a hypointense triangular band, with a regular edge and uniform width, surrounding the rectum posterolaterally (Fig 8a).

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Figure 7.  The transverse pubococcygeal plane connects the upper border of the symphysis pubis (P) with the os coccyx (C) and is the most important reference in evaluation of the pelvic musculature. Axial images should be obtained parallel to this plane (horizontal red line); coronal images should be obtained perpendicular to it (vertical red line).

The second relevant transverse plane follows a line joining the lowest points of the ischial tuberosities (ischial plane) and represents the deepest point of the funnel of the levator ani muscles in healthy patients. The ischial plane includes the base of the penis in males and, posteriorly, the oval EAS (Fig 8b). At this level, a traced line between the os ischii will divide the perineum into two triangles: one anterior or urogenital and one posterior or anal (33,35). The superficial transverse perineal muscles, the perineal body, and the EAS can be clearly observed here (Fig 8b). In transverse planes, the deep portion of the EAS can be distinguished from the puborectalis muscle, which is located directly adjacent but cranially (34). Coronal images should be obtained perpendicular to the pubococcygeal plane to allow correct identification of the levator hammock and the anal canal inside the EAS (Fig 8c). This plane provides the best view for evaluation of the anatomic relationship between the puborectalis mus-

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Figure 8.  Normal anatomy of the pelvic floor musculature. (a) Axial T2-weighted MR image in the pubococcygeal plane shows the rectum surrounded by the puborectalis muscle (arrows), which appears as a triangle with the apex directed posteriorly. (b) Axial T2-weighted MR image in the ischial plane shows the oval EAS (black arrows). Caudally, the rectum and anal canal should lie in the center of the EAS. The superficial transverse perineal muscles (white arrows) are shown at the anterior border of the EAS and extend laterally to the ischial rami (I). (c) Coronal T2-weighted MR image shows the levator ani muscle (arrowheads) supporting the pelvic floor and the rectal ampulla resting on the levator hammock, whereas the rectum and anal canal penetrate the EAS (arrows). (d) Midsagittal T2-weighted MR image shows the sphincteric muscles (white arrows) as a posterior curved bandlike structure, with the rectal ampulla () resting over the curved structure of the levator ani muscle (black arrow).

cle and the EAS (35). The puborectalis muscle is seen as a hypointense narrow strip surrounding the rectum, although the pubococcygeus and puborectalis muscles are difficult to differentiate (11,34). It is also difficult to distinguish the different portions of the EAS, which appears as a hypointense layer surrounding the caudal part of

the anal canal (Fig 8c). The deep portion of the EAS overlaps the puborectalis muscle bundles and appears as a hypointense structure with a long elliptical morphology.

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Figure 9.  Low ARM without a fistula in a boy. (a) Photograph shows perineal pearls at the midscrotal line (arrows) but no external opening. (b) VCUG image at day 1 shows no communication between the bladder (B) and distal rectum (R). The air-filled anal canal can be followed to the subcutaneous level (arrows), anterior to the normal position of the anus.

Table 5 Combined Spinal and Pelvic MR Imaging Protocol for Neonates and Infants with ARMs Sequences T2W TSE (spine) T1W TSE (spine) T1W TSE (conus) T2W TSE (pelvis) T2W TSE (pelvis) T2W TSE (pelvis) T1W TSE (pelvis) T1W TSE (pelvis) T1W TSE (pelvis)

Section Thickness (mm)

TR/TE (msec)

Flip Angle (degrees)

FOV (mm)

Plane

3.00 3.00 4.00 3.00 3.00 4.00 4.00 4.00 4.00

4100/103 800/11 819/12 8030/88 8030/88 5680/104 752/12 819/12 819/12

120 120 140 120 120 120 120 140 140

300 300 200 150 150 200 200 200 200

Sagittal Sagittal Axial Axial Coronal Sagittal Sagittal Axial Coronal

Note.—Performed at the authors’ institution with a Verio 3-T imaging unit (Siemens Medical Solutions, Erlangen, Germany). FOV = field of view, TE = echo time, T1W = T1-weighted, TR = repetition time, TSE = turbo spin-echo, T2W = T2-weighted.

