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The spectrum of anorectal malformations (ARMs) ranges from anal
stenosis to persistent cloaca. The term imperforate anus applies
to most of these malformations because the anal canal is malformed
and there is no visible normal anal opening onto the perineum. Anorectal anomalies
occur in as many as one in 4000 live births and are slightly more
common in boys. The most common defect is an imperforate anus with
a fistula between the distal anorectum and the urethra in boys,
or the vestibule in girls. The risk of a second child with an anorectal
malformation is approximately 1%.
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The embryologic development of the hindgut is discussed in Chapter 381. By 6 weeks of gestation, the
urorectal septum divides the cloaca into the anterior urogenital
sinus and posterior anorectal canal. Failure of the urorectal septum
to form results in a fistula between the bowel and urinary tract
(male) or the vagina or vestibule (female). The urorectal septum
divides the cloacal membrane into the urogenital and anal membranes.
Failure of the anal membrane to resorb results in an anal membrane
or stenosis. The perineum also contributes to development of the
external anal opening and genitalia by formation of cloacal folds
that extend from the anterior genital tubercle to the anal membrane.Breakdown
of the cloacal folds results in the external anal opening being
anterior to the external sphincter (ie, anteriorly displaced anus
or perineal fistula).
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Clinical Features
and Diagnostic Evaluation
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Anorectal malformations represent a wide spectrum of defects
and should be ultimately described in terms of the realized anatomy,
rather than “low or high” as is often customary
(Table 415-1). The terms high or low, are
used to generalize the location of the distal rectum or anal canal
relative to the perineum. High lesions are those in which the rectum
ends above the levator musculature and may or may not have a fistulous
connection to the vagina, prostatic urethra, bladder neck, or bladder.
Intermediate lesions are characterized by the rectal pouch ending
within the levator complex, with or without a fistula to the vestibule or the bulbar urethra
in boys. In low lesions, the rectal pouch has completely traversed
the levator musculature, and a fistula usually is evident on the
skin within the midline (perineal fistula). Rectal atresia refers
to an unusual lesion in which the lumen of the rectum is either
completely or partially interrupted, with the upper rectum being
dilated and the lower rectum consisting of a small anal canal. Anal
stenosis can be seen in both males and females and is characterized
by a normally positioned anal perineal orifice with a narrowed anal
canal. A persistent cloaca is a female defect in which the rectum,
vagina, and urethra all empty into a single, common channel.
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