7.6: Anorectal malformations

Arun Gupta, Manisha Jana, Devasenathipathy Kandasamy

Introduction

Anorectal malformations (ARM) are uncommon congenital malformations. They can either be found as an isolated anomaly or in association with other congenital malformations. The age of presentation and clinical presentation can have wide variation. In order to understand the complex anatomic details of the ARM, it is imperative to know the embryology and development of the normal anal canal.

Embryology

In the early embryonic life (4th–5th week), the primitive hindgut and the allantois communicate with each other at the cloaca. The membrane covering the cloaca at the caudal end is called cloacal membrane. During the 6th week of gestation, a sheet of mesenchyme (urorectal septum) grows between the allantois and hindgut. Eventually, lateral and midline folds develop in the cloaca, thereby dividing the common cavity into two parts: anteriorly urogenital sinus and posteriorly anorectum. Gradually, the embryo starts to lengthen and curve inwards. As a result, the distance between cloacal membrane and the tip of urorectal septum decreases and eventually, they meet each other. Consequently, the urogenital sinus and anorectum get completely separated (Fig. 7.6.1).

Fig. 7.6.1 Schematic diagram of normal development of rectum and anal canal. A: Early embryonic life (4 weeks); cloaca is formed and it communicates the allantois and hindgut. At the caudal end of it lies the cloacal membrane. Urorectal septum (URS) develops as a mesodermal ingrowth and grows caudally in to the cloaca (blue arrow). Simultaneously the embryo also undergoes inward folding and the cloacal membrane approaches the URS (red arrow). B: About 6/7 weeks, the URS and cloacal membrane meet each other and divide the cloaca in two halves: anterior urogenital sinus (UGS) and posterior rectum (R).

In a later stage of gestation (7th–8th week), the anal canal gets secondarily occluded by epithelial plugs, which again gets recanalized to form the lower anal canal.

Henceforth, it can be understood that any aberrations in the normal development at an earlier stage of gestation would result in a more severe degree and higher level of communication between urogenital system and anorectum. Any insult at a later stage of development will result in a low ARM, which will mostly have an abnormal anal opening in a normal position and no urogenital communication with the hindgut.

Classification

Over the years, many classification systems for ARM have been introduced. Three major classifications are:

• International classification, introduced in 1970 (Table 7.6.1)
• Wingspread classification, introduced in 1984 (Table 7.6.2)
• A more recent Krickenbeck classification, introduced in 2005 (Table 7.6.3).

TABLE 7.6.1

International Classification of ARMs

	Male	Female
High	Anorectal agenesis without fistula Anorectal agenesis with • Rectovesical fistula • Rectoprostatic urethral fistula • H or N type fistula* Rectal atresia	Anorectal agenesis without fistula Anorectal agenesis with • Rectovesical fistula • Cloacal anomalies • High rectovaginal fistula Rectal atresia H or N type fistula*
Intermediate	Anal agenesis without fistula Anal agenesis with rectobulbar urethral fistula Anorectal stenosis	Anal agenesis without fistula Anal agenesis with low rectovaginal fistula Anal agenesis with rectovestibular fistula Anorectal stenosis
Low #	1. Covered anus, complete 2. Anal stenosis 3. Anterior perineal anus 4. Anocutaneous fistula	1. Covered anus, complete 2. Anal stenosis 3. Anterior perineal anus 4. Anocutaneous fistula 5. Anovestibular fistula 6. Vulvar anus 7. Anovulvar anus

