What causes hydronephrosis?

Causes of Hydronephrosis

Hydronephrosis is primarily caused by urinary tract obstruction, with the most common etiologies being urinary stones, tumors, and ureteropelvic junction obstruction, requiring prompt evaluation to prevent permanent renal damage. 1, 2

Definition and Pathophysiology

Hydronephrosis refers to dilation of the renal collecting system, which can occur with or without obstruction. When left untreated, it can lead to:

• Permanent nephron loss
• Irreversible renal damage
• Acute kidney injury
• Chronic kidney disease
• Hypertension
• Growth retardation (in pediatric cases)

Common Causes of Hydronephrosis

Obstructive Causes

• Ureteropelvic Junction Obstruction (UPJO)

  • Accounts for 10-65% of cases
  • 90% of cases are unilateral 1
  • More common with increasing severity of hydronephrosis

• Urinary Calculi (Stones)

  • Common cause in adults
  • Can occur at any level of the urinary tract

• Tumors/Malignancies

  • Extrinsic compression from pelvic or retroperitoneal tumors
  • Intrinsic tumors of the urinary tract
  • Higher risk in patients with hematuria (33% vs 17.6%) 3
  • Significantly higher in patients with weight loss 3

• Primary Megaureter

  • Accounts for 5-10% of antenatal hydronephrosis cases
  • Diagnosed by persistent ureteral dilation (>7mm) 1

• Pregnancy

  • Physiologic hydronephrosis due to hormonal effects and mechanical compression
  • Usually asymptomatic and resolves after delivery 2

• Benign Prostatic Hyperplasia

  • Common cause in older males
  • Causes bladder outlet obstruction leading to bilateral hydronephrosis

Non-obstructive Causes

• Vesicoureteral Reflux (VUR)

  • Common in pediatric population
  • Can cause hydronephrosis without true obstruction

• Post-obstructive Dilatation

  • Residual dilatation after relief of obstruction

• Congenital Anomalies

  • Duplex collecting systems
  • Ectopic ureters
  • Ureteroceles 4

Diagnostic Approach

Initial Imaging

• Ultrasound

  • Highly sensitive (>90%) for detecting hydronephrosis
  • Can help localize level of obstruction
  • Evaluates renal size, echogenicity, and morphology 1
  • False negatives may occur with dehydration, early obstruction, or compression of collecting system

• CT Urography

  • Preferred initial imaging for adults with severe hydronephrosis
  • Comprehensive visualization of upper and lower urinary tracts 2

Functional Assessment

• MAG3 Renal Scan
  • Optimal for evaluating hydronephrosis with impaired renal function
  • Higher extraction fraction (40-50%) than DTPA
  • Provides information on split renal function and drainage
  • Preferred over DTPA for suspected obstruction 1, 2

Clinical Implications and Management

Indications for Intervention

Surgical intervention is indicated when:

• Differential renal function <40% on MAG3 scan
• T1/2 >20 minutes on diuretic renography
• Deteriorating function (>5% change on consecutive scans)
• Worsening drainage on serial imaging 1, 2

Prognosis Factors

• Duration of obstruction
• Degree of cortical thinning
• Baseline renal function
• Patient age
• Promptness of intervention 2

Special Considerations

• In adults: Even with severe hydronephrosis and <10% function, kidneys can recover after decompression, emphasizing the importance of avoiding premature nephrectomy 2

• In pregnancy: Asymptomatic hydronephrosis typically requires no intervention, while symptomatic cases may require intervention to prevent complications 2

• In pediatric patients: Initial ultrasound should be performed 48-72 hours after birth with follow-up based on severity 2

Common Pitfalls

• Mistaking non-obstructive causes (like VUR or post-obstructive dilatation) for true obstruction
• Delaying decompression, which can lead to permanent nephron loss
• Failure to recognize that hydronephrosis does not always indicate obstruction (can be due to distended bladder, reflux, pregnancy, or diuresis) 1
• Underestimating the recovery potential of severely affected kidneys