Pediatric Imperforate Anus (Anorectal Malformation) Workup

Laboratory tests

Complete blood cell (CBC) count, blood typing and screening, and serum electrolyte levels should be measured in all children with imperforate anus who require operation.

Urinalysis should be performed to determine the presence of a rectourinary fistula in all cases in which the diagnosis cannot be made based solely on the physical examination findings. If a child has a perineal fistula, vestibular fistula, or a single perineal orifice, urinalysis is unnecessary. Urinalysis is required in all other affected children.

Other studies

Although anorectal manometry has no role in the newborn period, it may be used in older children who suffer from varying degrees of incontinence. However, the findings are primarily of academic interest.

Imaging studies used in this condition include the following.

Radiography

Sacral radiography

Two views of the sacrum, posteroanterior and lateral, should be obtained to measure sacral ratios and to look for sacral defects, hemivertebrae, and presacral masses. This should be performed before surgery.

Lateral pelvic radiography at 24 hours

Children who could not be diagnosed based solely on physical examination findings, traditionally underwent invertography, which consisted of holding the baby upside down and using lateral radiography to observe the level of gas in the distal rectum. A similar, but more humane, approach is to wait 24 hours after birth to observe for possible maximal pelvic pouch distension and then to use cross-table lateral pelvic radiography with a radio-opaque marker on the anal dimple with the child in the prone position and the hips slightly raised. If the pouch is observed within 1 cm of the marker, some surgeons offer primary repair without colostomy. For pouches farther than 1 cm, colostomy is performed. This 1-cm guideline has been validated only using radiographic measurements and is not directly translatable for measurements made using ultrasonography. Currently, perineal ultrasonography has no role in diagnosis.

Ultrasonography

Prenatal ultrasonography may not always identify an imperforate anus; however, it can be used to identify the type of imperforate anus. ^[9] The diagnostic performance of ultrasonography using the pouch-perineum (P-P) distance (suprapubic, infracoccygeal, or perineal approach) to evaluate the type of imperforate anus and any associated anomalies is superior on the day after birth than that on the day of birth, ^[10]  particularly for low-type imperforate anus. ^[11]  In addition, the diagnostic accuracy of ultrasonography using the P-P distance alone for low-type imperforate anus appears to be comparable to that obtained by prone cross-table radiographic assessment (60% vs 45%, respectively). ^[12] However, when using P-P distance and fistula location, there is a significant improvement in the diagnostic accuracy of ultrasonography (90%) over that of prone cross-table radiography (45%). ^[12]

There does not appear to be a significant correlation between the distance between the distal rectal pouch and the perineum in low birth weight infants with imperforate anus. ^[13]

Abdominal ultrasonography is specifically used to examine the genitourinary tract and to look for any other masses. Hydronephrosis, hydrocolpos, presacral mass, abdominal mass, or any similar finding can profoundly affect management. This study should be performed before surgery and must be repeated after 72 hours because early ultrasonography findings may be insufficient to rule out hydronephrosis due to vesicoureteral reflux.

Spinal ultrasonography or magnetic resonance imaging (MRI)

All children with any form of anorectal malformation, even those considered minor, should undergo screening for spinal malformations. These lesions can be diagnosed using ultrasonography prior to the ossification of the spine. Ultrasonography should be performed as early as possible but is not essential prior to a newborn surgical procedure. Due to ossification, the use of ultrasound diminishes after 3-4 months of age, and by 6 months MRI is usually necessary. MRI may be required any time an ultrasound is suggestive or nondiagnostic.

Augmented-pressure distal colostography

This is the single most important diagnostic test used to clarify the anatomy in all children with malformations who require colostomy. It is personally performed by the colorectal surgeon in many centers. In a fluoroscopy suite, a balloon catheter is placed into the distal stoma, and the balloon is inflated. The catheter is pulled back, and water-soluble contrast is injected by hand. This pressure is required to overcome the pressure of the levator muscles and to allow the contrast to flow into the lowest part of the colon and reveal any fistula. In patients with a fistula to the urinary tract, the bladder often fills, and the study is continued to obtain as much information as is provided with voiding cystourethrography. If no fistula is present, the distal pouch has a rounded appearance, and no urinary extravasation is visible.

Voiding cystourethrography or micturating cystourethrography

These studies are not necessary if comprehensive distal colostography is performed. They are not recommended for primary evaluation of children with anorectal malformations because of poor sensitivity compared with that of distal colostography.

MRI

All children who have sacral defects on plain radiographs should undergo spine ultrasonography to rule out associated malformations, such as meningocele or meningomyelocele, teratoma, or mixed lesions. If ultrasonography findings are suggestive or nondiagnostic, MRI should be performed. All children who have suggestion of tethered cord on ultrasound, a nondiagnostic ultrasound, or have not had an ultrasound and are older than 6 months should undergo MRI.

