EMBRYOLOGY OF THE ANUS AND RECTUM
A series of developmental steps are involved in the formation of the normal anatomy of the lower end of the anus, rectum, and genitourinary (GU) tract. By the fourth week of development, the cloaca and a structure called the cloacal membrane are present. The cloaca is a normal structure in birds and is present for a short period of time during the development of the human before birth. A cloaca is a structure into which the colon, urinary tract and genitalia all drain and exit the body with a single opening. A human goes thorough a stage of development where a cloaca normally exists and then goes on to develop separate openings for the rectum, urinary tract and in girls, the vagina. This normal development is in much part due to the development of a structure called the cloacal membrane. If the membrane does not develop normally the cloaca may persist at birth in girls or boys will develop some variant of imperforate anus.
NORMAL ANORECTAL ANATOMY AND FUNCTION
Continence is related to normal function of the muscle sphincters surrounding the anus and rectum and the degree to which they are present and have appropriate nerve stimulation. Development of the sacrum occurs at around the same time as development of the anus, rectum, and sphincters. This is an important consideration because the nerves that are located near the sacrum supply the muscle sphincters that control continence. If the sacrum does not develop normally, these nerves may not develop or function normally either. During normal development there are sensory receptors in the lining of the anal canal that are important for continence. These may be absent in children with imperforate anus. Humans normally have 3 muscle groups around the anus and rectum that are important for continence, the external sphincter, internal sphincter, and levator complex. Children born with imperforate anus may have dysfunction or absence of any one of these components. The internal and external sphincters control our ability to squeeze the anus shut. The various portions of the levator ani muscle are shaped like a cone surrounding the anus and rectum. When these muscles squeeze they pull the rectum forward, increasing the angle of the bowel before it enters the anal canal. A proper rectoanal angle helps to maintain continence by preventing formed stool from entering the anal canal. The levator muscles are also supplied by nerves that are close to the sacrum. This is important because as a general rule, if a part of the sacrum is missing, the nerve that was associated with it is likely absent also.
Innervation of the Anus, Rectum, and their Sphincters
Nerve segments coming from portions of the sacrum supply the anus and rectum, urethra, bladder, and vagina, including the various components of the levator ani complex (the muscles of the pelvis). These nerves also serve as the skin sensory receptors of the anus and surrounding skin areas. The lining to the anal canal and skin around the anus are extremely sensitive to pain, touch, cold, pressure, tension, and friction. Evidence suggests that similar sensory receptors are located in the surrounding pelvic musculature. These receptors are able to distinguish between rectal contents that are solid, liquid, and gas. The anal canal and the rectum above the anal lining are mostly insensitive to pain but are sensitive to distention. Fecal continence is possible only when appropriate sensation from the anal lining, the rectal wall, and the surrounding muscles are received and processed normally within the brain and appropriate signals are then sent back out to the various muscles that control continence. Under normal circumstances, the anal canal is closed except when having a bowel movement. When defecation is to occur, abdominal pressure increases and causes the pelvic floor (levator complex) to drop down as the various muscles of continence relax. The feces are evacuated followed by return of the levator and continence muscles to baseline tone.
In very simplified terms, during development the rectum descends into the pelvis and is supposed to end at the anal opening. In both sexes, anorectal deformities are divided into high, intermediate, and low anomalies. These classifications depend on how far into the pelvis the rectum descended, its relation to the surrounding muscles and whether there is a fistula (abnormal connection) to the urinary tract in males or the vagina in females. The incidence of anorectal malformations is approximately 1 in 5000 live births. They occur slightly more commonly in boys, and boys are twice as likely as girls to have high or intermediate anomalies. In boys with an intermediate or high anomaly, 85% have a rectourinary fistula (abnormal connection between the rectum and urethra) In terms of low-lying anomalies, 35% of boys have a thin membrane covering the anal fistula, whereas 93% of girls have an external fistula. Although imperforate anus may occur as an isolated malformation, it may coexist with duodenal atresia, esophageal atresia, vertebral and renal anomalies, Down syndrome, and congenital heart disease in 50% to 60% of patients. The VATER or VACTERL syndrome (vertebral, anal, cardiac, tracheoesophageal, renal, and radial limb anomalies) occurs in approximately 15% of patients. A tethered cord and other types of spinal cord abnormalities are observed in half of patients with imperforate anus. Approximately 60% of patients with high or intermediate forms of imperforate anus have some form of associated genitourinary (GU) malformation or vesicoureteral reflux, a condition where urine goes back upstream from the bladder to the kidneys. The incidence of GU malformation with low anomalies is only 15% to 20%; however, associated anomalies of the GU tract are particularly important to recognize early if problems with the kidneys are to be avoided.
