🩺 FCPS Paediatrics TOACS · Imperforate Anus (Anorectal Malformation) · Nelson Chapter 392.1
📖 Nelson's Textbook of Pediatrics · Newborn emergency · Delayed meconium · Absent anal opening · VACTERL
📚 paeds.online – Paeds Online
🩺 OBSERVED/INTERACTIVE STATION · CPSP FORMAT · 8 MINUTES · NEWBORN: NO ANAL OPENING, DELAYED MECONIUM → ANORECTAL MALFORMATION
📋 Observed Station – “3‑day‑old newborn, no anal opening, not passed stool since birth”
👶🏻 Clinical Scenario (TOACS – read aloud / displayed):
A 3‑day‑old male newborn born at term via normal vaginal delivery (NVD) at a peripheral centre is brought to the emergency department because he has never passed meconium since birth. On examination, there is no visible anal opening. The abdomen is mildly distended but soft. A tiny amount of meconium is seen staining the perineal skin just anterior to the expected anal position – a perineal fistula is present. The baby is otherwise well, feeding poorly, and has no other obvious dysmorphism.
🎯 Task (examiner observed): Recognize the condition as an anorectal malformation (imperforate anus). Identify that this is a low lesion (perineal fistula). Describe the immediate management (confirm no other anomalies: spine, cardiac, renal). Explain that low lesions can undergo primary anoplasty without a colostomy. Discuss the need for VACTERL screening (spinal x‑ray, echocardiogram, renal ultrasound). Outline the long‑term prognosis (good bowel control).
📸 Diagram: Types of anorectal malformations (low vs high)
A 3‑day‑old male newborn born at term via normal vaginal delivery (NVD) at a peripheral centre is brought to the emergency department because he has never passed meconium since birth. On examination, there is no visible anal opening. The abdomen is mildly distended but soft. A tiny amount of meconium is seen staining the perineal skin just anterior to the expected anal position – a perineal fistula is present. The baby is otherwise well, feeding poorly, and has no other obvious dysmorphism.
🎯 Task (examiner observed): Recognize the condition as an anorectal malformation (imperforate anus). Identify that this is a low lesion (perineal fistula). Describe the immediate management (confirm no other anomalies: spine, cardiac, renal). Explain that low lesions can undergo primary anoplasty without a colostomy. Discuss the need for VACTERL screening (spinal x‑ray, echocardiogram, renal ultrasound). Outline the long‑term prognosis (good bowel control).
📸 Diagram: Types of anorectal malformations (low vs high)
Figure: Anorectal malformation – low lesion with perineal fistula (left) vs high lesion requiring colostomy (right).
⚠️ IMMEDIATE ACTIONS (Nelson Ch.392.1):
• Do NOT place a rectal thermometer or perform digital rectal exam – can cause false passage.
• Inspect perineum carefully – look for meconium on perineal skin (low lesion).
• Check for meconium in urine (high lesion with rectourethral fistula).
• Prone cross‑table lateral x‑ray at 24 hours (gas in distal rectum).
• Screen for VACTERL: vertebral, anorectal, cardiac, tracheoesophageal, renal, limb.
• Low lesion (perineal fistula) → primary anoplasty without colostomy.
• High lesion (no fistula or rectourinary) → protective colostomy, then posterior sagittal anorectoplasty (PSARP).
• Do NOT place a rectal thermometer or perform digital rectal exam – can cause false passage.
• Inspect perineum carefully – look for meconium on perineal skin (low lesion).
• Check for meconium in urine (high lesion with rectourethral fistula).
• Prone cross‑table lateral x‑ray at 24 hours (gas in distal rectum).
• Screen for VACTERL: vertebral, anorectal, cardiac, tracheoesophageal, renal, limb.
• Low lesion (perineal fistula) → primary anoplasty without colostomy.
• High lesion (no fistula or rectourinary) → protective colostomy, then posterior sagittal anorectoplasty (PSARP).
🧠 CLASSIFICATION (based on fistula / level of rectum):
• Low lesions (good prognosis): Perineal fistula, anal stenosis, covered anus. Meconium visible on perineum. Primary anoplasty.
• High lesions (variable prognosis): Rectourethral (bulbar/prostatic), rectovesical, rectovestibular (female), cloaca. No perineal meconium. Protective colostomy → PSARP.
• Currarino triad: Sacral agenesis + presacral mass + anorectal malformation (MNX1 gene).
• Sacral ratio predicts continence (normal sacrum → better outcome).
• Low lesions (good prognosis): Perineal fistula, anal stenosis, covered anus. Meconium visible on perineum. Primary anoplasty.
