What is hydronephrosis?

Definition of Hydronephrosis

Hydronephrosis is defined as dilation of the renal collecting system due to obstruction of urine outflow, which can lead to permanent nephron loss and irreversible renal damage if not adequately managed. 1

Pathophysiology and Classification

Hydronephrosis specifically describes dilation and swelling of the kidney that occurs when normal drainage of urine from the kidney to the bladder is impaired. It can be graded based on severity:

• Grade I (mild): Slight dilation of the renal pelvis without calyceal dilation
• Grade II (moderate): Moderate dilation of the renal pelvis with some calyceal dilation
• Grade III (severe): Significant dilation of the renal pelvis and calyces with preserved renal parenchyma
• Grade IV (severe): Extreme dilation with thinning of the renal parenchyma 1, 2

Diagnostic Imaging

Ultrasound is the primary initial diagnostic tool for hydronephrosis with several advantages:

• Highly sensitive (>90%) for detecting hydronephrosis
• Non-invasive and radiation-free
• Helps localize the level of obstruction
• Evaluates renal size, echogenicity, and morphology 1

For more detailed evaluation:

• CT urography is preferred for adults with severe hydronephrosis
• MAG3 renal scan is optimal for evaluating hydronephrosis with impaired renal function, providing information on split renal function and drainage
• MRI with gadolinium is useful for detailed evaluation of surrounding structures 1

Common Etiologies

Hydronephrosis can result from various causes:

1. Obstructive causes:

   • Urinary calculi (most common cause - 54.1% of cases) 3
   • Pelviureteric junction stenosis
   • Vesicoureteric junction obstruction
   • Urethral strictures
   • Benign prostatic hyperplasia

2. Non-obstructive causes:

   • Vesicoureteral reflux (7.3% of cases) 3
   • Pregnancy (physiologic hydronephrosis common after 20 weeks)
   • Neurogenic bladder

3. Malignant causes:

   • Infiltrating bladder cancer
   • Infiltrating prostatic carcinoma
   • Retroperitoneal malignancies 1, 3

Clinical Implications

Untreated hydronephrosis can lead to serious complications:

• Acute kidney injury
• Permanent nephron loss
• Irreversible renal damage
• Urinary tract infections
• Sepsis 1, 4

Management Considerations

The management approach depends on the severity, cause, and presence of complications:

• Mild asymptomatic hydronephrosis: May be monitored with serial ultrasounds
• Moderate to severe hydronephrosis: Requires urological consultation, especially if accompanied by acute kidney injury, infection, or intractable pain
• Decompression methods:
  • Percutaneous nephrostomy (PCN) - higher technical success rate (>95%) when stenting fails
  • Retrograde ureteral stenting - less invasive with fewer subsequent interventions
  • Regular stent exchanges (typically every 3 months) are necessary to prevent complications 1

Special Populations

• Pediatric patients: Initial ultrasound should be performed 48-72 hours after birth for antenatal hydronephrosis, with follow-up based on severity
• Pregnant patients: Co-management by urology and obstetrics is recommended for symptomatic cases 1

Prognosis depends on duration of obstruction, degree of cortical thinning, baseline renal function, patient age, and promptness of intervention, with better outcomes in younger patients and those with earlier decompression 1.