ARM or imperforate anus has been a well-known condition since antiquity. For many centuries, physicians created an orifice in the perineum of children with imperforate anus. Those that survived most likely suffered from a type of defect that would now be recognized as “low.” Those with a “high” defect did not survive that treatment. Amussat, in 1835, was the first individual who sutured the rectal wall to the skin edges, which could be considered the first anoplasty. During the first 60 years of the twentieth century, surgeons performed a perineal operation without a colostomy for the so-called low malformations. High imperforate anus was usually treated with a colostomy performed in the newborn period, followed by an abdominoperineal pull-through some time later in life, but surgeons lacked objective anatomic guidelines. Unfortunately this left many patients incontinent and was not an appropriate solution to the spectrum of malformations. The surgical approach to repairing these defects changed dramatically in 1980 with the introduction of the posterior sagittal approach, which allowed surgeons to view the anatomy of these defects clearly, to repair them under direct vision, and to learn about the complex anatomic arrangement of the junction of the rectum and genitourinary tract. It has become the predominant surgical method for anorectal anomalies. In cases when the rectum or the vagina is very high and an abdominal approach as well is needed, laparoscopy can be used in combination with the posterior sagittal approach [3].

The early embryologic development of the anorectum, the primitive urogenital sinus, and the caudal neural tube is closely related, which helps explain the associated malformations of these three systems. In early embryonic life, the distal portion of the hindgut, the primitive cloaca, is divided into dorsal and ventral parts by a coronal sheet of the mesenchyme, the urorectal septum, and separated from the amniotic cavity by the cloacal membrane. Most ARMs result from abnormal development of the urorectal septum. 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. 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. 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. 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. 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) [4].

ARMs are found as isolated congenital birth defects, as part of a syndrome or associated with other anomalies. Associated anomalies have been reported to occur in approximately 45–65 % of the patients, mostly of the urogenital tract, central nervous system, skeletal system (vertebrae), or the remaining gastrointestinal tract [1]. ARM has been reported to occur in families suggesting that there is a genetic component in its etiology. There appears to be a low rate of association in families, but some appear to have an autosomal dominant inheritance pattern. Consanguinity has been identified as leading to a higher incidence of ARM, particularly in countries in the Gulf and Middle East regions. In addition, familial Currarino associations are well established, and family members have been shown to have sacral anomalies without the full syndrome. In a small number of patients, genetic factors are clearly associated with ARM. Previous studies have suggested the importance of a locus on chromosome 7q39, which includes three genes: SHH, EN2, and HLXB9. These include Towne-Brock syndrome, FG syndrome, Kaufman-McKusick syndrome, and Lowe syndrome. In addition, ARM has been described in association with trisomy 8 mosaicism, as well as Down and fragile X syndromes. Till date, the accurate embryologic defect causing anorectal malformations still remains undetermined. With recent researches in the pathogenesis of anorectal malformations, the previous theories have been discarded. While in the past, defects in lateral fusion were thought to be causative, there is evidence from animal models and from detailed study of human fetuses with major anomalies that a deficiency in the dorsal component of the cloacal membrane and the adjacent dorsal cloaca is causative. A subsequent malfunction of the primitive streak and tail bud in the early development phase around 3–4 weeks has been proposed (yet to be clearly defined) as causation for associated anomalies of the pelvic floor [1, 5].

Based on the anatomy, various classifications have been proposed to define the pathology of these anorectal anomalies. The earliest classification dates back to 1953 when Gross proposed a simple differentiation based on the levator muscle, i.e., supralevator, for those above the levator ani, or infralevator anomalies, for those below the levator ani [6].

With advancement in the understanding of the pathology of the malformations, a need was felt to define these lesions more appropriately. During the centenary of the Royal Children’s Hospital in Melbourne, a new international classification was proposed in 1970. This classification utilized the concept of levator ani wherein anomalies above the levator were termed as high and those below were termed as low anomalies, but it also introduced intermediate anomalies which were known as translevator anomalies [7].

