Imperforate anus has been recognized since antiquity, but has only been treatable in all of its forms in the latter half of the past century. The first surgical anoplasty for a low-type of imperforate anus was performed by Amussat in Paris in 1835. The next significant advance did not occur until the work of Stephens, who described a combined sacral and abdominal perineal approach based on cadaveric dissections. This highlighted the importance of the puborectolia sling (Stephens, 1953). The modern approach to reconstruction of imperforate anus was pioneered by deVries and Pena in 1982, with the posterior sagittal approach to the whole spectrum of anorectal malformations using the posterior sagittal anorectoplasty (PSARP) (deVries and Pena, 1982; Pena and deVries, 1982).

Anomalies of the anus and rectum have usually been explained on the basis of an arrest of the caudal descent of the urorectal septum toward the cloacal membrane between 4 and 8 weeks of gestation (Fitzgerald and Fitzgerald, 1994). At 4 to 6 weeks the cloacal membrane becomes partitioned into the anterior urogenital sinus and the posterior anorectum by the cranial to caudal growth of mesoderm-derived urorectal septum. The urorectal septum fuses with the cloacal membrane at what then becomes the perineal body (Paidas and Pena, 1996). The mesoderm-derived urorectal septum is composed of the midline Tournex fold and two lateral Rathke folds. The lower third of the anal canal is derived from the ectoderm of the anal pit. Fusion of the hindgut mesoderm with the ectoderm occurs at the dentate line. Failure of Rathke folds to develop results in arrest of the inferior urorectal septum, resulting in a rectourethral fistula in the male and a persistent cloaca in the female. This arrest in Rathke folds usually occurs just below the paramesonephric duct, but a more caudal arrest in Rathke folds could result in a high rectovaginal fistula in a female. Failure of both Rathke and Tourneaux folds in males results in rectovesicular fistulas at the bladder neck. In the female it is more likely to result in cloacal anomaly or duplicated vagina and uterus (Paidas and Pena, 1996). The malalignment of Tourneaux and Rathke folds may also result in rectourethral fistula in males and vestibular fistula in females. Isolated imperforate anus without a fistula occurs from failure of the anal pit to form, and despite normal descent of Tourneaux and Rathke folds, the lack of an ectodermal anal pit results in imperforate anus. Failure of the anal membrane to resorb or incomplete resorption despite formation of the anal pit results in rectal atresia or stenosis, respectively. Defects in mesoderm at the level of the perineal body are thought to result in perineal fistula.

An alternative theory of the embryologic origin of anorectal malformation focuses on the dorsal cloacal membrane in observations in pigs with anorectal malformations (Van der Putte, 1986; Van der Putte and Neeteson, 1984). Van der Putte suggests that the division of the anal and urogenital systems is the result of a marked alteration in the shape of the cloaca by disproportionately strong growth of the mantle mesenchyme surrounding the urogenital compartment (Van der Putte, 2006).

As many as 50% of cases of anorectal malformations have associated anomalies (Table 76-1). Spinal or skeletal anomalies are present in 50% of cases, genitourinary anomalies in 58%, tracheoesophageal fistula in 10%, and cardiac anomalies in 5% (Sanders, 1996; Stoll et al., 2007). Imperforate anus is part of the VACTERL association and is not uncommon in trisomy 21 (Torres et al., 1998).

The incidence of imperforate anus is 1 in 5000 livebirths (Boocock and Donnai, 1987; Cuschieri and EUROCAT Working Group, 2001; Kiely and Pena, 1998; Schwoebel, 1984; Stephens and Smith, 1971). Most reports note a male preponderance of between 55% and 65% of cases (Kiely and Pena, 1998; Stephens, 1953; Weinstein, 1965). Imperforate anus has been linked to maternal diabetes mellitus, thalidomide exposure, ethanol intake, and assisted reproductive technology (Midrio et al., 2006).

It is unusual to make a diagnosis of imperforate anus on a prenatal sonographic examination. In a series of 69 cases of imperforate anus, only 11 (15.9%) were diagnosed prenatally (Brantberg et al., 2006). Similarly, Livingston et al. (2006) found that in a series of 95 children diagnosed postnatally with anorectal malformations, the correct diagnosis was made by antenatal sonography in only one child. Sanders (1996) has noted that the anus can be seen in the normal fetus as an echogenic dot on a transverse view at the level of the genitalia. He notes that this dot is absent in cases of imperforate anus. The reliability of the absence of this echogenic dot is unknown. It likely represents the echogenic sphincter complex, which may be present even if there is imperforate anus. More commonly, a dilated distal colon may be observed in the fetal pelvis (Guzman et al., 1995; Taipale et al., 2005) (Figure 76-1). But as Kaponis et al. (2006) have observed, bowel dilatation in anorectal malformations may be transient, occurring only during the first trimester (Gilbert et al., 2006).