What does hydronephrosis on ultrasound indicate?

Hydronephrosis on Ultrasound: Diagnostic Significance and Implications

Hydronephrosis on ultrasound primarily indicates dilation of the renal collecting system, most commonly due to urinary obstruction, which requires prompt evaluation to prevent permanent renal damage and potential nephron loss. 1

Grading and Appearance on Ultrasound

Hydronephrosis appears as anechoic (fluid-filled) dilation of the renal collecting system within the normally hyperechoic renal sinus. It can be classified according to severity:

• Mild (Grade I): Minimal dilation of the renal pelvis
• Moderate (Grade II): Confluent calices resulting in a "bear's paw" appearance
• Severe (Grade III): Extensive dilation with effacement of the renal parenchyma 1

Common Causes of Hydronephrosis

Hydronephrosis typically indicates one of several underlying conditions:

1. Obstructive causes:

   • Urolithiasis (kidney stones)
   • Ureteropelvic junction obstruction (UPJO)
   • Malignant obstruction (tumors)
   • Strictures
   • Retroperitoneal fibrosis
   • Bladder outlet obstruction (e.g., prostatic hyperplasia)

2. Non-obstructive causes:

   • Vesicoureteral reflux (VUR)
   • Pregnancy-related physiologic dilation (70-90% of pregnant patients)
   • Post-void residual dilation (transient)
   • Full bladder causing back-pressure 1, 2

Clinical Significance and Implications

The finding of hydronephrosis on ultrasound has several important clinical implications:

1. Risk of renal damage: Progressive dilation can lead to acute kidney injury and permanent nephron loss if not addressed 1

2. Need for further evaluation: Hydronephrosis requires additional diagnostic workup to determine the underlying cause and whether it represents true obstruction 1

3. Potential for intervention: Depending on severity and cause, intervention may be needed to relieve obstruction and preserve renal function 2

Important Caveats and Pitfalls

Several factors can affect the interpretation of hydronephrosis on ultrasound:

1. False positives:

   • Hydronephrosis may be mimicked by dilated renal vasculature, renal sinus cysts, and medullary pyramids (especially in young patients) 1
   • A distended bladder can cause mild hydronephrosis in normal healthy adults 1

2. False negatives:

   • Dehydration may mask the presence of obstruction 1
   • Small ureteral stones may not cause hydronephrosis 1

3. Technical limitations:

   • Patient habitus (obesity)
   • Bowel gas
   • Abdominal or rib tenderness
   • Empty bladder 1

Diagnostic Algorithm Following Detection of Hydronephrosis

When hydronephrosis is detected on ultrasound:

1. For mild hydronephrosis:

   • Consider post-void repeat ultrasound to rule out transient hydronephrosis (38.6% of cases may resolve after voiding) 3
   • Monitor with follow-up ultrasound in 1-6 months 1

2. For moderate to severe hydronephrosis:

   • Evaluate for VUR with voiding cystourethrography (VCUG), especially in pediatric patients 1
   • Consider CT urography for comprehensive visualization of upper and lower urinary tracts in adults 2
   • Consider diuretic renography (MAG3 scan) to evaluate drainage and differential renal function 2

3. For suspected obstruction:

   • Urgent decompression may be needed via percutaneous nephrostomy or retrograde ureteral stenting 2

Special Populations

1. Pregnant patients:

   • Asymptomatic hydronephrosis (70-90% of pregnant patients) typically requires no intervention
   • Symptomatic cases (0.2-4.7%) may require intervention to prevent preterm labor or maternal/fetal death 1, 2

2. Pediatric patients with antenatal hydronephrosis:

   • Initial ultrasound 48-72 hours after birth
   • Follow-up based on severity
   • VCUG for moderate to severe cases to rule out vesicoureteral reflux 2

In conclusion, hydronephrosis on ultrasound is a significant finding that warrants careful evaluation to determine its cause, severity, and need for intervention. While it commonly indicates obstruction, other non-obstructive causes must be considered, and proper follow-up is essential to prevent permanent renal damage.