At what Anteroposterior Diameter (APD) should we be alert for potential hydronephrosis in a child?

Anteroposterior Diameter Threshold for Hydronephrosis Alert in Children

An anteroposterior renal pelvic diameter (APRPD) greater than 15 mm should trigger clinical alert for severe hydronephrosis in children, as this measurement indicates moderate to severe hydronephrosis that may require more aggressive monitoring or intervention. 1, 2

Grading Systems and APRPD Thresholds

The Society for Fetal Urology (SFU) grading system is widely used to categorize hydronephrosis severity, with APRPD serving as an important complementary measurement:

• Mild hydronephrosis (SFU Grade 1-2): APRPD < 15 mm
• Moderate to severe hydronephrosis (SFU Grade 3-4): APRPD > 15 mm 2

The American College of Radiology (ACR) specifically includes an APRPD > 15 mm as a sign of severe hydronephrosis in postnatal imaging, which conforms to intermediate- to high-risk urinary tract dilation stratification 1.

Clinical Implications of APRPD > 15 mm

When APRPD exceeds 15 mm, clinicians should be alerted to:

• Higher risk of ureteropelvic junction obstruction (UPJO) - Lee et al. showed a 54% risk of UPJO in severe (>15 mm APRPD) urinary tract dilation 1
• Lower spontaneous resolution rate (only 29% for severe hydronephrosis compared to 64-73% for mild cases) 2
• Increased need for urological referral and potential surgical intervention 2
• Higher risk of urinary tract infection, even in the absence of vesicoureteral reflux 3

Management Algorithm Based on APRPD

1. APRPD < 15 mm (Mild to moderate hydronephrosis):

   • Follow-up ultrasound in 3-6 months
   • Lower risk of requiring surgical intervention
   • Higher likelihood of spontaneous resolution (64-73%)

2. APRPD > 15 mm (Severe hydronephrosis):

   • More frequent follow-up (every 1-3 months initially)
   • Consider functional studies (MAG3 renal scan preferred over DTPA)
   • Consider antibiotic prophylaxis due to increased UTI risk
   • Urological referral recommended
   • Higher likelihood of requiring surgical intervention

Important Considerations and Pitfalls

• APRPD is a dynamic parameter affected by hydration status, bladder filling, position, and respiration 4
• Initial postnatal ultrasound should be delayed until 48-72 hours after birth due to relative low urine production in newborns 1
• Exceptions requiring earlier imaging include severe bilateral hydronephrosis, bladder abnormalities, or oligohydramnios 1
• A full bladder can cause temporary mild hydronephrosis; scanning kidneys after voiding helps avoid artifactual results 2
• APRPD alone should not be the sole criterion for decision-making; it should be considered alongside other parameters such as calyceal dilation, renal parenchymal thickness, and renal function 1, 2

The threshold of 15 mm for APRPD represents an important clinical decision point that helps distinguish cases requiring closer monitoring and potential intervention from those that can be safely observed with less frequent follow-up.