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The role of radiology in children with anorectal anomalies;
European Journal of Radiology 26 (1998) 194 – 199
The role of radiology in children with anorectal anomalies; with particular emphasis on MRI K. McHugh * Radiology Department, Great Ormond Street Hospital for Children, London WC1N 6JH, UK Received 13 June 1997; received in revised form 16 June 1997; accepted 17 June 1997
Abstract Anorectal anomalies have a reported incidence of between 1 per 1000 and 1 per 9630 live births [1]. The international classification subdivides anorectal malformations into high, intermediate, low and miscellaneous deformities with emphasis on the sex of the child [2]. The classification is based on where the rectum terminates in relation to the levator ani muscles — above the levator is termed a high (supralevator) lesion, at the level of the levator intermediate, and below is a low or translevator anomaly. A modified classification has recently been proposed by Pena based on his anatomic observations during posterior sagittal anorectoplasty—the terms high, intermediate and low lesions continue to be used but with slightly different connotations [3]. Approximately 50% of all patients with anorectal anomalies have associated other congenital lesions. These lesions necessitate a variety of radiological investigations which will be outlined briefly. The pertinent muscular anatomy of the pelvic floor and recent advances in surgical techniques will be discussed. The particular role of MRI in the evaluation of the pre-operative newborn or infant prior to definitive pull-through repair surgery and the post-operative, older, paediatric patient with continuing problems will be reviewed. Reference to the other radiological options, and their usefulness, in the evaluation of anorectal malformations will be made throughout the text. © 1998 Elsevier Science Ireland Ltd. Keywords: Anorectal; Rectum; Levator ani muscle; Sagittal anorectoplasty
1. Associated lesions. Anorectal anomalies form part of the VACTERL complex. Often survival, and the quality of survival, is more dependent on the associated anomalies than on the anorectal lesion [2]. Cardiac, tracheo – oesophageal and limb abnormalities generally present with specific clinical manifestations and undergo radiological evaluation as appropriate. Associated genito – urinary tract abnormalities occur in 28–72% of cases [4]. Although a higher incidence of urologic anomalies occur in association with supralevator lesions, up to 30% of patients with low imperforate anus can have associated urologic anomalies, several of which are capable of causing significant morbidity [5].
* Tel.: +44 171 4059200.
All patients therefore with a congenital anorectal malformation should have a renal tract ultrasound examination as a screening test in the early newborn period. If this study shows hydronephrosis or a hydroureter, a full urological work-up should be performed before a colostomy is opened [6]. Newborn females with a cloaca may have a hydrocolpos which can need drainage. Micturating cystourethrography (MCUG) should really be performed on all patients as soon as possible to exclude or assess the degree of any associated vesico– ureteric reflux, and the MCUG can often also indicate the site of a recto–urinary fistula. The best study available, however, to demonstrate the precise location of the fistula continues to be a high pressure distal colostogram [7]. The first renal ultrasound examination in the neonatal period should now probably also include, if the
0720-048X/98/$19.00 © 1998 Elsevier Science Ireland Ltd. All rights reserved. PII S 0 7 2 0 - 0 4 8 X ( 9 7 ) 0 0 0 9 5 - 8
K. McHugh / European Journal of Radiology 26 (1998) 194–199
expertise is available, a spinal ultrasound examination to assess for intraspinal pathology and associated dysraphic abnormalities. Reports vary between a 2:1 ratio of vertebral anomalies in high versus low anorectal malformations generally, to one study which described the frequency of vertebral abnormalities in children with a high imperforate anus as 59%, and 19% with a low lesion [8]. All patients with anorectal malformations therefore should have plain radiographs of the lumbo–sacral spine. These radiographs should be examined closely for abnormalities which may indicate the presence of intraspinal pathology. When the radiographic or sonographic examination is abnormal then MRI can be used to accurately depict the likely associated intraspinal pathology such as cord tethering, caudal regression syndrome, hydromyelia or a lipoma of the filum terminale [9 – 11]. One recent series has shown that occult myelodysplasia occurs in children with all types of anorectal anomalies including those patients with normal spine radiographs [11]. In the absence of any neurological abnormality, however, normal radiographic and sonographic examinations of infants with anorectal malformations would appear to make routine MRI of the lumbo – sacral spine superfluous, although ideally all of these children should have MRI of the spine at some stage [9 – 11].
