The spectrum of anorectal malformations ranges from simple anal stenosis to the persistence of a cloaca; incidence ranges from one in 4000 to one in 5000 live births and is slightly more common in boys. The most common defect in both boys and girls is an imperforate anus with a fistula between the distal bowel and the urethra in boys or the vestibule of the vagina in girls. The risk for a couple having a second child with an anorectal malformation is approximately 1%.

By 6 weeks of gestation, the urorectal septum begins to move in a caudal direction to divide 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 (in boys) or the vagina (in girls). The urorectal septum divides the cloacal membrane into the urogenital and anal membranes. Complete or partial 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. The perineal body is formed by fusion of the cloacal folds between the anal and urogenital membranes. Breakdown of the cloacal membrane anywhere along its course results in the external anal opening being anterior to the external sphincter (ie, anteriorly displaced anus).

Management of anorectal anomalies requires that the level of the rectal pouch and presence of fistula to the urinary tract or vagina be determined; this is important in early management. Evaluating the location of a fistula can be performed at a later time.

Traditionally, the level of the end of the rectal pouch is determined by obtaining a lateral pelvic radiograph (ie, “invertogram”) after the infant is held upside-down for several minutes to allow air to pass into the rectal pouch. High lesions are above the levator and may or may not have a fistulous connection to the vagina or bladder/prostatic urethra in boys. Intermediate lesions are characterized by the rectal pouch ending within the levator, with or without a fistula to the vagina or a bulbous urethra in boys. In low lesions, the rectal pouch has completely traversed the levator musculature, and a fistula usually is evident on the skin within the midline (ie, anteriorly displaced anus). 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. A persistent cloaca is defined as a defect in which the rectum, vagina, and urethra all meet and fuse to form a single, common channel. In girls, the type of defect may be determined by the number of orifices at the perineum. A single orifice would be consistent with a cloaca. If two orifices are seen (ie, urethra and vagina), the defect is an imperforate anus or, less commonly, a persistent urogenital sinus comprising one orifice and a normal anus as the other orifice.

From a practical standpoint, the invertogram is difficult to perform, and it is not entirely predictive of the pouch level. A crying infant may appear from the increased intraabdominal pressure and downward displacement of the gas within the pouch to have a low anomaly. Furthermore, the gas may not have traversed to the very end of the pouch, thus giving the impression of a high lesion. Inspection of the perineum alone determines the pouch level in 80% of boys and 90% of girls. Clinically, if a fistula is seen anywhere on the perineal skin of a boy or external to the hymen of a girl, a low lesion can be assumed, which allows a primary perineal repair procedure to be performed. The majority of all other lesions are high or intermediate, and they require diversion through a sigmoid colostomy that is followed by a definitive repair procedure at a later date. If required, the level of the rectal pouch can be delineated more definitively by ultrasonography or magnetic resonance imaging. Perineal ultrasonography may be useful in determining the distance between the rectal pouch and the anal skin, although ultrasonography may be subject to the same pitfalls as the invertogram. In general, a lesion can be considered to be low if the distance from the rectal pouch to the skin, as determined by ultrasonography, is less than 1 cm. Magnetic resonance imaging accurately delineates the anatomy of the pouch relative to the levator ani musculature and sphincters; it is particularly useful during postoperative evaluation of the child with continence problems after repair.

Congenital anorectal anomalies often coexist with other lesions, and the VATER or VACTERL association must be considered. Bony abnormalities of the sacrum and spine occur in about one-third of patients with anorectal anomalies and consist of absent, accessory, or hemivertebrae and/or an asymmetric or short sacrum. The absence of two or more vertebrae is associated with poor prognosis in terms of bowel and bladder continence. Occult dysraphism of the spinal cord also may be present, and it consists of tethered cord, lipomeningocele, or fat within the filum. Genitourinary abnormalities other than the rectourinary fistula occur in 26 to 59% of patients. Vesicoureteral reflux and hydronephrosis are the most common abnormalities, but other findings such as horseshoe, dysplastic, or absent kidney as well as hypospadias or cryptorchidism also must be considered. In general, the higher the anorectal malformation, the more frequent the associated urologic abnormalities. In patients with persistent cloacas or rectovesical fistula, the likelihood of a genitourinary abnormality is approximately 90%. In contrast, the frequency is only 10% in children with low defects (ie, perineal fistula).

Evaluation for associated anomalies should include plain-film radiography of the chest and spine to exclude abnormalities of the heart as well as the vertebrae and sacrum. If a cardiac defect is suspected, echocardiography should be performed before any surgical procedure. Ultrasonography of the spine should be obtained to exclude occult dysraphism. Before feeding, a nasogastric tube should be placed, and its presence within the stomach confirmed, to exclude esophageal atresia. Radiographic evaluation of the urinary tract should include renal ultrasonography and voiding cystourethrography; a rectourinary fistula (if present) likely will be demonstrated by the latter procedure.

