Excerpt from Imperforate Anus

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Background

Anorectal malformations include a wide spectrum of defects in the development of the lowest portion of the intestinal and urogenital tracts. Many children with these malformations are said to have an imperforate anus because they have no opening where the anus should be. Although the term may accurately describe a child's outward appearance, it often belies the true complexity of the malformation beneath. When a malformation of the anus is present, the muscles and nerves associated with the anus often have a similar degree of malformation. The spine and urogenital tract may also be involved.

The affected organs are located deep in the pelvis and are not well visualized through abdominal incisions. Traditional surgical dictum did not allow for division of the posterior midline because this division of the muscle was believed to cause incontinence in the child. Therefore, surgeons approached these malformations using a combined abdominal, sacral, and perineal approach, with limited visibility. Such approaches have put continence at greater risk than simply cutting sphincter muscles to adequately visualize the malformation. This principle was central to the development of the surgical techniques currently used to repair these malformations.

In 1982, Peña et al reported the results of the use of a posterior sagittal surgical repair approach. Peña et al used the traditional approach with a sacral incision and made the incisions progressively larger in an attempt to adequately visualize the anatomy. Eventually, the entire posterior sagittal plane was opened, affording a full view of the complete malformation. This technique, referred to as posterior sagittal anorectoplasty (PSARP) or posterior sagittal anorectovaginourethroplasty (PSARVUP), has led to a more complete understanding of the anatomy of these children and of what is required to repair the malformations with optimal results.

After the procedure, many children still experience effects of their malformation in the form of urinary or fecal incontinence. Despite optimal surgical management, no adequate repair for poorly developed muscles or nerves has been developed. Bowel-management regimens can provide an excellent quality of life for these children when primary continence is not achievable.

Pathophysiology

The embryogenesis of these malformations remains unclear. The rectum and anus are believed to develop from the dorsal potion of the hindgut or cloacal cavity when lateral ingrowth of the mesenchyme forms the urorectal septum in the midline. This septum separates the rectum and anal canal dorsally from the bladder and urethra. The cloacal duct is a small communication between the 2 portions of the hindgut. Downgrowth of the urorectal septum is believed to close this duct by 7 weeks' gestation. During this time, the ventral urogenital portion acquires an external opening; the dorsal anal membrane opens later. The anus develops by a fusion of the anal tubercles and an external invagination, known as the proctodeum, which deepens toward the rectum but is separated from it by the anal membrane. This separating membrane should disintegrate at 8 weeks' gestation.

Interference with anorectal structure development at varying stages leads to various anomalies, ranging from anal stenosis, incomplete rupture of the anal membrane, or anal agenesis to complete failure of the upper portion of the cloaca to descend and failure of the proctodeum to invaginate. Continued communication between the urogenital tract and rectal portions of the cloacal plate causes rectourethral fistulas or rectovestibular fistulas.

The external anal sphincter, derived from exterior mesoderm, is usually present but has varying degrees of formation, ranging from robust muscle (perineal or vestibular fistula) to virtually no muscle (complex long–common-channel cloaca, prostatic or bladder-neck fistula).

Frequency

United States

Anorectal malformations occur in approximately 1 newborn per 5000 live births.

Mortality/Morbidity

Anorectal and urogenital malformations are rarely fatal, although some associated anomalies (cardiac, renal) can be life threatening. Intestinal perforation or postoperative septic complications in a newborn with imperforate anus can result in mortality or severe morbidity.

Morbidity generally arises from the following 2 sources:

• Malformation-related morbidity
  
  
  
  • Malformation-related morbidity relates to associated malformations of rectal motility, anorectal innervation, and sphincteric musculature. The most common morbidity in this category is constipation. Most children have mild malformations that commonly result in constipation for reasons that remain unclear. If left untreated, chronic constipation results in rectal dilation, which worsens the constipation. This becomes a vicious cycle, which, if untreated, results in fecal impaction and overflow pseudoincontinence or encopresis.
  
  
  • The most severe forms of malformation-associated morbidity are fecal and urinary incontinence. Higher malformations, such as long–common-channel cloacae and prostatic or bladder-neck fistulas, are associated with more frequent spinal malformations and poorer nerve and muscle formation, all of which increase the likelihood of fecal or urinary incontinence. Malformations that directly involve urinary sphincteric mechanisms, and, specifically, any malformation in which the rectum or vagina joins the urinary tract at the bladder neck, often results in either urinary incontinence or inability to completely void.


• Surgery-related morbidity
  
  
  • This can include standard complications such as line infections and pneumonia.
  
  
  • Wound infections or anastomotic breakdowns can occur in any intestinal surgery.
  
  
  • Children with imperforate anus are at greater risk for injury to surrounding pelvic organs because these organs may also be involved in the malformation in some unsuspected way.
  
  
  • During blind exploration in the pelvis, a dilated ureter can be mistaken for the rectum. Urethras can be opened or transected, and prostates or seminal vesicals can be easily injured. Dissection of these delicate structures can result in ischemia and possible stricture or complete stenosis.

Race

No known racial predilection has been reported.

Sex

No known sex predilection has been reported.

Age

Most children with an anorectal malformation are identified upon routine newborn physical examination. Delayed presentation is often the result of incomplete initial examination. Newborn anorectal and urogenital examination can be technically challenging and makes many practitioners uncomfortable.

• Subtle malformations, such as those in some children with perineal fistula that may look normal to the casual glance, may present months or years after birth when the child presents to a primary care provider for constipation or urinary tract infection and appears to have a small perineal body upon physical examination.
• Anorectal malformations in females with a normal-appearing anus who have absent vagina or persistent urogenital sinus may go undiagnosed for years because of examiner reluctance to separate the labia during physical examination. These malformations can be discovered upon evaluation for urinary tract infection or primary amenorrhea.

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