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pouch ending above the PC plane is regarded as a high anorectal anomaly; the rectum ending at the PC but not the I plane as an intermediate lesion; and the presence of the distal rectum in both the PC and I planes is considered a low anomaly [13]. The puborectal and EAS muscles can be subjectively assessed on all imaging planes in terms of size as good, mediocre or poor. Some studies have used actual measurements of the thickness of the sphincteric muscles as a guide to their overall development. As muscle thickness changes however, according to a patient’s age, subjective visual evaluation of the sphincteric muscles without strict measurements is generally adequate [14]. On postoperative MR studies, asymmetrical placement of the neorectum or pulled-through intestine through the levator sling, mesenteric fat inadvertently placed within the sling or passage of the rectum outside the levator sling are possible findings which are amenable to corrective surgery. Additionally in postoperative patients the angle formed where the rectum or pulled-through intestine courses anterior to the puborectalis muscle on sagittal images (anorectal angle) has been found in one study to be as important as the evaluation of the size of the levator and sphincteric muscles [14]. The anorectal angle should be less than 110° in normal subjects [14].
2. Practical anatomy The levator ani muscle lies in a plane between the symphysis pubis and the coccyx (PC Plane). This muscle comprises ileococcygeus and pubococcygeus including puborectalis. The puborectalis forms the most medial part of the levator hammock. The external anal sphincter (EAS) has three components which are the superficial, subcutaneous and the deep sphincter muscles. The deep fibres of the EAS blend imperceptibly into the inferior portion of the puborectalis. These anatomically inseparable muscle entities function invivo as a single coherent unit and all are important in normal continence [12]. For practical purposes two axial planes through the pelvis at the levels of the puborectalis and EAS can be defined simply on axial MR images [13]. The PC plane, through the symphysis pubis and coccyx, includes the cervix in a female or prostate in a male, and the puborectalis muscle. At this level the gluteus maximus muscles in normal subjects approximate each other over the coccyx. The other reference plane passes through the ischial rami (I plane) and includes the bulb of the penis and EAS (Fig. 1). The I plane defines the anal canal junction with the rectum. The normal EAS is oval shaped with its long axis in an anteroposterior direction. The lower limit of the rectal
Fig. 1. Two axial T1-weighted MR images at the level of the ischial rami (I plane) in a normal 16-month-old boy. External anal sphincter (EAS), curved arrow.
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K. McHugh / European Journal of Radiology 26 (1998) 194–199
3. Surgery An unequivocally low anorectal malformation is treated by a primary perineal operation in the neonatal period, but all higher anomalies generally undergo an initial colostomy followed sometime later by a definitive repair pull-through procedure. A major recent trend in the surgery of high and intermediate anomalies has been posterior sagittal anorectoplasty (PSARP) which utilises the EAS as well as the puborectal muscle [13,15]. Prior to this surgical procedure, the EAS was not thought to exist to any meaningful extent in these cases and so no real attempt was made to place the pulled-through rectum within the EAS. With a PSARP, the perineum is split through the centre of the EAS with electro-stimulation to guide the dissection [13]. This allows direct visualisation of all the relevant pelvic musculature with a possibility of tapering the pulledthrough bowel to fit the available sphincteric muscle. It is generally possible to immobilise the distal bowel adequately from below with this technique such that the terminal bowel is positioned within the voluntary sphincteric complex. The PSARP is now the generally accepted practice throughout much of the world for the treatment of high and intermediate anorectal anomalies. In the latter part of the 1980’s, reports were published of manometric and histologic evidence of a functional internal sphincter in the terminal bowel of children with anorectal atresias. At the termination of the blind rectal pouch an annular smooth muscle thickening can be found as a continuation of the circular smooth muscle layer. According to some authors, the commonly used term ‘fistula’ used in this context is erroneous and the bowel opening should rather be called an ectopic anus [16]. As it is generally accepted that no part of the terminal bowel should be resected unnecessarily, surgeons from Scandinavia have recently published a modification of the PSARP involving an internal sphincter saving technique [16]. All of these surgical procedures underscore the importance of the sphincteric muscle complex in achieving continence thus increasing the value of imaging, particularly MRI, which can display not only the presence but also the relative size of the puborectalis and EAS and their position in relation to the pulled-through neorectum.
