What is hydronephrosis?

Hydronephrosis: Definition, Causes, and Clinical Implications

Hydronephrosis is the urine-filled aseptic dilation of the renal pelvis and calyces, generally occurring in response to urinary obstruction or reflux, which can lead to acute kidney injury and permanent nephron loss if not promptly addressed. 1

Definition and Pathophysiology

• Hydronephrosis represents dilation of the collecting system of the kidney, which can be unilateral or bilateral 1
• It is typically a response to urinary obstruction or reflux rather than a disease itself 1
• Progressive dilation can lead to acute kidney injury and permanent nephron loss if not corrected in a timely manner 1

Classification

• Can be classified by cause (obstructive vs. non-obstructive) and by unilateral versus bilateral involvement 1
• Obstructive hydronephrosis can be further categorized by:
  • Level of obstruction (upper, mid, or lower urinary tract)
  • Complete versus partial obstruction
  • Intrinsic versus extrinsic obstruction 1

Common Causes

• Urolithiasis (kidney or ureteral stones) - most common cause 1, 2
• Malignant obstruction (bladder cancer, prostate cancer) 1
• Strictures of the ureter or urethra 1
• Upper urinary tract infection 1
• Traumatic or ischemic injury to the urinary tract 1
• Retroperitoneal fibrosis 1
• Bladder dysfunction or bladder outlet obstruction (prostatic hyperplasia) 1
• Mechanical compression (enlarged uterus, pelvic organ prolapse) 1
• Congenital anomalies (ureteropelvic junction obstruction, posterior urethral valves) 1, 3
• Pregnancy - occurs in 70-90% of pregnant patients, typically more prominent on the right side 1

Clinical Presentation

• May be identified incidentally on imaging studies ordered for unrelated reasons 1
• Can present with symptoms such as:
  • Flank or groin pain
  • Urinary tract infection
  • Nausea
  • Urinary urgency 1
• In pregnant patients, symptomatic hydronephrosis (occurring in 0.2-4.7%) may lead to preterm labor or maternal/fetal death if untreated 1
• Upper urinary tract deterioration in adults is often silent and detected incidentally 1

Diagnostic Evaluation

• Ultrasound is highly sensitive (>90%) for detecting hydronephrosis and bladder distension 1
• Color Doppler ultrasound can help differentiate a dilated renal pelvis from prominent renal veins and confirm the presence or absence of ureteral jets 1, 2
• CT urography provides comprehensive evaluation of both upper and lower urinary tracts 1, 4
• MR urography is an alternative, particularly in patients with renal impairment 4
• Diuretic renography (MAG3) can differentiate non-obstructive hydronephrosis from true functional obstruction 1, 5

Clinical Implications and Management

• Prompt treatment is essential to prevent permanent renal damage 1, 4
• Decompression via percutaneous nephrostomy or retrograde ureteral stenting is required if infection is present with obstruction 4
• Bilateral hydronephrosis or hydronephrosis in a solitary kidney requires more urgent intervention due to higher risk of acute kidney injury 5
• Regular monitoring of renal function and imaging follow-up are essential after intervention 4, 5

Important Considerations and Pitfalls

• Hydronephrosis does not always indicate obstruction; distended bladder, reflux, pregnancy, or diuresis may cause ureteral and collecting system dilation 1
• False-negative ultrasound studies may occur due to dehydration, early obstruction, or compression of the renal pelvis 1
• Delaying intervention in cases with infection or significant obstruction can lead to irreversible renal damage 4
• A distended bladder can cause artifactual hydronephrosis; ensuring proper bladder status during imaging is crucial 4