Male

Female

High

Anorectal agenesis without fistula

Anorectal agenesis with

• Rectovesical fistula
• Rectoprostatic urethral fistula
• H or N type fistula*

Rectal atresia

Anorectal agenesis without fistula

Anorectal agenesis with

• Rectovesical fistula
• Cloacal anomalies
• High rectovaginal fistula

Rectal atresia

H or N type fistula*

Intermediate

Anal agenesis without fistula

Anal agenesis with rectobulbar urethral fistula

Anorectal stenosis

Anal agenesis without fistula

Anal agenesis with low rectovaginal fistula

Anal agenesis with rectovestibular fistula

Anorectal stenosis

Low #

1. Covered anus, complete
2. Anal stenosis
3. Anterior perineal anus
4. Anocutaneous fistula

1. Covered anus, complete
2. Anal stenosis
3. Anterior perineal anus
4. Anocutaneous fistula
5. Anovestibular fistula
6. Vulvar anus
7. Anovulvar anus

^*Not described in original classification.

#Site of anal opening is usually low but variable (1 and 2: at normal anal site; 3 and 4: at perineal site; 5–7: at vulvar location).

TABLE 7.6.2

Wingspread Classification of ARMs

Level	Male	Female
High	Anorectal agenesis without fistula Anorectal agenesis with rectoprostatic urethral fistula Rectal atresia	Anorectal agenesis without fistula Anorectal agenesis with rectovaginal urethral fistula Rectal atresia
Intermediate	Anal agenesis without fistula Rectobulbar urethral fistula	Anal agenesis without fistula Rectovaginal fistula Rectovestibular fistula
Low	Anal stenosis Anocutaneous fistula	Anal stenosis Anocutaneous fistula Anovestibular fistula
Cloacal malformations	Rare	Rare

TABLE 7.6.3

Anorectal Anomalies: Krickenbeck Classification

Type	Male	Female
Intermediate/High	Anal or anorectal agenesis without fistula Rectal atresia Anal or anorectal agenesis • Rectourethral bulbar fistula • Rectourethral prostatic fistula • Rectovesical fistula	Anal or anorectal agenesis without fistula Rectal atresia Cloacal anomalies • Short canal (<3 cm) • Long canal (>3 cm)
Low	Anal stenosis Imperforate anus without fistula Anal agenesis with rectoperineal fistula	Anal stenosis Imperforate anus without fistula Anal agenesis with rectoperineal fistula Anal agenesis with rectovestibular fistula

Type

Male

Female

Intermediate/High

Anal or anorectal agenesis without fistula

Rectal atresia

Anal or anorectal agenesis

• Rectourethral bulbar fistula
• Rectourethral prostatic fistula
• Rectovesical fistula

Anal or anorectal agenesis without fistula

Rectal atresia

Cloacal anomalies

• Short canal (<3 cm)
• Long canal (>3 cm)

Low

Anal stenosis

Imperforate anus without fistula

Anal agenesis with rectoperineal fistula

Anal stenosis

Imperforate anus without fistula

Anal agenesis with rectoperineal fistula

Anal agenesis with rectovestibular fistula

International classification was considered by some to be too detailed and too complex. However, in the authors’ experience, this system should ideally be followed in a high-volume teaching institute. A simplified version of it was, therefore, considered and is known by the name of Wingspread classification (so called because the meeting to decide it was held at the Wingspread convention centre in Racine, Wisconsin). More recently, another classification was proposed in the Krickenbeck conference on ARM in 2005. There are some common features/differentiating points in all the classification systems, namely the level of bling rectal pouch and presence/absence of fistula.

Evaluation of a child with arm

Clinical findings

Evaluation of a child with suspected ARM starts with a thorough clinical examination. Careful perineal examination should be performed to look for the number and location of the orifices. If an external anal orifice is appreciable, a low anomaly should be suspected. The location of anal orifice may be variable (Table 7.6.1). To the contrary, a flat perineum (no anal dimple or orifice) suggests a high anomaly. If there is evidence of meconium at urethral orifice/history of meconium in urine, it indicates presence of an internal fistula. In all suspected cases of high malformation, colostomy should be done, followed by detailed evaluation for the delineation of internal anatomy by contrast studies (discussed later).