Computed tomography (CT) scanning

CT scanning presently plays no role in the routine evaluation of children with anorectal malformations.

Imperforate anus was historically classified based on the position of the distal-most aspect of the colon in relation to the levator ani muscles. Malformations at or above the levator muscle complex were defined as high anomalies. Infralevator lesions were termed low and were considered simpler and were associated with better prognosis. This system was based on the now obsolete Wingspread classification.

Information obtained from the posterior sagittal approach has led to an anatomic classification that lists malformations based on their specific anatomy. The following is a list of the most common malformations:

Perineal fistula

This malformation is associated with good prognosis, occurs in either sex, and involves a closed anus with a small connection opening on the perineal body.

Some babies with this malformation have a small loop of skin at the anal opening that resembles a bucket-handle. This is pathognomonic for perineal fistula.

Some boys may have no visible perineal opening but may accumulate mucous or meconium in the fistula, which can extend up the median raphe of the scrotum and resembles a black cord (meconium) or a string of pearls (mucous).

This malformation is amenable to primary neonatal pull-through.

Healthy girls who have normal-sized anal openings and small-appearing perineal bodies do not have perineal fistula. This is easily determined by measuring the size of the anus using Hegar dilators. The anus of an average-term newborn should be approximately 12 mm but varies with the size of the child. The "correct" size of the perineum is largely a matter of experience.

Vestibular fistula

This malformation is associated with good prognosis and is easily diagnosed upon physical examination based on the appearance of a small opening at the posterior aspect of the vestibule. The opening is external to the hymen and is, therefore, not vaginal.

The term vaginal fistula was commonly and incorrectly used to describe vestibular fistula. True solitary congenital rectovaginal fistula is exceedingly rare.

Vestibular fistula is safely treated with diverting colostomy, although some pediatric colorectal surgeons repair this malformation primarily in the newborn period without using colostomy.

Cloaca

Persistent cloaca is a malformation in females that encompasses a spectrum of defects, including the presence of a common channel that incorporates the urethra, vagina, and rectum.

The length of the common channel correlates with complexity and prognosis. Shorter channels (< 3 cm) have fewer associated malformations and carry a better prognosis. Longer channels have more complex malformations and poorer prognosis.

One half of all girls with this malformation have 2 hemivaginas and many have hydrocolpos.

This malformation is easily diagnosed upon physical examination based on the presence of a solitary perineal orifice. Females with this malformation often have very small-appearing labia.

All children with cloacae should undergo colostomy shortly after birth.

Bulbar urethral fistula

This malformation observed in boys is relatively common.

No fistula is observed upon physical examination, and urinalysis often shows meconium.

Colostomy is essential to relieve obstruction, prevent urinary soiling, and to allow for distal colostography, which clarifies the malformation for definitive surgical repair.

Prostatic urethral fistula

This malformation observed in boys is rarer than bulbar fistula and carries a poorer prognosis.

The diagnosis and treatment algorithm are identical to those of bulbar fistula, although the surgical procedures used differ.

Bladder-neck fistula

This rare malformation observed in males (10% of all malformations in males) carries a very poor prognosis.

Most patients with bladder-neck fistula require bowel-management regimens.

The diagnosis and treatment algorithm are identical to those of bulbar and prostatic fistulas, although the surgical procedures used differ.

This fistula is best approached abdominally.

Absent fistula

This malformation can occur in either sex, is somewhat rare, and is associated with a good prognosis. It is commonly associated with trisomy 21.

Diagnosis is primarily by exclusion.

Lateral pelvic radiography is performed in babies who have no external evidence of fistula, who pass no meconium after 24 hours, and who have no meconium in the urine.

If the pelvic rectal pouch is within 1 cm of the anal dimple, a primary pull-through may be performed. In these instances, a fistula is unlikely but should be definitely excluded using the proper surgical technique.

If the surgeon opts for colostomy, the absence of a fistula is confirmed using distal colostography.

Cloacal exstrophy

This extremely rare malformation can occur in either sex but is most common in boys. It encompasses a spectrum that includes variant forms of covered exstrophy. The classic form is devastatingly complex.

Affected children have an omphalocele and a large extrophied cloacal plate on their lower abdominal wall. They have 2 hemibladders separated by an intestinal plate, often with prolapsed terminal ileum that proceeds distally to include an extrophied urethral plate flanked by 2 hemiphallic or hemiclitoral structures.

All children with cloacal exstrophy have some degree of pubic symphysis diastasis and may have a spinal malformation, most commonly myelocystocele.

Thankfully, this complex malformation is rare but it has devastating implications on quality of life.