4. Newborn period Although the international classification subdivides into high, intermediate and low anorectal anomalies, the major subdivision of clinical relevance in the first few days of life is whether the anomaly is of a supralevator or translevator type. Decisions to be made during this period are whether the patient needs a colostomy
or whether other urinary or vaginal diversions to prevent likely sepsis or acidosis are necessary [6]. An inappropriate perineal approach to a high anorectal lesion can have disastrous results with irreparable damage to the sphincter muscle complex [6]. It should be emphasised, however, that meticulous perineal inspection with urinalysis, without recourse to any radiologic imaging, provides sufficient diagnostic information to the clinician in up to 80–90% of newborn males and females [6]. In the 10–20% of neonates with equivocal clinical findings, radiology can contribute to the diagnostic process despite the fact that all radiologic investigations at this age have significant inherent drawbacks. A prone lateral plain radiograph (an ‘invertogram’ should never be performed) which attempts to estimate the location of the levator ani is limited by not being able to display the levator muscle directly. Straining or crying can cause the puborectal muscle to move 2–3 cm in the same patient [17]. Meconium in the distal rectum can lead to the erroneous impression of a high lesion with both plain radiography and ultrasound also. Efforts to classify anorectal malformations into supralevator or translevator types based on a radiographic pubo-coccygeal line or an ‘M’ point on the ischium indicating the plane of the levator sling are notoriously inaccurate [17,18]. Estimations based on the distance of the rectum from an anal skin marker as to the type of anomaly are also unreliable. A plain film can however, help identify lumbo–sacral anomalies and occasionally in males with a high lesion gas can be seen in the bladder due to a recto–vesical or recto(prostatic) –urethral fistula. Injection of iodinated contrast via the perineum with or without ultrasound guidance to delineate the distal rectum suffers similar limitations to plain radiographic techniques and is not widely practised. CT is limited by scanning only in the axial plane and by difficulties in differentiating muscle from rectal wall particularly in neonates, but really is no longer justifiable because of the high dose of ionising radiation inherent in the technique. MRI has been used in the neonatal period prior to pull-through surgery and may have a limited role to play in some newborns with equivocal findings [15,19]. Meconium, due to its lipid content, is uniformly hyperintense on T1 weighted sequences in the immediate newborn period (Fig. 2) [15,19]. MR can be performed soon after birth, reducing the risk of perforation from an over-distended colon. Examination during sedation or sleep may give a more accurate estimation of the true level of the levator sling [13]. MR allows direct visualisation of the distal rectum and related musculature without additional ionising radiation, but with multiplanar capabilities. Associated lesions such as sacrococcygeal hypoplasia, lumbar spine or renal
K. McHugh / European Journal of Radiology 26 (1998) 194–199
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fistulas are best detected in boys with MCUG, or after the initial colostomy by a distal loopogram study [7]. Fistulas have been demonstrated on MRI in older infants after the colostomy procedure with injection of an agent such as vaseline oil into the distal colon [4].
5. Post-operative evaluation
Fig. 2. Sagittal T1-weighted image of a one-day-old newborn showing typically hyperintense T1 signal meconium with a recto–urethral fistula (arrow). Sacrococcygeal hypoplasia is also evident [15].
anomalies can be evaluated. The information provided by MR can be of some value to paediatric surgeons in their discussions with parents, in the planning of a pull-through procedure and in predicting likely future continence [15]. One study by this author, however, showed that MRI in the neonatal period does have some drawbacks [15]. In one patient distension of the distal rectal pouch with meconium lead to the erroneous interpretation on the MRI study of a high anorectal anomaly as a low lesion. This baby passed meconium per urethram soon after the MRI examination. Meconium per urethram indicated a fistula between the rectum and the urinary tract, prompting the need for a protective colostomy, and emphasising the importance of a thorough clinical evaluation. The high T1 signal meconium in all nine newborn infants in our and another study however allowed for easy differentiation of rectal contents from rectal wall and musculature [15,19]. In utero assessment of possible anorectal malformations may be facilitated in the future by the uniformly hyperintense T1 signal meconium acting as an MR contrast agent. MRI appears to have a low sensitivity to the detection of a fistula in the neonatal period and it should be stressed that
Due to multifactorial influences on rectal continence, the technical adequacy or failure of a surgical pull-through procedure may not be clinically apparent for many years after surgery [20]. A persisting problem with faecal incontinence or constipation, however, despite an apparently successful pull-through operation is the usual indication for further imaging in older children with congenital anorectal malformations. MRI is undoubtedly the optimal imaging modality for assessing these patients who are under consideration for re-operation, and is without the risk of ionising radiation. Axial images are generally best for assessing the siting of the pulled-through rectum and also for evaluating the puborectalis and EAS. A catheter can be placed within the rectum to facilitate its identification. The levator hammock is best evaluated on coronal images (Fig. 3). An additional advantage of MRI is that it can survey for other associated lesions in the spine, spinal cord or renal tract. MR can provide useful information regarding development of the pelvic musculature which can be helpful to the surgeon in predicting likely functional outcome. Many patients with persistent problems
Fig. 3. Coronal T1-weighted image through the posterior pelvis showing the normal levator hammock (arrow). R, rectal ampulla.