Initial Management of the Newborn with Imperforate Anus
The first decision in the assessment of the newborn with an anorectal anomaly is whether a colostomy is required. A colostomy is when the surgeon sutures a portion of the intestine to the abdominal wall and the child passes stool into a plastic bag for a period of time. This is usually a temporary colostomy. Although it is always better to err on the side of performing a colostomy, there are many anomalies in which one is unnecessary. Examination of the perineum (the area of the body around where the anus is supposed to be) is important because it may provide evidence of whether the imperforate anus is a high, intermediate or low lesion. If possible it is best to wait 24 hours prior to any surgery to allow progression of gas or meconium through the bowel down as far as it can pass before finalizing the assessment.
Imperforate Anus in Boys
The distinction between a low or intermediate/ high anomaly can often be assessed by clinical examination, the presence of gas in the bladder or meconium in the urine, and sometimes the use of X-rays. If an external opening is visible in the perineum, one can predict the presence of a low anomaly or it may be possible to see meconium just below the skin of the perineum. A skin formation called a “bucket handle,” or an apparently well-formed anus and gluteal crease suggest the presence of a low anomaly. Sometimes after 24 hours of life, a speck of meconium may be seen indicating the location of a tiny fistulous orifice. A fistula is an abnormal connection between two parts of the body, in this case from the rectum to an abnormal opening in the skin. In patients in whom the anal opening appears to be normal, but there is intestinal obstruction or passage of small-caliber stools, gentle probing of the anus may show an anal membrane, anorectal stenosis (narrowing), or rectal atresia ( a complete blockage).
If a boy has an associated cleft scrotum (a scrotum that seems to be in two halves) or significant hypospadias (the opening of the urethra in the penis is not in a normal location), one should suspect a high or intermediate malformation. If there is complete absence of the gluteal fold (the crease between the buttocks), if no anal wink can be found and if there are abnormalities to the sacrum a high anomaly is usually present.
In addition to the preceding studies, the patient should be examined for components of the VACTERL syndrome. Ultrasound evaluation of the kidneys and ureters should be performed to diagnose a potential obstruction to the flow of urine from the kidneys. In boys, this is treated best at the time of surgery for imperforate anus. A voiding cystourethrogram (VCUG) should be performed before initial discharge home to rule out vesicoureteral reflux (flow of urine backward from the bladder to the kidney) so that appropriate therapy can be started. Ultrasound or MRI evaluation for a spinal cord problem (called tethered cord) also should be performed at some point but is not necessary right away.
Patients with an exam suggestive of abnormal nerves in the pelvis, such as myelomeningocele and sacral malformations should be evaluated for neurogenic bladder. These patients do not have normal nerve supply to the bladder and the bladder may be large and empty poorly. Determining whether a patient has a neurogenic bladder is important because treatment may be necessary to avoid problems with the kidneys and infection in the urine.
Imperforate Anus in Girls
The internal anatomy of girls with imperforate anus usually can be predicted from a careful study of the visible openings in the perineum, the area around her bottom. Three openings typically are seen. Usually the urethral (where urine comes out) and vaginal openings are normal, and the third opening represents a fistula (an abnormal opening) either at the level of the normal anus or more commonly as an opening at an abnormal site in front of where the anus should be. This third opening may not be obvious and can be difficult to see.
If only two openings are seen in the perineum, including the urethra and vagina, and meconium (baby stool) is seen coming from the vagina, an anomaly called a rectovaginal fistula or cloacal variant is present. It is not possible to determine under these circumstances whether the abnormal connection between the rectum and vagina, called a fistula, is high or low in the vagina. At times, it is possible to pass a catheter into the fistula and to do an x-ray study or to perform endoscopy to obtain further information. An endoscopy uses a small telescope to see inside the rectum, vagina or urethra and bladder. In either case, an intermediate/high anomaly can be presumed. These babies need a colostomy (sewing part of the bowel to the abdominal wall and allowing the child to have bowel movements into a plastic bag) so that 1. they are able to pass stool at all and 2. so that surgical repair can be safely done. The colostomy is temporary.
If neither a fistula nor meconium is noted on or just beneath the perineum, ultrasound or an x-ray may be done to see if the rectum is near the skin where the anus was supposed to be. If it appears to be less than 1 cm away from the skin it may be possible to avoid a colostomy and do surgery to correct the problem, called an anoplasty. In most cases, however, an intermediate or high anomaly should be presumed and the rectum will be farther away from the skin. In these babies a colostomy should be performed.
If only one opening is seen, it must be assumed that there is a problem called a cloaca. A cloaca is a common opening for the urethra, vagina, and rectum. Usually urine and meconium exit from a common opening. These are generally complicated anomalies that require colostomy and, at times, other procedures. Many studies may be needed to help figure out the anatomy. All girls with imperforate anus should be screened for vesicoureteral reflux and other severe associated malformations.