• High lesions (variable prognosis): Rectourethral (bulbar/prostatic), rectovesical, rectovestibular (female), cloaca. No perineal meconium. Protective colostomy → PSARP.
• Currarino triad: Sacral agenesis + presacral mass + anorectal malformation (MNX1 gene).
• Sacral ratio predicts continence (normal sacrum → better outcome).
💡 Examiner instruction (observed station): Candidate must (1) diagnose imperforate anus, (2) classify as low lesion (perineal fistula), (3) explain that primary anoplasty can be performed without colostomy, (4) describe VACTERL screening (spinal US/x‑ray, echocardiogram, renal US, rule out TEF), (5) discuss timing of surgery (early neonatal period), (6) mention prognosis (good bowel control with low lesions).
🔬 Diagnostic Workup & Classification (Anorectal Malformations)
1 Physical exam
Inspect perineum: look for anal opening, meconium on skin, perineal fistula, single orifice (cloaca).
Inspect perineum: look for anal opening, meconium on skin, perineal fistula, single orifice (cloaca).
2 Prone cross‑table lateral x‑ray
At 24 hours (gas reaches distal rectum). Measure distance from rectal gas to perineal skin. >1 cm suggests high lesion.
At 24 hours (gas reaches distal rectum). Measure distance from rectal gas to perineal skin. >1 cm suggests high lesion.
3 Abdominal ultrasound
Renal anomalies (hydronephrosis, agenesis), presacral mass (Currarino).
Renal anomalies (hydronephrosis, agenesis), presacral mass (Currarino).
4 Echocardiogram & spine x‑ray
VACTERL screening: cardiac defects, vertebral anomalies.
VACTERL screening: cardiac defects, vertebral anomalies.
5 Voiding cystourethrogram (VCUG)
If rectourinary fistula suspected (meconium in urine).
If rectourinary fistula suspected (meconium in urine).
6 MRI spine (if tethered cord suspected)
Sacral anomalies, spinal dysraphism.
Sacral anomalies, spinal dysraphism.
📋 VACTERL ASSOCIATION (screen all ARM patients):
• V – Vertebral anomalies (hemivertebrae, scoliosis).
• A – Anorectal malformation.
• C – Cardiac (VSD, ASD, tetralogy of Fallot).
• TE – TracheoEsophageal fistula / esophageal atresia.
• R – Renal anomalies (renal agenesis, hydronephrosis, VUR).
• L – Limb (radial anomalies, polydactyly).
→ Newborn with ARM needs evaluation for all these systems.
• V – Vertebral anomalies (hemivertebrae, scoliosis).
• A – Anorectal malformation.
• C – Cardiac (VSD, ASD, tetralogy of Fallot).
• TE – TracheoEsophageal fistula / esophageal atresia.
• R – Renal anomalies (renal agenesis, hydronephrosis, VUR).
• L – Limb (radial anomalies, polydactyly).
→ Newborn with ARM needs evaluation for all these systems.
🩺 Prognostic factors for bowel continence:
• Sacral development (most important): Normal sacrum → good prognosis; absent sacrum → poor.
• Type of lesion: Low lesions (perineal fistula) → 90% continent; high lesions → 40-60% need bowel management.
• Associated spinal anomalies (tethered cord).
• Surgical technique: PSARP with preservation of sphincter complex.
• Sacral development (most important): Normal sacrum → good prognosis; absent sacrum → poor.
• Type of lesion: Low lesions (perineal fistula) → 90% continent; high lesions → 40-60% need bowel management.
• Associated spinal anomalies (tethered cord).
• Surgical technique: PSARP with preservation of sphincter complex.
🗨️ Examiner Q&A · Imperforate Anus · Classification · VACTERL · Surgical Management
❓ Q1 (Examiner): “What is the most likely diagnosis in this 3‑day‑old with no anal opening and meconium on the perineal skin?”
✅ Imperforate anus with a low lesion (perineal fistula). The presence of meconium on the perineal skin indicates a fistula that opens onto the perineum, which is characteristic of a low anorectal malformation. These patients have an excellent prognosis for bowel continence.
❓ Q2 (Examiner): “How do you differentiate a low lesion from a high lesion clinically and radiologically?”
✅ Low lesion: Meconium visible on perineum, well‑developed gluteal fold, anal dimple present. Prone cross‑table x‑ray shows rectal gas within <1 cm of perineal skin.
High lesion: Flat perineum, no meconium, may have meconium in urine (fistula to urethra/bladder). X‑ray shows rectal gas >1-2 cm from skin. Requires colostomy.
High lesion: Flat perineum, no meconium, may have meconium in urine (fistula to urethra/bladder). X‑ray shows rectal gas >1-2 cm from skin. Requires colostomy.