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Fig. 4. Post-operative complications amenable to corrective surgery. (a) Axial MR at the level of the ischium shows an asymmetric hypoplastic EAS (curved arrow), and neorectum (arrow) with a catheter in-situ misplaced outside the EAS; (b) mesenteric fat (arrow) has been inadvertently brought down with the rectum which can interfere with normal sphincteric function [13] (reprinted with permission, Sato et al.).
have hypoplastic muscles for which little can be done surgically, although augmentation with striated muscle transplantation, such as a gracilis sling, for an underdeveloped puborectalis to provide an adequate voluntary sphincter mechanism has been described [20]. Misplacement of the neorectum outside the puborectal sling (Fig. 4(a)), anterior misplacement within the EAS or asymmetric positioning within the levator muscle can all be easily recognised on MRI. It should be noted that the puborectalis muscle can appear asymmetric in thickness on either side of the rectum because of chemical shift artefact occurring at the junction between muscle and fat. Care must be taken, therefore, not to mistake chemical shift artefact for asymmetric musculature or asymmetric positioning of the pullthrough rectum in the puborectalsis sling [20]. Mesenteric fat inadvertently brought down with the rectum into the sphincteric muscles at operation is easily visualised on axial MRI, particularly on T1-weighted images as an area of hyperintensity around the neorectum (Fig. 4(b)). This fat can interfere with muscle action on the rectum and may have to be surgically removed [13]. The information provided by MRI can greatly decrease the amount of dissection at repeat surgery and thus helps to minimise muscle damage during a further corrective procedure. One series has suggested that measurement of an anorectal angle in the sagittal plane is as important as evaluation of the development of the levator and sphinteric muscles in the assessment of children with post-operative faecal incontinence [14]. The anorectal angle is created by the continuous contraction of the puborectalis muscle at rest, which is one of the major factors affecting faecal continence. Poor clinical scores of incontinence were found in patients with wider anorectal angles irrespective of the relative development of the puborectalis or EAS, but further studies need to be performed to validate these findings [14].
6. Conclusion The vast majority of anorectal malformations can be managed without recourse to radiological investigations in the neonatal period. Renal and probably spinal ultrasound in the early newborn period is, however, routinely recommended. Very occasionally other imaging such as a combination of plain radiography and MRI can be of help in planning the pull-through procedure and may be useful in predicting likely future continence. In the post-operative older patient, MRI is the technique ‘par excellence’ in the assessment of the pull-through procedure particularly as the technical adequacy or failure of surgery may not be clinically apparent for many years.
References [1] Kohda E, Fujioka M, Ikawa H, Yokoyama J. Congenital anorectal anomaly: CT evaluation. Radiology 1985;157:349–52. [2] Durham-Smith E. The bath water needs changing, but don’t throw out the baby: an overview of anorectal anomalies. J Ped Surg 1987;22:335 – 48. [3] Currarino G. The various types of anorectal fistula in male imperforate anus. Pediatr Radiol 1996;26:512 – 22. [4] Taccone A, Martucciello G, Dodero P, Delliacqua A, Marzoli A, Salomone G, Jasonni V. New concepts in preoperative imaging of anorectal malformation. Pediatr Radiol 1992;22:196–9. [5] Misra D, Mushtaq I, Drake DP, Kiely EM, Spitz L. Associated urologic anomalies in low imperforate anus are capable of causing significant morbidity; a 15-year experience. Urology 1996;48:281 – 3. [6] Pena A. Management of anorectal malformations in the newborn period. World J Surg 1993;17:385 – 92. [7] Gross GW, Wolfson PJ, Pena A. Augmented pressure colostogram in imperforate anus with fistula. Pediatr Radiol 1991;21:560 – 1. [8] Tunnell WP, Austin JC, Barnes PD, Reynolds A. Neuroradiologic evaluation of sacral abnormalities in imperforate anus complex. J Ped Surg 1987;22:58 – 61.