SURGICAL MANAGEMENT

The newborn infant with a low lesion can have a primary, single-stage repair procedure on the perineum without need for a colostomy. Three basic approaches may be used. For anal stenosis with a normal location of the anal opening, only simple dilatation is necessary. This should be performed daily (12-Fr size for newborn infants), and the size of the dilator should be increased progressively. Over several months, the anus ultimately will admit an index finger easily, and the dilatations can be discontinued. If the anal opening is anterior to the external sphincter (ie, anteriorly displaced anus) with a small distance between the opening and the center of the external sphincter, and the perineal body also is intact, a cutback anoplasty is performed. A cut is made from the anal orifice to the central part of the anal sphincter, thus enlarging the anal opening. Alternatively, if there is a large distance between the anal opening and the central portion of the external anal sphincter, the aberrant anal opening is transposed to the correct position, and the perineal body is reconstructed.

Infants with intermediate or high lesions require a colostomy as the first part of a three-stage reconstruction. The colon is completely divided in the sigmoid region, with the proximal bowel as the colostomy and the distal bowel as a mucous fistula. Complete division of the bowel minimizes fecal contamination into the area of a rectourinary fistula, and it may lessen the risk of urosepsis. Furthermore, the distal bowel can be evaluated radiographically to determine the location of the rectourinary fistula. The second-stage procedure usually is performed 3 to 6 months later; it consists of surgically dividing the rectourinary or rectovaginal fistula with a “pull-through” of the terminal rectal pouch into the normal anal position. A posterior sagittal approach allows the central position of the anal sphincter to be identified by electrical stimulation of the perineum. An incision is made in the midline extending from the coccyx to the external sphincter. The rectum is identified, and the fistula to the vagina or urinary tract (if present) is divided. The rectum is then mobilized, and the perineal musculature (ie, levator ani and parasagittal fibers of the external sphincter complex) reconstructed. The patient is left with the protective colostomy to afford healing of the new anal anastomosis. In boys, a urinary catheter remains in place to maintain the lumen of the urethra after repair of the rectourinary fistula. The third and final stage is performed a few months after the second stage, and it consists of colostomy closure. Anal dilatations are begun 2 weeks after the pull-through procedure and continue for several months after the colostomy closure. A 12-Fr dilator is used for newborns, which is increased up to 14- or 16-Fr for older infants.

In patients with a persistent cloaca, the surgical approach generally is the same as that with high imperforate anus. The urethra is created from the tubularized old urogenital sinus, and the vagina and rectum are carefully separated and placed into the appropriate location. Often, the vagina will not reach the perineal skin, and a vaginal augmentation procedure can be performed using flaps of the perineal or labial skin or a segment of small intestine.

OUTCOME

Major morbidity in patients with anorectal malformations is related to associated anomalies. Fecal continence is the major goal regarding correction of the defect. Prognostic factors for continence include the level of the pouch and whether the sacrum is normal. The best results are seen in patients with low lesions and a normal sacrum, with incontinence reported to occur in up to 40% of patients at long-term follow-up. In patients with intermediate lesions and a normal sacrum, soiling was reported to occur in 50 to 75%. In high lesions, the rate of incontinence is nearly 100%. Despite the fairly high rate of incontinence, 84% of adult patients were satisfied with their level of cleanliness. In general, patients who have continued problems with either constipation or soiling following the repair of an anorectal anomaly should begin a bowel-training program with daily enemas. The goal of this postoperative program is to keep the lower rectum decompressed while controlling the need to defecate. Success or failure in achieving continence cannot be judged until after the age of 10 years. Anorectal biofeedback may improve continence in some children with low to intermediate lesions.

Mark Lane Welton, MD

REFERENCES

_{HASSINK EA , RIEU PN , SEVERIJNEN RS , et al: Are adults content or continent after repair for high anal atresia? A long-term follow-up study in patients 18 years of age and older. Ann Surg 218:196-200, 1993}

PENA A: Posterior sagittal anorectoplasty: results in the management of 332 cases of anorectal malformations. Pediatr Surg Int 3:94-104, 1988

PENA A: Imperforate anus and cloacal malformations. In: ASHCRAFT KW , HOLDER TA , eds: Pediatric Surgery, 2nd ed. Philadelphia, WB Saunders, 1993:372-392

RINTALA R , MILDH L , LINDAHL H: Fecal continence and quality of life in adult patients with an operated low anorectal malformation. J Pediatr Surg 27:902-905, 1992