❓ Q3 (Examiner): “What is the initial surgical management for a low lesion (perineal fistula)?”
✅ Primary anoplasty (perineal repair) without a colostomy. The fistula is mobilized and the rectum is brought down to the perineal skin. This is usually performed in the neonatal period. No stoma is needed, and the prognosis for bowel control is excellent.
❓ Q4 (Examiner): “What is the management for a high lesion (e.g., rectourethral fistula in a male)?”
✅ Protective descending colostomy in the newborn period, followed by posterior sagittal anorectoplasty (PSARP) at 3-6 months of age. PSARP involves a midline sagittal incision to place the rectum within the sphincter complex. After repair, anal dilatations are performed to prevent stricture.
❓ Q5 (Examiner): “What is the Currarino triad? Which gene is associated?”
✅ Currarino triad: 1) Sacral agenesis (partial or complete), 2) Presacral mass (anterior meningocele, teratoma, dermoid, or enteric cyst), 3) Anorectal malformation (anal stenosis or imperforate anus).
Gene: MNX1 (HLXB9) mutation (autosomal dominant). Associated with hereditary sacral agenesis.
Gene: MNX1 (HLXB9) mutation (autosomal dominant). Associated with hereditary sacral agenesis.
❓ Q6 (Examiner): “What is the most important predictor of long‑term fecal continence in children with ARM?”
✅ Sacral development (sacral ratio). A well‑formed sacrum with normal number of vertebrae (≥ S4) predicts good sphincter function. An absent or hypoplastic sacrum is associated with poor bowel control. Other factors: type of lesion (low > high), quality of PSARP, and presence of tethered cord.
❓ Q7 (Examiner): “What associated anomalies must be screened for in a newborn with imperforate anus?”
✅ VACTERL screen:
• Spine x‑ray (vertebral anomalies).
• Echocardiogram (cardiac defects).
• Esophageal atresia / TEF – rule out with NG tube (cannot pass).
• Renal ultrasound (hydronephrosis, agenesis).
• Limb examination (radial anomalies).
Also screen for tethered cord (MRI spine if sacral anomaly or neurologic signs).
• Spine x‑ray (vertebral anomalies).
• Echocardiogram (cardiac defects).
• Esophageal atresia / TEF – rule out with NG tube (cannot pass).
• Renal ultrasound (hydronephrosis, agenesis).
• Limb examination (radial anomalies).
Also screen for tethered cord (MRI spine if sacral anomaly or neurologic signs).
❓ Q8 (Examiner): “When do you perform the prone cross‑table lateral x‑ray, and what does it show?”
✅ Performed at 24 hours of age (allows air to reach the distal rectum). The infant is placed prone with hips flexed, and a radiopaque marker is placed on the perineal skin. The distance from the most distal gas bubble to the marker is measured. <1 cm = low lesion (usually can be repaired without colostomy). >1-2 cm = high lesion (requires colostomy).
❓ Q9 (Examiner): “What is the risk of urinary tract injury during ARM repair? How is it avoided?”
✅ Risk of injury to the urethra or bladder during dissection of rectourinary fistula. Avoided by: 1) Preoperative colostomy, 2) Intraoperative placement of a Foley catheter, 3) Using the PSARP approach with careful identification of the fistula under direct vision, 4) Dividing the fistula close to the rectal wall. Post‑operative urethral calibration or cystoscopy may be performed.
❓ Q10 (Examiner): “The parents ask about future bowel control. What do you tell them for a low lesion?”
✅ “For a low lesion (perineal fistula), the prognosis is excellent. More than 90% of children achieve normal bowel control (continence) by school age. They may have constipation (common), which is managed with stool softeners and bowel training. Very rarely do they need a bowel management program. High‑lesion patients have a lower rate of spontaneous continence and may need enemas or an antegrade continence enema (ACE) procedure.”
📢 Examiner probe (counseling): “The parents are extremely distressed. How would you counsel them?” → Explain that anorectal malformation is a surgically correctable condition. A low lesion has an excellent outcome with normal continence. Reassure them that associated anomalies will be systematically evaluated. Involve pediatric surgeon, genetics, and social support.
📘 Nelson's Textbook of Pediatrics – Chapter 392.1: Anorectal Malformations (Core Summary)
📖 Definition
Anorectal malformations (ARM) = spectrum of congenital defects of the distal anus and rectum, ranging from anal stenosis to cloaca.
Anorectal malformations (ARM) = spectrum of congenital defects of the distal anus and rectum, ranging from anal stenosis to cloaca.
📊 Incidence
1 in 3000-5000 live births. Slight male predominance.
