🩻 TOACS FCPS Station · Invertogram for Anorectal Malformations

Nelson · 22nd Ed · “Prone cross‑table lateral x‑ray, low vs high lesion, colostomy”
⏱️ 7 minutes · Examiner-led · Observed station

📷 Invertogram (Prone Cross‑Table Lateral X‑ray)

Invertogram for Anorectal Malformations – prone cross-table lateral x-ray
Figure 1 · Prone cross‑table lateral invertogram · Anorectal Malformation

🔍 Key radiographic features:

  • ✓ Prone position with pelvis elevated (invertogram view)
  • ✓ Air-filled distal rectum visible as a gas shadow
  • ✓ Distance from rectal gas to perineal skin – measured on the x-ray
  • ✓ Rectal gas level ≥1 cm above perineum → high lesion (needs colostomy)
  • ✓ Rectal gas <1 cm from perineum → low lesion (primary anoplasty)

📋 Clinical scenario (examiner prompt)

A newborn male is noted to have no anal opening on the first day of life. The abdomen is distended and he has passed no meconium. The perineum appears flat with no visible fistula. A prone cross‑table lateral x‑ray (invertogram) is performed at 24 hours of age to assess the level of the anorectal malformation. The image shows the distal rectal gas shadow ≥1 cm above the perineal skin.

No anal opening Flat perineum Rectal gas ≥1 cm Distended abdomen

🧑‍⚕️ Examiner tasks · TOACS

1. Interpret the invertogram – classify the lesion as low or high.

2. Describe the technique – prone position, 24‑hour timing, measurement.

3. Identify the most likely diagnosis (high ARM with rectourethral fistula).

4. Discuss surgical management (colostomy → PSARP).

⚠️ Key concept: The invertogram (prone cross‑table lateral x‑ray) determines the level of the anorectal malformation. Rectal gas ≥1 cm from perineal skin indicates a high lesion → needs a protective colostomy followed by PSARP at 3‑6 months. Gas <1 cm = low lesion → primary perineal anoplasty.

🎯 Expected answers (for examiners)

  • • Interpretation: High anorectal malformation (rectal gas ≥1 cm from perineum)
  • • Technique: Prone position, pelvis elevated, 24-hour film (allows gas to reach distal rectum)
  • • Diagnosis: High ARM – likely rectourethral fistula (most common in males)
  • • Management: Protective colostomy (neonatal), then PSARP at 3-6 months

⚡ Quick FCPS‑style MCQ

On an invertogram for anorectal malformation, a rectal gas shadow ≥1 cm from the perineal skin indicates:

A. High lesion – needs colostomy B. Low lesion – primary anoplasty C. Normal finding D. Currarino triad

📌 Topic summary ·

Invertogram
Prone cross‑table lateral x‑ray at 24 hours
High lesion
Gas ≥1 cm from perineum → colostomy
Low lesion
Gas <1 cm from perineum → primary anoplasty
Most common (males)
Rectourethral fistula
Most common (females)
Rectovestibular fistula
Definitive repair
PSARP (Pena procedure) at 3‑6 months
FeatureLow LesionHigh Lesion
PerineumMeconium visible/fistulaFlat, no fistula
InvertogramGas <1 cm from perineumGas ≥1 cm from perineum
Initial surgeryPrimary anoplastyProtective colostomy
Definitive repairNone neededPSARP at 3‑6 months
Continence prognosisExcellent (90%)Variable (40‑60%)
Source: Nelson Textbook of Pediatrics 22nd Ed · TOACS FCPS station · Invertogram for Anorectal Malformations.