K. McHugh / European Journal of Radiology 26 (1998) 194–199 [9] Beek FJA, Boemers TML, Witkamp TD, van Leeuwen MS, Mali WP, Bax NMA. Spine evaluation in children with anorectal malformations. Pediatr Radiol 1995;25:S28–32. [10] Heij HA, Nievelstein RAJ, de Zwart I, Verbeeten BWJM, Valk J, Vos A. Abnormal anatomy of the lumbosacral region imaged by magnetic resonance in children with anorectal malformations. Arch Dis Child 1996;74:441–4. [11] Long FL, Hunter JV, Mahboubi S, Kalmus A, Templeton JM Jr. Tethered cord and associated vertebral anomalies in children and infants with imperforate anus: evaluation with MR imaging and plain radiography. Radiology 1996;200:377–82. [12] DeVries PA, Cox KL. Surgery of anorectal anomalies. Surg Clin North Am 1985;65:1139–69. [13] Sato Y, Pringle KC, Bergman RA, Yuh WTC, Smith WL, Soper RT, Franken EA Jr. Congenital anorectal anomalies: MR imaging. Radiology 1988;168:157–62. [14] Fukuya T, Honda H, Kubota M, Kawashima A, Hayashi T, Tateshi Y, Shono T, Suita S, Masuda K. Postoperative MRI
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evaluation of anorectal malformations with clinical correlation. Pediatr Radiol 1993;23:583 – 6. McHugh K, Tam P, Dudley N. Pre-operative MRI of anorectal anomalies in the newborn period. Pediatr Radiol 1995;25:S33–6. Husberg B, Lindahl H, Rintala R, Frenckner B. High and intermediate imperforate anus; results after surgical correction with special respect to internal sphincter function. J Ped Surg 1992;27:185 – 9. Berdon WE, Baker DH, Santulli TV, Amoury R. The radiologic evaluation of imperforate anus. Radiology 1968;90:466–71. Cremin BJ. The radiological assessment of anorectal anomalies. Clin Radiol 1971;22:239 – 50. Sachs TM, Applebaum H, Touraj T, Taber P, Darakjian A, Colleti P. Use of MRI in evaluation of anorectal anomalies. J Ped Surg 1990;25:817 – 21. Vade A, Reyes H, Wilbur A, Gyi B, Spigos D. The anorectal sphincter after rectal pull-through surgery for anorectal anomalies: MRI evaluation. Pediatr Radiol 1989;19:179 – 83.
The role of radiology in children with anorectal anomalies; with particular emphasis on MRI K. McHugh * Radiology Department, Great Ormond Street Hospital for Children, London WC1N 6JH, UK Received 13 June 1997; received in revised form 16 June 1997; accepted 17 June 1997
Abstract Anorectal anomalies have a reported incidence of between 1 per 1000 and 1 per 9630 live births [1]. The international classification subdivides anorectal malformations into high, intermediate, low and miscellaneous deformities with emphasis on the sex of the child [2]. The classification is based on where the rectum terminates in relation to the levator ani muscles — above the levator is termed a high (supralevator) lesion, at the level of the levator intermediate, and below is a low or translevator anomaly. A modified classification has recently been proposed by Pena based on his anatomic observations during posterior sagittal anorectoplasty—the terms high, intermediate and low lesions continue to be used but with slightly different connotations [3]. Approximately 50% of all patients with anorectal anomalies have associated other congenital lesions. These lesions necessitate a variety of radiological investigations which will be outlined briefly. The pertinent muscular anatomy of the pelvic floor and recent advances in surgical techniques will be discussed. The particular role of MRI in the evaluation of the pre-operative newborn or infant prior to definitive pull-through repair surgery and the post-operative, older, paediatric patient with continuing problems will be reviewed. Reference to the other radiological options, and their usefulness, in the evaluation of anorectal malformations will be made throughout the text. © 1998 Elsevier Science Ireland Ltd. Keywords: Anorectal; Rectum; Levator ani muscle; Sagittal anorectoplasty
1. Associated lesions. Anorectal anomalies form part of the VACTERL complex. Often survival, and the quality of survival, is more dependent on the associated anomalies than on the anorectal lesion [2]. Cardiac, tracheo – oesophageal and limb abnormalities generally present with specific clinical manifestations and undergo radiological evaluation as appropriate. Associated genito – urinary tract abnormalities occur in 28–72% of cases [4]. Although a higher incidence of urologic anomalies occur in association with supralevator lesions, up to 30% of patients with low imperforate anus can have associated urologic anomalies, several of which are capable of causing significant morbidity [5].
* Tel.: +44 171 4059200.