The midsagittal plane allows evaluation of the sphincteric muscles posterior to the prostate in males or the vagina in females along with the curved structure of the levator ani muscle (11) (Fig 8d). In this orientation, both the EAS and the

puborectalis muscle have an irregular aspect. The EAS appears as a hypointense layer surrounding the anal canal both anteriorly and posteriorly. The anterior part of the EAS is lower than the posterior part and extends ventrally, connecting with the bulbocavernous muscle in males and the perineal body; the posterior part extends to and connects

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Figure 10.  Low ARM with a perineal fistula in a boy. (a) Photograph shows normal position of the imperforate anus (arrowhead) and the external opening of a fistula (arrow). (b) Midsagittal T2-weighted MR image shows the rectoperineal fistula (arrowhead). The anteriorly displaced anorectum (arrow) is seen below the level of the levator ani muscle. (c) Axial T2-weighted MR image at the perineal level shows good development of the EAS (arrow). Note the ventral interruption of the sphincter fibers (arrowheads), a finding that corresponds to the fistula.

with the coccyx. Here, the EAS lies posterior and caudal to the puborectalis muscle (34). In neonates and infants with ARMs, a combined protocol of pelvic and spinal MR imaging should include T1- and T2-weighted images of the pelvis in axial planes parallel to the pubococcygeal line, coronal planes perpendicular to the pubococcygeal line, and sagittal planes. Moreover, sagittal T1- and T2-weighted images of the whole spine and posterior cranial fossa and axial and sagittal T1- and T2-weighted images of the lumbar spine are required. The usual protocol used at our hospital is shown in Table 5.

Pelvic MR Imaging in Boys with ARMs

In one study, imperforate anus without a fistula occurred in only 5% of newborn males with ARMs (Fig 9), with most cases (>50%) in children with Down syndrome (10). A perineal fistula indicates a low type of ARM (Fig 10). Other clinical signs indicating low ARMs are the presence of perineal pearls (Fig 9a), a corrugated perineum, or a perineal bucket-handle skin fold.

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Figure 11.  Intermediate or high ARM with a recto–urethral bulbar fistula in a boy. (a) Image from colostography shows a rectourethral fistula (arrows) at the level of the bulbar urethra. (b) Sagittal T2-weighted MR image shows the rectal pouch (white arrow) ending at the level of the levator ani muscle and the fistula (black arrow) extending anteriorly to the bulbar urethra. (c) Axial T2-weighted MR image at the pubococcygeal level shows an anteriorly located anorectum (white arrow) with an asymmetric and underdeveloped puborectalis muscle (black arrows).

In a high percentage of cases, boys with low ARMs present with associated genital anomalies, including micropenis, hypospadias, and ambiguous genitalia (4). At MR imaging, these children usually have sphincteric muscles of normal or almost-normal morphology, grade of development, and size; the rectum is usually located within most of the sphincter mechanism, and only the lowest part of the rectum is anteriorly mislocated (Fig 10b). Rectal atresia and stenosis occur in approximately 1% of cases, with a normal anal canal and normal sphincter morphology at MR imaging in most cases.

The most commonly seen ARM variant in male patients is the intermediate or high type with a rectourethral fistula (Fig 11). Clinically, most children with recto–urethral bulbar fistulas present with almost normal perineal and gluteal anatomy, with a well-developed levator ani muscle group and well-developed EAS muscles at MR imaging. However, underdeveloped muscles are occasionally observed (Fig 11c).

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Figure 12.  High ARM with a rectovesical fistula in a boy. (a) Image from colostography at 11 months shows the proximally located rectal pouch and a communication with the posterior wall of the urinary bladder (arrow). (b) Midsagittal T2-weighted MR image shows the high position of the rectal pouch and the trajectory of the fistula (arrow) into the posterior bladder wall. Partial agenesis of the sacrum is also seen. (c, d) Axial T2-weighted MR images show an atrophic puborectalis muscle (arrows in c) and a diminutive EAS complex (arrowheads in d).

