Chapter 42 Imperforate Anus
PATHOPHYSIOLOGY
The congenital malformation known as imperforate anus (anorectal agenesis) involves the anus, the rectum, or the junction between the two. There are two classifications of imperforate anus, related to the placement of the distal end of the colon (the rectum). In high lesions, the rectum ends above the puborectalis sling, the main muscle complex responsible for sphincter control and fecal continence. In low lesions, the rectum has traversed the puborectalis sling, with an abnormal location in the perineum. Affected infants can be expected to have rectal continence after repair.
Along with the imperforate anus, the following may also occur:
1. In girls, a fistula may be present between the rectum and the vagina.
2. In boys, a fistula may be present between the rectum and the urinary tract at the scrotum.
The appearance of the defect varies, depending on its severity. A less-involved imperforate anus appears as a deep anal dimple and exhibits strong muscular reaction to pinprick, which indicates innervation of that area. More severe involvement is initially seen as a flat perineum with no dimple and poor muscular response to pinprick, a result of defective perineal innervation and muscle formation. A highly involved defect includes other anomalies as well (Table 42-1). The infant may initially be seen with poorly developed labia, undescended testicles, or ambiguous genitalia. Outcomes are favorable after definitive surgery is performed.
| Type | Incidence (%)* |
|---|---|
| Esophageal atresia | 10–13 |
| Intestinal atresia | 4 |
| Intestinal malrotation | 4 |
| Cardiovascular defects | 12–22 |
| Skeletal deformities (spina bifida, agenesis of sacrum) | 6 |
| Genitourinary anomalies (renal agenesis, hypospadias, epispadias) | 20–54 |
INCIDENCE
1. Congenital anomalies of the anus and rectum are relatively common. Minor anomalies occur in approximately 1 per 500 live births; major anomalies occur in 1 per 5000 live births.
2. Most studies report a male preponderance of 55% to 65%. High anomalies occur more often in males, whereas low lesions occur more often in females.
3. Of affected infants, 20% to 75% have an associated anomaly, with genitourinary (GU) tract malformations found most frequently (20% to 54%), tracheoesophageal fistula occurring in 10% of infants, and cardiovascular malformations in 12% to 22%.
4. The associated anomalies are usually the cause of death.
5. There are no reported racial differences in disease incidence or severity of disease.
CLINICAL MANIFESTATIONS
1. Physical examination showing absent external anal opening, flat perineum with a short sacrum
2. Failure to pass meconium within first 24 hours after birth
3. Caregiver inability to locate anus to take infant’s rectal temperature
4. Passage of meconium through fistula or misplaced anus
5. Gradual distention and signs of bowel obstruction if no fistula present
COMPLICATIONS
1. Renal and GU: hyperchloremic acidosis, continuing urinary tract infection, urethral damage (result of surgical procedure), problems or delays associated with toilet training
2. Gastrointestinal: constipation, eversion of anal mucosa, stenosis (result of contraction of scar from anastomosis), impactions and constipation (result of sigmoid dilation), problems or delays associated with toilet training, incontinence (result of anal stenosis or impaction), prolapse of anorectal mucosa (results in persistent seepage and incontinence), recurrent fistulas (result of tension in surgical site and infection)
LABORATORY AND DIAGNOSTIC TESTS
1. Visual and digital rectal examination is generally diagnostic.
2. If a fistula is present, urine may be examined for meconium epithelial cells.
3. Radiographic study after infant kept in knee-chest position for approximately 30 minutes. May demonstrate air collected in the blind-ending rectum at or near the perineum; may be misleading if the rectum is filled with meconium, which prevents air from reaching the end of the rectal pouch.
