Anorectal malformations, imperforate anus and Hirschsprung disease

Overview

Both conditions cause low intestinal obstruction in the newborn: abdominal distension, failure to pass meconium normally and, later, bilious vomiting.

• An anorectal malformation (ARM) is a structural problem: no normal anus.
• Hirschsprung disease is a functional problem: an anus is present, but the distal bowel has no nerve cells and cannot relax.

Every newborn examination must include inspection of the perineum and a note of when meconium was first passed.

Anorectal malformations (imperforate anus)

Definition

A spectrum in which the anus is absent (imperforate) or the rectum opens through a fistula that does not pass through the sphincter muscle complex and opens away from the normal anal site (onto the perineum, urethra, bladder, vestibule or vagina).

The incidence is about 1 in 4000–5000 births. Abnormalities of the sacrum and the urinary and genital tracts frequently coexist.

Classification (simplified)

The old high / low division depends on where the rectum ends in relation to the pelvic floor (levator ani / puborectalis).

Associated anomalies

• VACTERL (vertebral, anorectal, cardiac, tracheo-oesophageal, renal, limb).
• Sacral anomalies and tethered spinal cord (affect continence).
• Genitourinary anomalies (very common, especially with high lesions).
• Check: echocardiogram, renal ultrasound, spinal ultrasound/X-ray of the sacrum, pass an NG tube to exclude oesophageal atresia.

Clinical assessment of the newborn

Inspect the perineum carefully and wait. In the first 24 hours the rectum fills with gas and meconium is pushed through any fistula.

Investigations

• Cross-table lateral prone X-ray (prone, hips flexed) after about 24 hours, with a marker on the anal dimple: the distance from the rectal gas to the skin shows whether the lesion is high or low. The older invertogram (baby held upside down, Wangensteen–Rice) works on the same idea but is now rarely used.
• Perineal ultrasound to measure the gap.
• Later: distal loopogram (contrast through the colostomy) to show the fistula before definitive surgery.

Management

1. First 24 hours: nil by mouth, NG tube, IV fluids and antibiotics; look for associated anomalies.
2. Low lesion (perineal fistula): anoplasty in the neonatal period (or simple dilatation of some fistulas).
3. High or uncertain lesion: divided colostomy in the newborn period, then definitive repair after a few months: posterior sagittal anorectoplasty (PSARP, Peña procedure), separating any fistula from the urinary tract, with abdominal (or laparoscopic) mobilisation if very high.
4. After repair: a programme of anal dilatations, then colostomy closure.
5. Cloaca: complex reconstruction of rectum, vagina and urethra.

Outcome

• Bowel control depends on the level of the lesion and the state of the sacrum and spine.
• Low lesions tend to cause constipation; high lesions tend to cause faecal incontinence.
• Children with poor control can achieve social continence with a bowel management programme or antegrade colonic enemas through an appendicostomy (Malone procedure).

Hirschsprung disease

Definition and cause

Congenital absence of ganglion cells in both the myenteric (Auerbach) and submucosal (Meissner) plexuses of the distal bowel, with thickened (hypertrophic) nerve trunks.

• Cause: failure of neural crest cells (vagal origin) to migrate all the way down the gut (craniocaudal migration). So the aganglionic segment always starts at the anus and extends upwards without skip areas.
• The aganglionic bowel is in tonic spasm and cannot relax: a functional obstruction. The normal bowel above it dilates (megacolon).
• Between them is a funnel-shaped transition zone.

Extent

(Figures from Bailey & Love.)

Epidemiology and genetics

• About 1 in 5000 live births; boys about 4:1 (less marked in long-segment disease).
• RET proto-oncogene mutations on chromosome 10 (also endothelin pathway genes).
• Associated with Down syndrome and other syndromes (e.g. Waardenburg); familial cases occur.
• MEN 2 shares the RET link.

Clinical features

Newborn (most cases):

• Delayed passage of meconium (none in the first 24–48 hours).
• Abdominal distension and bilious vomiting (low obstruction).
• On rectal examination: tight empty rectum, then an explosive gush of gas and stool when the finger is withdrawn ("squirt" or "blast" sign), with temporary relief.

Older child:

• Chronic severe constipation from birth, not responding to laxatives.
• Gross distension, visible peristalsis, failure to thrive, ribbon-like stools.
• Empty rectum on examination, unlike functional constipation where the rectum is full of hard stool and there is soiling.

Hirschsprung-associated enterocolitis

The most dangerous complication, before or even after surgery:

• Fever, explosive foul diarrhoea, distension, lethargy, sometimes bloody stool, shock and perforation.
• Treat with rectal washouts (saline), IV fluids, antibiotics (including anaerobic cover), and decompression; a stoma if not settling.

Investigations

Management

• Initial: decompression with rectal washouts (and NG tube); treat enterocolitis; or a stoma in normally innervated bowel (levelling biopsies at operation confirm ganglion cells).
• Definitive: resect the aganglionic segment and bring normal ganglionic bowel down to the anus (pull-through). Named operations:
  • Swenson: rectum removed down to just above the anus, end-to-end anastomosis;
  • Duhamel: normal colon brought down behind the rectum, rectum left in place;
  • Soave: endorectal pull-through within a stripped rectal muscle cuff;
  • Transanal endorectal pull-through (now common for short-segment disease), with or without laparoscopic assistance.
• Can be done in one stage or after a stoma.

Outcome

Most children end up with good bowel control. A sizeable minority still have ongoing constipation, soiling or incontinence, or recurrent enterocolitis. Other complications include anastomotic leak or stricture.

Quick revision

• ARM: boys rectourethral (bulbar) fistula; girls rectovestibular fistula; single opening = cloaca.
• Meconium on perineum = low; in urine = high (urinary fistula).
• Wait 24 hours; lateral prone X-ray or perineal ultrasound.
• Low: anoplasty. High: colostomy, then PSARP.
• Low lesions constipate; high lesions leak.
• Hirschsprung: no ganglion cells from the anus upwards; neural crest failure.
• 75% rectosigmoid; boys 4:1; RET, Down syndrome.
• Delayed meconium, distension, bilious vomiting; squirt sign.
• Suction biopsy: no ganglion cells, raised acetylcholinesterase.
• Pull-through (Swenson, Duhamel, Soave, transanal); beware enterocolitis.

Written by The House Job from standard references (Bailey & Love; Basis of Paediatrics).

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