In patients with high rectourinary communications (recto–urethral prostatic or rectovesical fistulas), clinical examination often shows an underdeveloped anal dimple, flat perineum, and poorly visible midline groove. The sphincteric muscles are frequently asymmetric and highly

atrophic (Fig 12). Gastrointestinal tract anomalies and sacral dysplasia are seen in a high percentage of cases in this group (1,10).

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Figure 13.  Low ARM with a rectovestibular fistula in a girl. (a) Photograph shows the external vestibular opening (arrow) of a rectovestibular fistula. (b) Image from colostography after catheterization of the fistula shows the position of the rectal pouch, which is located below the pubococcygeal line.  = fecaloma, arrow = normal position of the anus. (c) Axial T2-weighted MR image at the level of the pubococcygeal plane shows the ventrally mislocated anorectum (arrows). (d) Caudal image shows the EAS complex (arrows) posterior to the fistula.

Pelvic MR Imaging in Girls with ARMs

In girls, clinical inspection of the perineum allows identification of perineal and vestibular fistulas (Fig 13), both of which are considered low types

of ARMs. In these patients, the rectum and vagina are well separated in most cases. Rectovestibular fistula is by far the most common ARM seen in girls. In most of these patients, not only the levator ani muscle but also the EAS is well developed and demonstrates normal or almostnormal size and morphology at MR imaging (Fig

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Figure 14.  Cloacal anomaly in a girl. (a) VCUG image after placement of a catheter at the unique external opening of the cloacal anomaly shows opacification of the bladder and genital duplication (arrows), but the rectal pouch is not opacified. (b) Midsagittal T2weighted MR image shows the rectal pouch above the level of the puborectalis muscle (arrow). Note the dysplastic morphology of the sacrum. (c) Caudal axial image shows only one external opening and hypotrophic EAS fibers (arrows).

13c, 13d). There is a high prevalence of vesicoureteral reflux in these patients (4). The diagnosis of cloacal anomaly is clearly established in the presence of a unique external opening at the perineum that represents the confluence of the distal rectum, vagina, and urethra. It is important to identify the length of the com-

mon channel because the shorter the common channel, the better the prognosis and the lower the prevalence of associated anomalies. Interpretation of MR images may be difficult in cases of cloacal anomaly (Figs 14, 15) because of the high prevalence of associated urologic and principally genital anomalies (duplication of the genital tract, septation of the uterus and vagina, hydrocolpos) (1,4,36). Most patients with cloacal anomaly have extremely atrophic and underdeveloped levator ani muscles and an almost unrecognizable EAS (Figs 14, 15) (1,10).

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Figure 15.  Caudal regression syndrome (cloacal anomaly with genital duplication, partial sacral agenesis, spinal cord anomalies, horseshoe kidney, and vesicoureteral reflux) in a girl. (a) VCUG image shows bilateral grade 3 vesicoureteral reflux and almost complete sacral agenesis (arrow). (b) Axial T2-weighted MR image shows a horseshoe kidney (). (c) Sagittal MR image shows a tethered spinal cord with typical rounded morphology of the conus medullaris (arrow). (d) Axial T2-weighted MR image shows the rectal pouch (R) between duplicate genitalia (arrows). The puborectalis muscle is highly atrophic, the os coccyx is not seen, and the gluteal fold is underdeveloped.

Complexes and Syndromic ARMs

Frequently, ARMs do not appear as a single entity but as part of a syndrome or multisystemic process. In these cases, prognosis and therapy

are limited, and imaging studies should mandatorily include MR imaging of the spine, spinal cord, and pelvic region. Table 2 shows the most relevant clinical syndromes and associations that have ARMs among their manifestations, with the VACTERL association, caudal regression syn-