All patients therefore with a congenital anorectal malformation should have a renal tract ultrasound examination as a screening test in the early newborn period. If this study shows hydronephrosis or a hydroureter, a full urological work-up should be performed before a colostomy is opened [6]. Newborn females with a cloaca may have a hydrocolpos which can need drainage. Micturating cystourethrography (MCUG) should really be performed on all patients as soon as possible to exclude or assess the degree of any associated vesico– ureteric reflux, and the MCUG can often also indicate the site of a recto–urinary fistula. The best study available, however, to demonstrate the precise location of the fistula continues to be a high pressure distal colostogram [7]. The first renal ultrasound examination in the neonatal period should now probably also include, if the
0720-048X/98/$19.00 © 1998 Elsevier Science Ireland Ltd. All rights reserved. PII S 0 7 2 0 - 0 4 8 X ( 9 7 ) 0 0 0 9 5 - 8
K. McHugh / European Journal of Radiology 26 (1998) 194–199
expertise is available, a spinal ultrasound examination to assess for intraspinal pathology and associated dysraphic abnormalities. Reports vary between a 2:1 ratio of vertebral anomalies in high versus low anorectal malformations generally, to one study which described the frequency of vertebral abnormalities in children with a high imperforate anus as 59%, and 19% with a low lesion [8]. All patients with anorectal malformations therefore should have plain radiographs of the lumbo–sacral spine. These radiographs should be examined closely for abnormalities which may indicate the presence of intraspinal pathology. When the radiographic or sonographic examination is abnormal then MRI can be used to accurately depict the likely associated intraspinal pathology such as cord tethering, caudal regression syndrome, hydromyelia or a lipoma of the filum terminale [9 – 11]. One recent series has shown that occult myelodysplasia occurs in children with all types of anorectal anomalies including those patients with normal spine radiographs [11]. In the absence of any neurological abnormality, however, normal radiographic and sonographic examinations of infants with anorectal malformations would appear to make routine MRI of the lumbo – sacral spine superfluous, although ideally all of these children should have MRI of the spine at some stage [9 – 11].
195
pouch ending above the PC plane is regarded as a high anorectal anomaly; the rectum ending at the PC but not the I plane as an intermediate lesion; and the presence of the distal rectum in both the PC and I planes is considered a low anomaly [13]. The puborectal and EAS muscles can be subjectively assessed on all imaging planes in terms of size as good, mediocre or poor. Some studies have used actual measurements of the thickness of the sphincteric muscles as a guide to their overall development. As muscle thickness changes however, according to a patient’s age, subjective visual evaluation of the sphincteric muscles without strict measurements is generally adequate [14]. On postoperative MR studies, asymmetrical placement of the neorectum or pulled-through intestine through the levator sling, mesenteric fat inadvertently placed within the sling or passage of the rectum outside the levator sling are possible findings which are amenable to corrective surgery. Additionally in postoperative patients the angle formed where the rectum or pulled-through intestine courses anterior to the puborectalis muscle on sagittal images (anorectal angle) has been found in one study to be as important as the evaluation of the size of the levator and sphincteric muscles [14]. The anorectal angle should be less than 110° in normal subjects [14].
2. Practical anatomy The levator ani muscle lies in a plane between the symphysis pubis and the coccyx (PC Plane). This muscle comprises ileococcygeus and pubococcygeus including puborectalis. The puborectalis forms the most medial part of the levator hammock. The external anal sphincter (EAS) has three components which are the superficial, subcutaneous and the deep sphincter muscles. The deep fibres of the EAS blend imperceptibly into the inferior portion of the puborectalis. These anatomically inseparable muscle entities function invivo as a single coherent unit and all are important in normal continence [12]. For practical purposes two axial planes through the pelvis at the levels of the puborectalis and EAS can be defined simply on axial MR images [13]. The PC plane, through the symphysis pubis and coccyx, includes the cervix in a female or prostate in a male, and the puborectalis muscle. At this level the gluteus maximus muscles in normal subjects approximate each other over the coccyx. The other reference plane passes through the ischial rami (I plane) and includes the bulb of the penis and EAS (Fig. 1). The I plane defines the anal canal junction with the rectum. The normal EAS is oval shaped with its long axis in an anteroposterior direction. The lower limit of the rectal
Fig. 1. Two axial T1-weighted MR images at the level of the ischial rami (I plane) in a normal 16-month-old boy. External anal sphincter (EAS), curved arrow.
