The spectrum of anorectal malformations (ARMs) ranges from anal stenosis to persistent cloaca. The term imperforate anus applies to the vast majority of these malformations, whereby the anal canal is malformed and there is no visible normal anal opening onto the perineum.
PATHOGENESIS AND EPIDEMIOLOGY
By 6 weeks of gestation, the urorectal septum divides the cloaca into the anterior urogenital sinus and posterior anorectal canal. Failure of the urorectal septum to form results in a fistula between the bowel and urinary tract (male) or the vagina or vestibule (female). The urorectal septum divides the cloacal membrane into the urogenital and anal membranes. Failure of the anal membrane to resorb results in an anal membrane or stenosis. The perineum also contributes to development of the external anal opening and genitalia by formation of cloacal folds that extend from the anterior genital tubercle to the anal membrane. Breakdown of the cloacal folds results in the external anal opening being anterior to the external sphincter (ie, anteriorly displaced anus or perineal fistula).
Anorectal anomalies as a whole occur in as many as 1 in 4000 live births and are slightly more common in boys. The most common defect is an imperforate anus with a fistula between the distal anorectum and the urethra in boys or the vestibule in girls. The risk for a couple having a second child with an ARM is approximately 1%.
ARMs represent a wide spectrum of defects and should ultimately be described in terms of the realized anatomy, rather than “low or high” as is often customary (Table 410-1). The terms “high” and “low” are used to generalize the location of the distal rectum or anal canal relative to the perineum. High lesions are those in which the rectum ends above the levator musculature and may or may not have a fistulous connection to the vagina, prostatic urethra, bladder neck, or bladder. Intermediate lesions are characterized by the rectal pouch ending within the levator complex, with or without a fistula to the vestibule or the bulbar urethra in boys. In low lesions, the rectal pouch has completely traversed the levator musculature, and a fistula is usually evident on the skin within the midline (perineal fistula). Rectal atresia refers to an unusual lesion in which the lumen of the rectum is either completely or partially interrupted, with the upper rectum being dilated and the lower rectum consisting of a small anal canal. Anal stenosis can be seen in both males and females and is characterized by a normally positioned anal perineal orifice with a narrowed anal canal. A persistent cloaca is a female defect in which the rectum, vagina, and urethra all empty into a single, common channel.
TABLE 410-1TYPES OF ANORECTAL ANOMALIES AND RECOMMENDED SURGICAL TREATMENT