Initial Treatment in the Neonate
Male and female newborns who have low malformations where the rectum almost came out in the proper place in the form of an anocutaneous fistula, an anovestibular fistula, or anal stenosis usually can be treated initially with dilation. Your doctor can describe these terms more fully as needed. In the past, anocutaneous fistulas regularly were dilated and repair performed several months later, but in more recent years, most pediatric centers have been performing surgical correction shortly after birth unless there is some reason not to. Male and female infants suspected of having intermediate or high-lying deformities and infants whose internal anatomy cannot be determined with certainty should have a colostomy performed. At the time the colostomy is performed, all meconium should be washed out to avoid contamination of the urinary tract. Surgical correction can be performed when the infant is believed to be thriving and of appropriate size. Timing is based on medical judgment rather than on an arbitrary interval, but typically is performed at any point beyond 8 to 10 weeks of age. Most boys require a colostomy, but most newborn girls with imperforate anus are able to have an anoplasty performed because most have low-lying lesions.
Repair of Low-Lying Lesions
In the setting of a thin layer of skin covering the normal rectal opening, some surgeons prefer to puncture the membrane followed by enlargement of the tract with a dilator, whereas others do a formal operation called an anoplasty. In the case of other low-lying anomalies, an operation is required. The goal of the operation is to move the anal opening back into its normal position and to create space between the anal opening and the vagina. One of two operations may be used for this. One is called a cutback procedure and the other is an anal transposition. Another method is called a Y-V anoplasty.
In many girls and rarely in boys, the front part of the anus would lie too far outside the muscle spincter that allows continence using the Y-V anoplasty technique. Anal transposition is recommended under these circumstances. Typically an anorectal transposition is performed at 3 to 4 weeks of age.
Approximately 3 weeks after anoplasty, anal dilations should be started and performed once daily by the parents for several weeks to prevent narrowing of the new anal opening. Dilations are weaned slowly over the next few months as long as stricture formation (narrowing of the opening) is not observed.
Repair of Intermediate and High Anomalies
Most patients with intermediate and high forms of imperforate anus are recognized shortly after birth. As previously mentioned, all of these infants should have a sigmoid colostomy performed. At the point that infants are believed to be of sufficient size and condition, a distal colostogram (x-ray study of the intestine) should be performed. This will show the exact site of any abnormal connection between the bowel and urinary tract or vagina.
Although numerous surgical approaches have been proposed for the repair of intermediate and high-lying anomalies in boys and girls, currently the posterior sagittal anorectoplasty (PSARP) procedure described by de Vries and Pena is the preferred technique (Fig. 61-18). This approach is also useful for repair of complicated cloacal malformations and for remedial procedures for previously operated patients who are incontinent. At times, an abdominal approach is required in addition to the posterior sagittal approach. A PSARP includes an incision from near to tail bone to near the scrotum or vagina. In the depths of this incision the rectum can be found and its abnormal connection to the urinary tract or vagina if one exists. This connection is repaired and the rectum and the muscles around it are replaced in as normal a position as possible. A new anus is created by sewing the rectum to the skin. In most patients, the entire procedure can be done with this incision from behind, but occasionally an abnormalities higher in the abdomen will require an abdominal incision as well. Two to three weeks later, the new anus is dilated gently with a dilator, and after 6-8 weeks or so, another colostogram may be performed if needed to verify that complete healing has occurred. At that point, the colostomy may be closed. Some surgeons may prefer a laparoscopic approach to some of these repairs
Current mortality rates are low after repair of imperforate anus, and most of these deaths are attributable to problems with other organ systems, particularly the cardiovascular system and central nervous system. Sepsis (overwhelming infection) is occasionally a problem in patients with complicated high anomalies involving the genitor-urinary system. Mortality in patients is in the range of 5% for low anomalies and 15% for high anomalies. After PSARP, the main complications have been prolapse of anal mucosa (the lining of the rectum comes out of the anus), anal stenosis (narrowing of the new anus as it heals), and incontinence. Rare patients have been found to have associated Hirschsprung’s disease (another congenital problem that makes if difficult for babies to have a bowel movement).
Patients who have had repair of imperforate anus must be followed carefully for most of their lives. Greater than 90% of patients who have had a simple anoplasty performed for low-lying lesions are continent, but constipation is a frequent problem, and appropriate management must be provided to avoid the development of future problems such as a very stretched out rectum. This can cause even further problems with constipation because the rectum will not work properly. In patients who have had a PSARP performed, the results in terms of continence are related directly to the level of the anomaly, high, intermediate, low or cloaca. Patients with intermediate-level imperforate anus anomalies have a higher incidence of continence than patients with high-lying anomalies. Similarly, patients missing muscle or lacking normal nerve stimulation to the muscles around the rectum and patients with abnormalities to the sacrum have a poor prognosis for continence. Satisfactory continence (which may include the need for a bowel management program) is reported in approximately 75% of patients with intermediate forms of imperforate anus, 65% in boys and girls with high-lying lesions, and 20% or less in girls with cloacal anomalies.
Article and graphics adapted from O'Neill: Principles of Pediatric Surgery. © 2003, Elsevier.