In the female child, if there is only one perineal opening, a cloacal malformation is likely. If two orifices are visible in the vulva with no anal orifice, then one of them is of the urethral, while the other one is of the vaginal orifice. Meconiun would come from the latter due to the associated recto-vaginal fistula. Colostomy is indicated in both the situations.

If three orifices are visible, then two of them are respectively, of the urethra and the vagina, both at their normal site, while the third one is the bowel which could be at the normal anal site, or it might open as a fistulous track anteriorly at an abnormal site – either in the perineum or in the vestibule.

Imaging evaluation

Two most important investigations in the evaluation of ARM are invertography and contrast studies.

Invertography

Invertography is a procedure described back in 1930 by Wangensteen and Rice. This involved obtaining a lateral radiograph of the neonate, held upside down. An ideal time to obtain this radiograph is about 6–8 hours after birth, so as to allow the intestinal gas to reach the rectum. The thighs of the baby should be flexed, and X-ray beam should be centred at the greater trochanter. Anal dimple should be marked by barium paste. A properly done invertogram should have the following:

• It should be true lateral; the ischial bones of both sides should superimpose.
• The terminal bowel loop should be well distended by air, or else the interpretation might be fallacious.

Interpretation.

On an invertogram, the distal part of the blind-ending bowel loop should be localized in relation to some specific bony landmarks, namely:

1. Using PC line and I line: This method was described by Stephens and Smith in 1971. PC line corresponds to pubococcygeal line (drawn between tip of coccyx till the midpoint of posterior margin of symphysis pubis). I line refers to the ischial line. Another third line is the anal pit line (caudalmost line) (Fig. 7.6.2). If the gas bubble in blind-ending bowel loop lies below the I line, it is termed as ‘low’ anomaly. Location of gas bubble between PC and I line suggests an ‘intermediate’ anomaly. Gas bubble lying above the level of PC line gets classified as ‘high’ anomaly.
2. Using ‘M’ line: In neonates where identifying the unossified coccyx is difficult, this alternative method described by Cremin can be used. ‘M line’ or ‘M point’ is located at the junction of upper two-third and lower one-third of the ischium. Anomalies are then divided into ‘high’ and ‘low’ types with no intermediate category (Figs. 7.6.3 and 7.6.4).

Fig. 7.6.2 Classification of ARM on invertography. High type (A), intermediate (B) and low type (C).

Fig. 7.6.3 Diagrammatic representation of fallacies of PC line. A: Normal PC line has to be drawn from the tip of coccyx (C) and midpoint of pubic bone (P). If the coccyx is not ossified, the line can be wrongly drawn from CX to PX, thus making the classification inaccurate. B: Using M line (drawn through the junction of upper two-thirds and lower one-third of ischium) as a landmark makes this classification more accurate.

Fig. 7.6.4 Pitfalls of using PC line as a reference for measurement. In a neonate with unossified coccyx, drawing PC line is not accurate. Hence, M line (red line) can be drawn at the junction of upper two-thirds and lower one-third of ischium (marked in yellow dotted line). The example was that of a high ARM.

Additional points to be evaluated on an invertogram are: presence/absence of spinal anomalies; and presence of gas in urinary bladder/vagina.

Invertography is reported to be very accurate in differentiating a low-type fistula from intermediate/high type. However, the accuracy depends on how well the study has been performed. While interpreting invertogram, one must be aware of the pitfalls of this study (Table 7.6.4).

TABLE 7.6.4

Pitfalls of Invertography

Study done too early (<6 hours after birth)	Bowel gas does not reach terminal part; falsely high level of anomaly on imaging
Meconium plugging the terminal bowel	False diagnosis of a high level of anomaly on imaging, as the gas does not reach terminal part
Excessive crying	Contraction of the puborectalis sling leads to a false diagnosis of a high level of anomaly on imaging
Presence of a patent fistula with urinary tract	Gas may escape through the fistula; delineating the level of anomaly might be difficult