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K. McHugh / European Journal of Radiology 26 (1998) 194–199
3. Surgery An unequivocally low anorectal malformation is treated by a primary perineal operation in the neonatal period, but all higher anomalies generally undergo an initial colostomy followed sometime later by a definitive repair pull-through procedure. A major recent trend in the surgery of high and intermediate anomalies has been posterior sagittal anorectoplasty (PSARP) which utilises the EAS as well as the puborectal muscle [13,15]. Prior to this surgical procedure, the EAS was not thought to exist to any meaningful extent in these cases and so no real attempt was made to place the pulled-through rectum within the EAS. With a PSARP, the perineum is split through the centre of the EAS with electro-stimulation to guide the dissection [13]. This allows direct visualisation of all the relevant pelvic musculature with a possibility of tapering the pulledthrough bowel to fit the available sphincteric muscle. It is generally possible to immobilise the distal bowel adequately from below with this technique such that the terminal bowel is positioned within the voluntary sphincteric complex. The PSARP is now the generally accepted practice throughout much of the world for the treatment of high and intermediate anorectal anomalies. In the latter part of the 1980’s, reports were published of manometric and histologic evidence of a functional internal sphincter in the terminal bowel of children with anorectal atresias. At the termination of the blind rectal pouch an annular smooth muscle thickening can be found as a continuation of the circular smooth muscle layer. According to some authors, the commonly used term ‘fistula’ used in this context is erroneous and the bowel opening should rather be called an ectopic anus [16]. As it is generally accepted that no part of the terminal bowel should be resected unnecessarily, surgeons from Scandinavia have recently published a modification of the PSARP involving an internal sphincter saving technique [16]. All of these surgical procedures underscore the importance of the sphincteric muscle complex in achieving continence thus increasing the value of imaging, particularly MRI, which can display not only the presence but also the relative size of the puborectalis and EAS and their position in relation to the pulled-through neorectum.
4. Newborn period Although the international classification subdivides into high, intermediate and low anorectal anomalies, the major subdivision of clinical relevance in the first few days of life is whether the anomaly is of a supralevator or translevator type. Decisions to be made during this period are whether the patient needs a colostomy
or whether other urinary or vaginal diversions to prevent likely sepsis or acidosis are necessary [6]. An inappropriate perineal approach to a high anorectal lesion can have disastrous results with irreparable damage to the sphincter muscle complex [6]. It should be emphasised, however, that meticulous perineal inspection with urinalysis, without recourse to any radiologic imaging, provides sufficient diagnostic information to the clinician in up to 80–90% of newborn males and females [6]. In the 10–20% of neonates with equivocal clinical findings, radiology can contribute to the diagnostic process despite the fact that all radiologic investigations at this age have significant inherent drawbacks. A prone lateral plain radiograph (an ‘invertogram’ should never be performed) which attempts to estimate the location of the levator ani is limited by not being able to display the levator muscle directly. Straining or crying can cause the puborectal muscle to move 2–3 cm in the same patient [17]. Meconium in the distal rectum can lead to the erroneous impression of a high lesion with both plain radiography and ultrasound also. Efforts to classify anorectal malformations into supralevator or translevator types based on a radiographic pubo-coccygeal line or an ‘M’ point on the ischium indicating the plane of the levator sling are notoriously inaccurate [17,18]. Estimations based on the distance of the rectum from an anal skin marker as to the type of anomaly are also unreliable. A plain film can however, help identify lumbo–sacral anomalies and occasionally in males with a high lesion gas can be seen in the bladder due to a recto–vesical or recto(prostatic) –urethral fistula. Injection of iodinated contrast via the perineum with or without ultrasound guidance to delineate the distal rectum suffers similar limitations to plain radiographic techniques and is not widely practised. CT is limited by scanning only in the axial plane and by difficulties in differentiating muscle from rectal wall particularly in neonates, but really is no longer justifiable because of the high dose of ionising radiation inherent in the technique. MRI has been used in the neonatal period prior to pull-through surgery and may have a limited role to play in some newborns with equivocal findings [15,19]. Meconium, due to its lipid content, is uniformly hyperintense on T1 weighted sequences in the immediate newborn period (Fig. 2) [15,19]. MR can be performed soon after birth, reducing the risk of perforation from an over-distended colon. Examination during sedation or sleep may give a more accurate estimation of the true level of the levator sling [13]. MR allows direct visualisation of the distal rectum and related musculature without additional ionising radiation, but with multiplanar capabilities. Associated lesions such as sacrococcygeal hypoplasia, lumbar spine or renal
K. McHugh / European Journal of Radiology 26 (1998) 194–199
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fistulas are best detected in boys with MCUG, or after the initial colostomy by a distal loopogram study [7]. Fistulas have been demonstrated on MRI in older infants after the colostomy procedure with injection of an agent such as vaseline oil into the distal colon [4].