1 in 3000-5000 live births. Slight male predominance.
📝 Classification
Low lesions (perineal fistula) – good prognosis. High lesions (rectourethral, rectovesical, cloaca) – colostomy + PSARP.
Low lesions (perineal fistula) – good prognosis. High lesions (rectourethral, rectovesical, cloaca) – colostomy + PSARP.
🩺 Diagnosis
Newborn exam → no anal opening. Prone cross‑table x‑ray at 24h. VACTERL screen essential.
Newborn exam → no anal opening. Prone cross‑table x‑ray at 24h. VACTERL screen essential.
💊 Low lesion management
Primary anoplasty (neonatal). No colostomy. Excellent continence (90%).
Primary anoplasty (neonatal). No colostomy. Excellent continence (90%).
💊 High lesion management
Protective colostomy → PSARP at 3-6 months → anal dilatations → bowel management program if needed.
Protective colostomy → PSARP at 3-6 months → anal dilatations → bowel management program if needed.
⚠️ Associated anomalies
VACTERL, Currarino triad (sacral agenesis + presacral mass), tethered cord, vesicoureteral reflux.
VACTERL, Currarino triad (sacral agenesis + presacral mass), tethered cord, vesicoureteral reflux.
📈 Prognosis (continence)
Low lesion: >90%; high lesion: 40-60% (may need enemas/ACE). Sacral development is key predictor.
Low lesion: >90%; high lesion: 40-60% (may need enemas/ACE). Sacral development is key predictor.
💢 Complications
Constipation (most common), soiling (fecal incontinence), stricture, recurrence of fistula, tethered cord.
Constipation (most common), soiling (fecal incontinence), stricture, recurrence of fistula, tethered cord.
📖 Nelson's Textbook Reference (22nd Edition, Chapter 392.1):
“Anorectal malformations are diagnosed at birth by inspection of the perineum. Low lesions (perineal fistula) can be repaired with a primary anoplasty without colostomy. High lesions (rectourethral, rectovesical, cloaca) require a colostomy followed by posterior sagittal anorectoplasty (PSARP). Associated anomalies are common, and a VACTERL screen is mandatory. The prognosis for bowel control depends on the level of the lesion and the development of the sacrum.”
“Anorectal malformations are diagnosed at birth by inspection of the perineum. Low lesions (perineal fistula) can be repaired with a primary anoplasty without colostomy. High lesions (rectourethral, rectovesical, cloaca) require a colostomy followed by posterior sagittal anorectoplasty (PSARP). Associated anomalies are common, and a VACTERL screen is mandatory. The prognosis for bowel control depends on the level of the lesion and the development of the sacrum.”
⭐ TOACS TAKE‑HOME POINTS (Imperforate Anus – Nelson 392.1):
1. Low lesion (perineal fistula): meconium visible on perineum → primary anoplasty, excellent continence.
2. High lesion: flat perineum, no perineal meconium → colostomy → PSARP at 3-6 months.
3. VACTERL screen is mandatory (spine, heart, TE fistula, renal, limbs).
4. Currarino triad: sacral agenesis + presacral mass + ARM (MNX1 gene).
5. Sacral ratio predicts continence (normal sacrum = good).
6. Do NOT perform digital rectal exam in undiagnosed ARM (risk of false passage).
7. Prone cross‑table x‑ray at 24 hours helps classify low vs high.
8. PSARP (Pena procedure) is the standard repair for high lesions.
9. Constipation is common after repair → treat with stool softeners.
10. Bowel management program (enemas, ACE) for refractory soiling in high lesions.
1. Low lesion (perineal fistula): meconium visible on perineum → primary anoplasty, excellent continence.
2. High lesion: flat perineum, no perineal meconium → colostomy → PSARP at 3-6 months.
3. VACTERL screen is mandatory (spine, heart, TE fistula, renal, limbs).
4. Currarino triad: sacral agenesis + presacral mass + ARM (MNX1 gene).
5. Sacral ratio predicts continence (normal sacrum = good).
6. Do NOT perform digital rectal exam in undiagnosed ARM (risk of false passage).
7. Prone cross‑table x‑ray at 24 hours helps classify low vs high.
8. PSARP (Pena procedure) is the standard repair for high lesions.
9. Constipation is common after repair → treat with stool softeners.
10. Bowel management program (enemas, ACE) for refractory soiling in high lesions.
🔗 Additional resources:
• Paeds.online – FCPS Paediatrics TOACS station library
• VACTERL association guidelines, ARM network international.
• Paeds.online – FCPS Paediatrics TOACS station library
• VACTERL association guidelines, ARM network international.