TOACS FCPS Station · Invertogram: Low Variety Anorectal Malformations

📷 Prone Cross‑Table Lateral Invertogram

Invertogram showing low variety anorectal malformation with rectal gas <1 cm from perineum

Figure 1 · Prone cross‑table lateral x‑ray · Low Anorectal Malformation

🔍 Key radiographic features:

✓ Rectal gas ≤1 cm from perineal skin – low lesion (good prognosis)
✓ Meconium visible on perineum – perineal fistula
✓ No significant gap between rectal gas and perineum
✓ Normal sacrum – associated with good continence
✓ Gas in distal rectum – confirms patency of fistula

📋 Clinical scenario (examiner prompt)

A newborn is noted to have no anal opening but there is meconium visible on the perineal skin at the fourchette (in a female) or scrotal raphe (in a male). The abdomen is not distended and the infant is passing urine normally. A prone cross‑table lateral invertogram is performed at 24 hours of age. The image shows the distal rectal gas shadow ≤1 cm above the perineal skin.

Meconium visible Rectal gas ≤1 cm Perineal fistula No abdominal distention

🧑‍⚕️ Examiner tasks · TOACS

1. Identify the diagnosis from the invertogram and clinical context.

2. Describe the invertogram findings (gas ≤1 cm from perineum, perineal fistula).

3. Explain the significance (low lesion, good prognosis, primary anoplasty).

4. Discuss management (primary perineal anoplasty, screening for VACTERL).

⚠️ Key concept: A low variety anorectal malformation is defined by rectal gas ≤1 cm from the perineal skin on invertogram. These lesions have a perineal fistula (meconium visible) and are associated with good prognosis for bowel continence. Primary perineal anoplasty is the treatment of choice.

🎯 Expected answers (for examiners)

• Diagnosis: Low variety anorectal malformation (perineal fistula)
• Invertogram findings: Rectal gas ≤1 cm from perineal skin, meconium visible on perineum
• Significance: Low lesion with good sphincter development → excellent prognosis for continence
• Management: Primary perineal anoplasty (within 24-48 hours), screen for VACTERL

⚡ Quick FCPS‑style MCQ

On invertogram, a rectal gas shadow ≤1 cm from the perineal skin with meconium visible on the perineum indicates:

A. High anorectal malformation (needs colostomy) B. Low anorectal malformation (primary anoplasty) C. Cloaca D. Currarino triad

Feature	Low Lesion	High Lesion
Invertogram	Gas ≤1 cm from perineum	Gas ≥1 cm from perineum
Perineum	Meconium visible	Flat, no meconium
Initial surgery	Primary anoplasty	Colostomy
Continence	Excellent (90%)	Variable (40-60%)

🩻 TOACS FCPS Station · Invertogram: Low Variety Anorectal Malformations

📷 Prone Cross‑Table Lateral Invertogram

🧑‍⚕️ Examiner tasks · TOACS

📌 Topic summary ·