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drome, and the Currarino triad being perhaps the best known and most relevant entities. The VACTER or VACTERL association is a group of sporadic, nonrandom birth defects. It consists of the following congenital anomalies: vertebral, anal, and cardiac anomalies; tracheoesophageal fistula; and renal and limb anomalies (37,38). At least three of these types of anomalies must be present for the diagnosis to be established (Fig 6c). The prevalence of VACTERL syndrome is estimated at one per 10,000–40,000 live births. Most cases (>90%) are sporadic; the rest show evidence of an inherited, familial component. Although prenatal diagnosis of VACTERL syndrome remains rare, it may be indicated at US by the presence of a single umbilical artery and polyhydramnios in association with vertebral, renal, limb, or cardiac malformations. Other features, such as lack of a gastric bubble in some cases of tracheoesophageal fistula or a dilated colon secondary to imperforate anus, may be present but are more difficult to detect (37,38). In children with VACTERL syndrome and ARMs, the associated anomalies may be life threatening in the first days of life, thus retarding investigation and therapy of the anal malformation. Therefore, a prospective colostomy is often performed, independent of the type of ARM detected. Patients with VACTERL syndrome have a high prevalence of spinal dysraphism, tethered cord syndrome, and genitourinary conditions (16,39). Indeed, 63% of patients with VACTERL syndrome and ARMs also have tethered cord syndrome. If urogenital anomalies are also present, the prevalence of tethered cord syndrome increases to 86%. MR imaging of the spinal cord is thus mandatory in all patients with VACTERL syndrome (39). Caudal regression syndrome is a rare and sporadic neural tube defect characterized by developmental anomalies of the distal spine, in association with complete or partial agenesis of the sacrum, pelvic deformity, and abnormalities of the neural tube, urogenital and digestive organs, heart, and limbs (Fig 15). The syndrome manifests clinically as a total neurologic deficit of the lower limbs as well as absence of bladder and bowel control. Embryologically, caudal regression syndrome is a consequence of abnormal development of

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the structures derived from the caudal mesoderm in early embryonic life (weeks 4–7 of gestation) (16). Its prevalence is approximately one per 60,000 births, with a male predominance (maleto-female ratio, 2.7:1). Most cases are sporadic, but a strong association with maternal diabetes has been well documented, with more than 22% of cases seen in diabetic mothers and with the syndrome detected in more than 1% of pregnancies in diabetic mothers (40). Prenatal US shows sudden interruption of the distal spine due to absence of the thoracic vertebrae, lumbar vertebrae, or sacrum, with fusion of the pelvic bones and a characteristic froglike position of the lower limbs (40,41). After birth, MR imaging of the spine and spinal cord is required to demonstrate the level of spinal agenesis. The most frequently observed medullary anomalies are a typical wedge-shaped cord terminus, separation of the anterior and posterior spinal roots of the cauda equina, and tethered cord syndrome (39) (Fig 15c). High-type ARMs with underdeveloped, atrophic, and asymmetric sphincteric muscles are often observed at pelvic MR imaging. Currarino triad is a very rare disorder. It was described in 1981 (42) and consists of an ARM (commonly anal stenosis or agenesis), a sacrococcygeal defect, and a presacral mass (most often an anterior meningocele; teratoma; or neurenteric, dermoid, or epidermoid cyst), which causes secondary tethered cord syndrome (Fig 16). The Currarino triad results from a common developmental defect of the notochord in the early phases of embryogenesis, with incomplete separation of the endodermal and ectodermal layers, ventral failure of vertebral fusion, and consequent persistence of an open communication between the gut and sacral spine (43). The disorder is autosomal dominant with incomplete penetrance and variable expressivity. Recently, several gene defects localized at 7q36 and the HLXB9 gene have been identified (44,45). Children with the Currarino triad present with intractable constipation. A lateral radiograph of the pelvis shows the sacral anomaly, typically with preservation of the first sacral vertebra and asymmetric dysplasia of the distal sacrum, which is shaped like a sickle or scimitar

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Figure 16.  Currarino triad (high ARM with recto–urethral prostatic fistula confirmed at surgery, partial sacral agenesis, tethered cord, and anterior lipomyelomeningocele) in a boy. (a) VCUG image shows a normal urethra without demonstration of the fistula. Note the large fecaloma () and partial sacral agenesis (arrow). (b) Sagittal T2-weighted MR image of the spine shows a tethered cord (arrowhead), the partial sacral agenesis (white arrow), and an associated presacral mass (black arrow), which corresponds to an anterior lipomyelomeningocele. (c, d) Coronal (c) and axial (d) T2-weighted MR images show the heterogeneous presacral mass, which extends between the fibers of the highly atrophic puborectalis muscle (white arrows).