The best known classification of ARMs is the Wingspread classification of 1984 (Wisconsin). This classification distinguished between high, intermediate, and low anomalies in the male and female, with special groups established for cloacal and rare malformations. High-type anorectal malformations were agenesis without fistula in both sexes. The low-type malformations were classified as anovestibular fistula in the female and, in both sexes, as anocutaneous fistula and anal stenosis. This classification was widely accepted over the years and was based on detailed embryological and anatomic studies performed especially by Stephens et al. and Kelly on anatomic sections and radiographic investigations. They recognized that the pubococcygeal line extending from the upper border of the os pubis to the os coccyx corresponds with the attachment of levator ani muscles to the pelvic wall, separating high-type malformations lying above the levator muscle and intermediate and low forms of anorectal agenesis lying below this anatomic line. Furthermore, in healthy individuals, the lowest point of the ischial tuberosity, the so-called I-point, represents the deepest point of the funnel of the levator ani muscles. Therefore, every blind rectal pouch, lying between the pubococcygeal line and the I-point, was classified as an intermediate anomaly and could be treated by a posterior sagittal anorectoplasty (PSARP). Low lesions below the I-point could be easily managed from a perineal approach. Because of these anatomic relations, the Wingspread classification had a significant impact on the choice of surgical approach. However, some details of the Wingspread classification remained questionable. Therefore, in 1995, Peña proposed a classification based on the type of the fistula present. He distinguished between perineal, vestibular, bulbar, prostatic, and bladder neck fistulas, imperforate anus without fistula, vaginal fistulas, cloacal fistulas, and rectal atresia or stenosis (Table 24.1).

This descriptive and fistula-related grouping became widely accepted over the past decade. The advantage of the classification of Peña is that the type of the fistula provides information not only about localization of the blind pouch but also on the anticipated extent of mobilization of the atretic rectal segment necessary to perform a sacro- or abdominosacroperineal pull-through. It is important to remember that the course of the fistula may vary from one individual to another and can be ascending or descending and of shorter or longer length so that the confluence of the fistula with the urogenital tract or perineum may differ from the lowest point of the blind pouch. This is especially true if the fistula arises from a higher level of the blind-ending rectum and not from its lowermost point. Therefore, the classification of Peña does not distinguish between rectovestibular and anovestibular fistulas. By closely comparing both classifications, that is, the Wingspread classification and the suggestions of Peña, it becomes clear that there is no real contradiction between them. Perineal and vestibular fistulas could be regarded as low malformations, bulbar fistulas, and imperforate anus without a fistula, and most of the vaginal fistulas may be regarded as intermediate-type anomalies, and prostatic and bladder neck fistulas are considered high-type imperforate anus. The same is true for rectal agenesis or stenosis. In addition, rare/regional variants, despite being frequent in certain geographic areas of the world, are not alluded to in either classifications. More recently, the Krickenbeck Conference of 2005 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. It has gained overall popularity in the international community of pediatric surgeons. This classification itself seemed a logical sequel to the Wingspread classification. It distinguishes five types of fistulas: rectoperineal, rectovestibular, rectourethral bulbar, rectourethral prostatic, and rectovesical. Cloacal malformations and the absence of fistulas, anal stenosis, and rare regional variants complete this classification. The extremely rare rectovaginal fistula is considered a variant of cloacal anomaly (Table 24.2) [4, 8].

Cloacal anomaly is a complex anatomic disorder that manifests as a unique external perineal opening with a short or long common canal for the genital, urinary, and digestive systems. Isolated rectovaginal fistulas are extremely rare and are considered a variant of cloacal anomaly. The Wingspread and Krickenbeck classifications are very similar. The Wingspread classification allows location of the blind rectal pouch. The Krickenbeck classification is more descriptive and is clinically oriented; 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 (Tables 24.3 and 24.4) [8].