Chapter 336 Surgical Conditions of the Anus and Rectum

336.1 Anorectal Malformations

In understanding the spectrum of anorectal anomalies, it is necessary to consider the importance of the sphincter complex, a mass of muscle fibers surrounding the anorectum (Fig. 336-1). This complex is the combination of the puborectalis, levator ani, external and internal sphincters, and the superficial external sphincter muscles, all meeting at the rectum. Anorectal malformations are defined by the relationship of the rectum to this complex and include varying degrees of stenosis to complete atresia. The incidence is 1/3,000 live births. Significant long-term concerns focus on bowel control and urinary and sexual functions.

Figure 336-1 Normal anorectal anatomy in relation to pelvic structures. A, Male. B, Female.

(From Peña A: Atlas of surgical management of anorectal malformations, New York, 1989, Springer-Verlag, p 3.)

Embryology

The hindgut forms early as the part of the primitive gut tube that extends into the tail fold in the 2nd wk of gestation. At about day 13, it develops a ventral diverticulum, the allantois or primitive bladder. The junction of allantois and hindgut become the cloaca, into which the genital, urinary, and intestinal tubes empty. This is covered by a cloacal membrane. The urorectal septum descends to divide this common channel by forming lateral ridges, which grow in and fuse by the middle of the 7th wk. Opening of the posterior portion of the membrane (the anal membrane) occurs in the 8th wk. Failures in any part of these processes can lead to the clinical spectrum of anogenital anomalies.

Imperforate anus can be divided into low lesions, where the rectum has descended through the sphincter complex, and high lesions, where it has not. Most patients with imperforate anus have a fistula. There is a spectrum of malformation in boys and girls. In boys, low lesions usually manifest with meconium staining somewhere on the perineum along the median raphe (Fig. 336-2A). Low lesions in girls also manifest as a spectrum from an anus that is only slightly anterior on the perineal body to a fourchette fistula that opens on the moist mucosa of the introitus distal to the hymen (Fig. 336-3A). A high imperforate anus in a boy has no apparent cutaneous opening or fistula, but it usually has a fistula to the urinary tract, either the urethra or the bladder (see Fig. 336-2B). Although there is occasionally a rectovaginal fistula, in girls, high lesions are usually cloacal anomalies in which the rectum, vagina, and urethra all empty into a common channel or cloacal stem of varying length (see Fig. 336-3B). The interesting category of boys with imperforate anus and no fistula occurs mainly in children with trisomy 21.

Figure 336-2 Imperforate anus in males. A, Low lesions. B, High lesions.

(From Peña A: Atlas of surgical management of anorectal malformations, New York, 1989, Springer-Verlag, pp 7, 26.)

Figure 336-3 Imperforate anus in females. A, Vestibular fistula. B, Cloaca.

(From Peña A: Atlas of surgical management of anorectal malformations, New York, 1989, Springer-Verlag, pp 50, 60.)

Associated Anomalies

There are many anomalies associated with anorectal malformations (Table 336-1). The most common are anomalies of the kidneys and urinary tract in conjunction with abnormalities of the sacrum. This complex is often referred to as the caudal regression syndrome. Boys with a rectovesical fistula and patients with a persistent cloaca have a 90% risk of urologic defects. Other common associated anomalies are cardiac anomalies and esophageal atresia with or without tracheoesophageal fistula. These can cluster in any combination in a patient. When combined, they are often accompanied by abnormalities of the radial aspect of the upper extremity and are termed the VATERR (vertebral, anal, tracheal, esophageal, radial, renal) or VACTERL (vertebral, anal, cardiac, tracheal, esophageal, renal, limb) anomalad.

A good correlation exists between the degree of sacral development and future function. Patients with an absent sacrum usually have permanent fecal and urinary incontinence. Spinal abnormalities and different degrees of dysraphism are often associated with these defects. Tethered cord occurs in ~25% of patients with anorectal malformations. Untethering of the cord can lead to improved urinary and rectal continence in some patients, although it seldom reverses established neurologic defects. The diagnosis of spinal defects can be established in the 1st 3 mo of life by spinal ultrasound. In older patients, MRI is needed.

Manifestations and Diagnosis

Low Lesions

Examination of a newborn includes the inspection of the perineum. The absence of an anal orifice in the correct position leads to further evaluation. Mild forms of imperforate anus are often called anal stenosis or anterior ectopic anus. These are probably imperforate anus with a perineal fistula. The normal position of the anus on the perineum is approximately halfway (0.5 ratio) between the coccyx and the scrotum or introitus. Although symptoms, primarily constipation, have been attributed to anterior ectopic anus (ratio <0.34 in girls, <0.46 in boys), many patients have no symptoms.

If no anus or fistula is visible, there may be a low lesion or “covered anus.” In these cases, there are well-formed buttocks and often a thickened raphe or “bucket handle.” After 24 hr, meconium bulging may be seen, creating a blue or black appearance. In these cases, an immediate perineal procedure can often be performed, followed by a dilatation program.