(46). Spinal MR imaging allows identification and characterization of the presacral mass and demonstration of the tethered cord (Fig 16b)

(34,46). At pelvic MR imaging, the sphincteric muscles are usually highly atrophic. In some cases, the presacral mass may invade the muscle fibers (Fig 16c, 16d), complicating surgical procedures.

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Conclusion

ARMs are a complex group of congenital anomalies involving the distal anus and rectum. They result from abnormal development of the urorectal septum in prenatal life. The Krickenbeck Conference of 2005 established a new classification of ARMs based not only on the level of the rectal pouch but also the presence or absence of fistulas and their description, factors helpful in determining the most appropriate surgical approach. Imaging plays a key role in evaluation of ARMs. In the first days of life, clinical and imaging findings facilitate early classification of ARMs and allow a decision about whether to perform an immediate colostomy. In children with intermediate and high types of ARMs, preoperative pelvic MR imaging after the neonatal period allows accurate evaluation of the morphology and grade of development of the sphincteric muscle complex. This information helps orient the medical and surgical teams as to the postoperative prognosis for continence. Acknowledgments.—We thank Ludovic Perrin, MD,

for graphic illustrations and Nicolas Chevrey for technical help.

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Alamo et al  511 9. Niedzielski JK. Invertography versus ultrasonography and distal colostography for the determination of bowel-skin distance in children with anorectal malformations. Eur J Pediatr Surg 2005;15(4): 262–267. 10. Peña A, Hong A. Advances in the management of anorectal malformations. Am J Surg 2000;180(5): 370–376. 11. Sato Y, Pringle KC, Bergman RA, et al. Congenital anorectal anomalies: MR imaging. Radiology 1988; 168(1):157–162. 12. McHugh K. The role of radiology in children with anorectal anomalies: with particular emphasis on MRI. Eur J Radiol 1998;26(2):194–199. 13. Nievelstein RA, Vos A, Valk J. MR imaging of anorectal malformations and associated anomalies. Eur Radiol 1998;8(4):573–581. 14. Nievelstein RA, Vos A, Valk J, Vermeij-Keers C. Magnetic resonance imaging in children with anorectal malformations: embryologic implications. J Pediatr Surg 2002;37(8):1138–1145. 15. Nievelstein RA, Hartwig NG, Vermeij-Keers C, Valk J. Embryonic development of the mammalian caudal neural tube. Teratology 1993;48(1):21–31. 16. Nievelstein RA, Valk J, Smit LM, Vermeij-Keers C. MR of the caudal regression syndrome: embryologic implications. AJNR Am J Neuroradiol 1994;15(6): 1021–1029. 17. Moore KL, Persaud TVN, eds. The developing human. Philadelphia, Pa: Saunders, 1998. 18. Nievelstein RA, van der Werff JF, Verbeek FJ, Valk J, Vermeij-Keers C. Normal and abnormal embryonic development of the anorectum in human embryos. Teratology 1998;57(2):70–78. 19. Peña A. Anorectal malformations. Semin Pediatr Surg 1995;4(1):35–47. 20. deVries PA, Peña A. Posterior sagittal anorectoplasty. J Pediatr Surg 1982;17(5):638–643. 21. Georgeson KE, Inge TH, Albanese CT. Laparoscopically assisted anorectal pull-through for high imperforate anus: a new technique. J Pediatr Surg 2000;35(6):927–930; discussion 930–931. 22. Bischoff A, Levitt MA, Lim FY, Guimarães C, Peña A. Prenatal diagnosis of cloacal malformations. Pediatr Surg Int 2010;26(11):1071–1075. 23. Calvo-Garcia MA, Kline-Fath BM, Levitt MA, et al. Fetal MRI clues to diagnose cloacal malformations. Pediatr Radiol 2011;41(9):1117–1128. 24. Alamo L, Laswad T, Schnyder P, et al. Fetal MRI as complement to US in the diagnosis and characterization of anomalies of the genito-urinary tract. Eur J Radiol 2010;76(2):258–264. 25. Haber HP, Seitz G, Warmann SW, Fuchs J. Transperineal sonography for determination of the type of imperforate anus. AJR Am J Roentgenol 2007;189 (6):1525–1529. 26. Le Bayon AG, Carpentier E, Boscq M, Lardy H, Sirinelli D. Imaging of anorectal malformations in the neonatal period [in French]. J Radiol 2010;91 (4):475–483.

