Anorectal Malformations
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Direct answer
Anorectal malformations are congenital failures of the terminal bowel to reach the perineum, opening instead through a fistula to the urinary tract in males or the genital tract and perineum in females. Classification turns on the levator complex and pubococcygeal line: high lesions (rectoprostatic fistula in males, cloaca in females) need colostomy followed by pull-through, while low lesions (rectoperineal fistula) can often be repaired by anoplasty. The first 24 hours decide the route — inspection for meconium at the perineum, in the urine or from the vestibule, with a prone cross-table lateral film at 24 hours localising gas relative to the pubococcygeal line. Definitive repair is the posterior sagittal anorectoplasty, and every baby is screened for associated VACTERL defects.
What you must remember
- Classification logic: high (supralevator) lesions communicate with the bladder neck or prostatic urethra in males, or cloaca in females; intermediate lesions open at the bulbar urethra or low vagina; low lesions open on the perineum — the modern Krinkenbeck classification names the fistula itself.
- Male fistulas: rectoprostatic urethral (high), rectobulbar urethral (intermediate), rectoperineal (low), rarely rectovesical.
- Female fistulas: rectovestibular fistula is the commonest female malformation, with rectoperineal (low), rectovaginal, and cloaca — a single common channel for urethra and vagina — as the high end.
- Bedside clues: a flat perineum with no anal dimple and no midline raphe pigmentation suggests a high lesion; meconium in the urine or ballooning of the perineum confirms a urinary fistula; the female perineum is inspected for three versus two orifices.
- Imaging: prone cross-table lateral shoot-through radiograph (the invertogram) at 24 hours — rectal gas above the pubococcygeal line means high, below it low; perineal ultrasound and MRI refine, and echo and renal ultrasound screen for associated defects.
- Management pathway: low lesions — primary anoplasty or limited PSARP in the newborn; high and intermediate lesions — divided sigmoid colostomy in the first 48 hours, PSARP at around 3-6 months, colostomy closure after healing.
- Cloaca specifics: length of the common channel (over about 3 cm) predicts the need for laparotomy or laparoscopy and urological reconstruction; tethered cord and uterine anomalies coexist.
Sorting high from low in the first 48 hours
Examine the baby warm and undisturbed under a light. In a male, look for an anal dimple that puckers on perineal stimulation — its presence with a visible meconium-stained track means a low lesion suitable for primary repair. A flat, featureless perineum with meconium tinting the urine (dipstick or the "oil-drop" sign in the diaper) declares a rectourethral fistula: diverting colostomy first. When the perineum is ambiguous, wait the full 24 hours of life, then obtain the prone cross-table lateral film with the baby head-down and the hips flexed: the column of rectal gas is measured against the pubococcygeal line and the ischial line — above the PC line is high, between PC and ischial intermediate, below low. In females, count the orifices: three separate openings with meconium from the vestibule means a rectovestibular fistula (these can often be repaired primarily in expert hands); one common opening means cloaca, and the channel length determines whether reconstruction is perineal or abdominal. In parallel, echo-cardiography, renal ultrasound and a vertebral survey complete the VACTERL screen before any anaesthetic.
Exam favourites from this topic
Meconium per urethram equals rectourethral fistula; a single perineal orifice in a female equals cloaca until defined; a "flat bottom" without an anal dimple equals a high lesion. The invertogram is examined as technique as much as reading — 24 hours of life, prone, head down, hips flexed, cross-table beam — because a film taken too early underestimates the gas level. The colostomy question tests judgement: a divided (not loop) sigmoid colostomy with separated stomas keeps urine and faeces apart and prevents prolapse of the distal limb through a communal opening. PSARP, credited to Pena, maps the sphincter complex by electrical stimulation and delivers the bowel within it through a midline incision onto the closed fistula. Long-term, constipation dominates low lesions and soiling high ones, and the bowel management programme, not repeat surgery, is what tames both.
Frequently asked questions
How is a high anorectal malformation distinguished from a low one?
By perineal inspection for the anal dimple and meconium location, and by the prone cross-table lateral radiograph at 24 hours — rectal gas above the pubococcygeal line indicates a high lesion.
What is the initial management of a high anorectal malformation?
A diverting divided sigmoid colostomy in the first days of life, followed by posterior sagittal anorectoplasty at around three to six months of age.
What does meconium in the urine signify?
A rectourethral fistula — a high malformation in a male, requiring colostomy and later pull-through.
Which is the commonest anorectal malformation in females?
Rectovestibular fistula, in which the bowel opens just inside the vestibule behind the vagina.
What is the VACTERL association?
The non-random clustering of vertebral, anorectal, cardiac, tracheo-oesophageal, renal and limb anomalies — mandating systematic screening in every affected newborn.