In a boy, the perineal (cutaneous) fistula can track anteriorly along the median raphe across the scrotum and even down the penile shaft. This is usually a thin track, with a normal rectum often just a few millimeters from the skin. Extraintestinal anomalies are seen in <10% of these patients.

In a girl, a low lesion enters the vestibule or fourchette (the moist mucosa outside the hymen but within the introitus). In this case, the rectum has descended through the sphincter complex.

Children with a low lesion can usually be treated initially with perineal manipulation and dilation. Visualizing these low fistulas is so important in the evaluation and treatment that one should avoid passing a nasogastric tube for the 1st 24 hr to allow the abdomen and bowel to distend, pushing meconium down into the distal rectum.

High Lesions

In a boy with a high imperforate anus, the perineum appears flat. There may be air or meconium passed via the penis (urethra) when the fistula is high, entering the bulbar or prostatic urethra, or even the bladder. In rectobulbarurethral fistulas (the most common in boys), the sphincter mechanism is satisfactory, the sacrum may be underdeveloped, and an anal dimple is present. In rectoprostaticurethral fistulas, the sacrum is poorly developed, the scrotum may be bifid, and the anal dimple is near the scrotum. In rectovesicular fistulas, the sphincter mechanism is poorly developed and the sacrum is hypoplastic or absent. In boys with trisomy 21, all the features of a high lesion may be present, but there is no fistula, the sacrum and sphincter mechanisms are usually well developed, and the prognosis is good.

In girls with high imperforate anus, there may be the appearance of a rectovaginal fistula. A true rectovaginal fistula is rare. Most are either the fourchette fistulas described earlier or are forms of a cloacal anomaly.

Persistent Cloaca

In persistent cloaca, the embryologic stage persists in which the rectum, urethra, and vagina communicate in a common orifice, the cloaca. It is important to realize this, because the repair often requires repositioning the urethra and vagina as well as the rectum. Children of both sexes with a high lesion require a colostomy before repair.

Rectal Atresia

Rectal atresia is a rare defect occurring in only 1% of anorectal anomalies. It has the same characteristics in both sexes. The unique feature of this defect is that affected patients have a normal anal canal and a normal anus. The defect is often discovered while rectal temperature is being taken. An obstruction is present about 2 cm above the skin level. These patients need a protective colostomy. The functional prognosis is excellent because they have a normal sphincteric mechanism (and normal sensation), which resides in the anal canal.

Approach to the Patient

Evaluation includes identifying associated anomalies (see Table 336-1). Careful inspection of the perineum is important to determine the presence or absence of a fistula. If the fistula can be seen there, it is a low lesion. The invertogram or upside-down x-ray is of little value, but a prone cross-table lateral plain x-ray at 24 hr of life (to allow time for bowel distention from swallowed air) with a radioopaque marker on the perineum can demonstrate a low lesion by showing the renal gas bubble <1 cm from the perineal skin. A plain x-ray of the entire sacrum, including both iliac wings, is important to identify sacral anomalies and the adequacy of the sacrum. An abdominal-pelvic ultrasound and voiding cystourethrogram (VCUG) must be performed. The clinician should also pass a nasogastric tube to identify esophageal atresia and should obtain an echocardiogram. In boys with a high lesion, the VCUG often identifies the rectourinary fistula. In girls with a high lesion, more invasive evaluation, including vaginogram and endoscopy, is often necessary for careful detailing of the cloacal anomaly.

Good clinical evaluation and a urinalysis provide enough data in 80-90% of male patients to determine the need for a colostomy. Voluntary sphincteric muscles surround the most distal part of the bowel in cases of perineal and rectourethral fistulas, and the intralumenal bowel pressure must be sufficiently high to overcome the tone of those muscles before meconium can be seen in the urine or on the perineum. The presence of meconium in the urine and a flat bottom are considered indications for the creation of a colostomy. Clinical findings consistent with the diagnosis of a perineal fistula represent an indication for an anoplasty without a protective colostomy. Ultrasound is valuable not only for the evaluation of the urinary tract, but it can also be used to investigate spinal anomalies in the newborn and to determine how close to the perineum the rectum has descended.

More than 90% of the time, the diagnosis in girls can be established on perineal inspection. The presence of a single perineal orifice is a cloaca. A palpable pelvic mass (hydrocolpos) reinforces this diagnosis. A vestibular fistula is diagnosed by careful separation of the labia, exposing the vestibule. The rectal orifice is located immediately behind the hymen within the female genitalia and in the vestibule. A perineal fistula is easy to diagnose. The rectal orifice is located somewhere between the female genitalia and the center of the sphincter and is surrounded by skin. Less than 10% of these patients fail to pass meconium through the genitalia or perineum after 24 hr of observation. Those patients can require a prone cross-table lateral film.