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radiographics.rsna.org 36. Laterza RM, De Gennaro M, Tubaro A, Koelbl H. Female pelvic congenital malformations. I. Embryology, anatomy and surgical treatment. Eur J Obstet Gynecol Reprod Biol 2011;159(1):26–34. 37. Solomon BD. VACTERL/VATER association. Orphanet J Rare Dis 2011;6:56–67. 38. Solomon BD, Raam MS, Pineda-Alvarez DE. Analysis of genitourinary anomalies in patients with VACTERL (vertebral anomalies, anal atresia, cardiac malformations, tracheo-esophageal fistula, renal anomalies, limb abnormalities) association. Congenit Anom (Kyoto) 2011;51(2):87–91. 39. Kuo MF, Tsai Y, Hsu WM, Chen RS, Tu YK, Wang HS. Tethered spinal cord and VACTERL association. J Neurosurg 2007;106(3 suppl):201–204. 40. Stroustrup Smith A, Grable I, Levine D. Caudal regression syndrome in the fetus of a diabetic mother. Radiology 2004;230(1):229–233. 41. Singh SK, Singh RD, Sharma A. Caudal regression syndrome: case report and review of literature. Pediatr Surg Int 2005;21(7):578–581. 42. Currarino G, Coln D, Votteler T. Triad of anorectal, sacral, and presacral anomalies. AJR Am J Roentgenol 1981;137(2):395–398. 43. Turgut M, Cullu E, Ulucan H. Incomplete Currarino triad as an embryological variant: case report and review of the literature. J Neurosurg 2006;105 (6 suppl):504–507. 44. Yates VD, Wilroy RS, Whitington GL, Simmons JC. Anterior sacral defects: an autosomal dominantly inherited condition. J Pediatr 1983;102(2):239–242. 45. Emans PJ, van Aalst J, van Heurn EL, et al. The Currarino triad: neurosurgical considerations. Neurosurgery 2006;58(5):924–929. 46. Low G, Irwin GJ, Haddock G, Maroo SV. Currarino triad: characteristic appearances on magnetic resonance imaging and plain radiography. Australas Radiol 2006;50(3):249–251.

TM

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Teaching Points

March-April Issue 2013

Anorectal Malformations: Finding the Pathway out of the Labyrinth Leonor Alamo, MD • Blaise J. Meyrat, MD • Jean-Yves Meuwly, MD • Reto A. Meuli, MD, PhD • François Gudinchet, MD RadioGraphics 2013; 33:491–512 • Published online 10.1148/rg.332125046 • Content Codes:

Page 493 In early embryonic life, the terminal portion of the hindgut—the primitive cloaca—is divided into dorsal and ventral parts by a coronal sheet of mesenchyme—the urorectal septum—and separated from the amniotic cavity by the cloacal membrane. Most ARMs result from abnormal development of the urorectal septum. Page 495 The Wingspread and Krickenbeck classifications are very similar. The Wingspread classification allows location of the blind rectal pouch. The Krickenbeck classification is more descriptive; its most important advantage is the preoperative identification and anatomic evaluation of not only the rectal pouch but also any fistulas. This information allows the surgeon to anticipate the extent of mobilization of the atretic rectal segment required during surgery and helps determine the most appropriate surgical approach for each case. Page 497 Imaging studies in the first 2 days of life should include radiography of the thorax, spine, and pelvis along with cardiac, perineal, abdominal, pelvic, and spine US to detect possible associated anomalies. Page 499 High-pressure distal colostography is the most effective imaging technique for demonstrating fistulas. Page 500 Two important reference transverse planes are used with MR imaging for evaluation of ARMs. The first is the pubococcygeal plane, which extends from the upper border of the os pubis to the os coccyx. As described by Stephens et al, this plane corresponds to the attachment level of the levator ani muscle to the pelvic wall.