In a child with unilateral asymmetrical polydactyly (one extra digit large, the opposite side a small extra digit), is abdominal ultrasound and KUB (kidney‑ureter‑bladder) screening required?

Abdominal Ultrasound Screening for Asymmetric Polydactyly

Yes, at least one baseline abdominal ultrasound is recommended for this child with asymmetric polydactyly to assess for renal asymmetry and rule out isolated lateralized overgrowth (ILO), which carries tumor risk. 1

Clinical Reasoning

The key concern here is distinguishing simple polydactyly from polydactyly associated with isolated lateralized overgrowth (ILO), which carries a lifetime malignancy risk of up to 5% (primarily Wilms tumor and hepatoblastoma). The asymmetric presentation—one side with a large extra digit and the opposite side with a small extra digit—raises the possibility of underlying lateralized overgrowth rather than simple bilateral polydactyly.

Why Screening Matters

According to the 2025 ACMG guidelines on ILO 1:

• At least one baseline abdominal ultrasound is recommended for children with idiopathic ILO because renal asymmetry is a critical marker for tumor risk estimation
• Renal asymmetry ≥1 cm difference in kidney size indicates tumor risk may exceed 1%, warranting ongoing surveillance every 3 months until age 7
• Symmetric kidneys (<1 cm difference) suggest tumor risk <1%, and ongoing screening may not be needed after the baseline study

The Decision Algorithm

1. Perform baseline abdominal/renal ultrasound to measure kidney sizes bilaterally

2. Assess for renal asymmetry:

   • If kidneys differ by ≥1 cm: Follow ILO tumor screening protocol (abdominal ultrasound every 3 months until age 3, then renal ultrasound every 3 months until age 7) 1
   • If kidneys differ by <1 cm: Tumor risk is likely <1%; ongoing screening may not be indicated, though family preference should guide this decision 1

3. Evaluate for broader overgrowth: If any overgrowth extends beyond the wrist/ankle or involves the trunk, follow full Beckwith-Wiedemann syndrome tumor screening guidelines regardless of molecular testing results 1

4. Consider genetics referral for comprehensive molecular evaluation to identify specific syndromes (BWS, Simpson-Golabi-Behmel syndrome, Perlman syndrome) that would clarify precise tumor risk and guide management 1, 2

Important Caveats

Common Pitfalls to Avoid

• Don't assume isolated polydactyly: While most polydactyly is isolated and benign 3, 4, asymmetric presentation warrants evaluation for underlying overgrowth syndromes
• Don't delay the baseline ultrasound: Early identification of renal asymmetry allows for timely tumor surveillance during the highest-risk period (ages 0-7 years)
• Don't overlook syndromic associations: Polydactyly can be associated with Ellis-van Creveld syndrome (ulnar polydactyly) 3 or Simpson-Golabi-Behmel syndrome (which includes postaxial polydactyly and carries Wilms tumor risk) 2

Risk-Benefit Considerations

The 2025 ACMG guidelines acknowledge that tumor screening in low-risk individuals increases healthcare costs and potential parental anxiety from false positives. However, most parents view screening as decreasing their worry rather than increasing burden 1. Given that:

• The baseline ultrasound is non-invasive
• Wilms tumor detected early has excellent outcomes
• Renal asymmetry findings will definitively guide whether ongoing surveillance is needed

The benefit of one baseline study clearly outweighs the minimal risk and cost.

When Molecular Testing Is Indicated

If the baseline ultrasound shows renal asymmetry ≥1 cm or if there are other features suggesting a syndrome, pursue comprehensive molecular testing including affected tissue biopsy if needed. Identifying a molecular diagnosis can significantly reduce unnecessary screening if a low-risk condition is confirmed, decrease parental anxiety, and enable access to emerging targeted therapies 1.