5. Post-operative evaluation
Fig. 2. Sagittal T1-weighted image of a one-day-old newborn showing typically hyperintense T1 signal meconium with a recto–urethral fistula (arrow). Sacrococcygeal hypoplasia is also evident [15].
anomalies can be evaluated. The information provided by MR can be of some value to paediatric surgeons in their discussions with parents, in the planning of a pull-through procedure and in predicting likely future continence [15]. One study by this author, however, showed that MRI in the neonatal period does have some drawbacks [15]. In one patient distension of the distal rectal pouch with meconium lead to the erroneous interpretation on the MRI study of a high anorectal anomaly as a low lesion. This baby passed meconium per urethram soon after the MRI examination. Meconium per urethram indicated a fistula between the rectum and the urinary tract, prompting the need for a protective colostomy, and emphasising the importance of a thorough clinical evaluation. The high T1 signal meconium in all nine newborn infants in our and another study however allowed for easy differentiation of rectal contents from rectal wall and musculature [15,19]. In utero assessment of possible anorectal malformations may be facilitated in the future by the uniformly hyperintense T1 signal meconium acting as an MR contrast agent. MRI appears to have a low sensitivity to the detection of a fistula in the neonatal period and it should be stressed that
Due to multifactorial influences on rectal continence, the technical adequacy or failure of a surgical pull-through procedure may not be clinically apparent for many years after surgery [20]. A persisting problem with faecal incontinence or constipation, however, despite an apparently successful pull-through operation is the usual indication for further imaging in older children with congenital anorectal malformations. MRI is undoubtedly the optimal imaging modality for assessing these patients who are under consideration for re-operation, and is without the risk of ionising radiation. Axial images are generally best for assessing the siting of the pulled-through rectum and also for evaluating the puborectalis and EAS. A catheter can be placed within the rectum to facilitate its identification. The levator hammock is best evaluated on coronal images (Fig. 3). An additional advantage of MRI is that it can survey for other associated lesions in the spine, spinal cord or renal tract. MR can provide useful information regarding development of the pelvic musculature which can be helpful to the surgeon in predicting likely functional outcome. Many patients with persistent problems
Fig. 3. Coronal T1-weighted image through the posterior pelvis showing the normal levator hammock (arrow). R, rectal ampulla.
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Fig. 4. Post-operative complications amenable to corrective surgery. (a) Axial MR at the level of the ischium shows an asymmetric hypoplastic EAS (curved arrow), and neorectum (arrow) with a catheter in-situ misplaced outside the EAS; (b) mesenteric fat (arrow) has been inadvertently brought down with the rectum which can interfere with normal sphincteric function [13] (reprinted with permission, Sato et al.).
have hypoplastic muscles for which little can be done surgically, although augmentation with striated muscle transplantation, such as a gracilis sling, for an underdeveloped puborectalis to provide an adequate voluntary sphincter mechanism has been described [20]. Misplacement of the neorectum outside the puborectal sling (Fig. 4(a)), anterior misplacement within the EAS or asymmetric positioning within the levator muscle can all be easily recognised on MRI. It should be noted that the puborectalis muscle can appear asymmetric in thickness on either side of the rectum because of chemical shift artefact occurring at the junction between muscle and fat. Care must be taken, therefore, not to mistake chemical shift artefact for asymmetric musculature or asymmetric positioning of the pullthrough rectum in the puborectalsis sling [20]. Mesenteric fat inadvertently brought down with the rectum into the sphincteric muscles at operation is easily visualised on axial MRI, particularly on T1-weighted images as an area of hyperintensity around the neorectum (Fig. 4(b)). This fat can interfere with muscle action on the rectum and may have to be surgically removed [13]. The information provided by MRI can greatly decrease the amount of dissection at repeat surgery and thus helps to minimise muscle damage during a further corrective procedure. One series has suggested that measurement of an anorectal angle in the sagittal plane is as important as evaluation of the development of the levator and sphinteric muscles in the assessment of children with post-operative faecal incontinence [14]. The anorectal angle is created by the continuous contraction of the puborectalis muscle at rest, which is one of the major factors affecting faecal continence. Poor clinical scores of incontinence were found in patients with wider anorectal angles irrespective of the relative development of the puborectalis or EAS, but further studies need to be performed to validate these findings [14].
6. Conclusion The vast majority of anorectal malformations can be managed without recourse to radiological investigations in the neonatal period. Renal and probably spinal ultrasound in the early newborn period is, however, routinely recommended. Very occasionally other imaging such as a combination of plain radiography and MRI can be of help in planning the pull-through procedure and may be useful in predicting likely future continence. In the post-operative older patient, MRI is the technique ‘par excellence’ in the assessment of the pull-through procedure particularly as the technical adequacy or failure of surgery may not be clinically apparent for many years.
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