What does a dual left renal collecting system with mild hydronephrosis of the upper pool collecting system mean?

Understanding Dual Left Renal Collecting System with Mild Hydronephrosis of the Upper Pole

This finding describes a common congenital anatomic variant where the left kidney has two separate collecting systems (duplex kidney), and the upper portion shows mild fluid accumulation—a condition that typically requires monitoring but may need intervention if complications develop. 1

What This Anatomic Finding Means

A dual (duplex) collecting system is a congenital anomaly where a single kidney has two separate ureters arising from it, each draining a different portion of the kidney. 2, 3

• The upper pole collecting system drains the upper portion of the kidney, while the lower pole system drains the lower portion 2, 3
• This occurs in approximately 1-7% of the general population and is usually an asymptomatic normal variant 1
• Mild hydronephrosis of the upper pole means there is slight fluid accumulation and dilation in the upper collecting system, suggesting partial obstruction or impaired drainage 2, 3

Clinical Significance and Risk Assessment

The presence of hydronephrosis in a duplex system is clinically significant because it indicates abnormal drainage that can lead to complications. 2, 3

• Duplex kidneys with hydronephrosis are at increased risk for recurrent urinary tract infections, vesicoureteral reflux (VUR), ureteropelvic junction obstruction, and ureterocele 2, 4, 3
• Progressive dilation can lead to acute kidney injury and permanent nephron loss if not corrected 1, 5
• The upper pole moiety in duplex systems is particularly prone to obstruction and may have unequal renal function compared to the lower pole 3

Immediate Evaluation Required

Check for signs of infection immediately, as infected obstructed systems require urgent decompression. 6, 5

• Obtain urinalysis to evaluate for pyuria and bacteriuria, which would indicate urinary tract infection requiring urgent intervention 6, 2
• Measure serum creatinine and estimated GFR to assess overall renal function 6
• If fever, flank pain, or signs of infection are present with obstruction, urgent decompression via percutaneous nephrostomy or retrograde ureteral stenting is required 6, 5

Recommended Diagnostic Workup

Further imaging is necessary to determine the cause of upper pole hydronephrosis and assess for complications. 1, 6

• CT urography is the preferred next imaging study for comprehensive evaluation of the genitourinary tract to identify the specific cause of obstruction 1, 6
• MR urography is an alternative when radiation exposure is a concern 6
• Voiding cystourethrography (VCUG) should be considered to evaluate for vesicoureteral reflux, which occurs in approximately 30% of patients with duplicated systems and hydronephrosis 1
• Diuretic renal scan (MAG3) should be performed to assess split renal function between the upper and lower poles and confirm functional obstruction 1, 6

Management Algorithm

The management approach depends on severity, presence of infection, and renal function:

1. If infection is present: Urgent decompression is mandatory to prevent sepsis and irreversible renal damage 6, 5

2. If mild hydronephrosis without infection:

   • Initiate prophylactic antibiotics to prevent recurrent UTIs 3
   • Arrange urgent urology follow-up within 1-2 weeks 3
   • Obtain follow-up ultrasound in 1-6 months to monitor progression 1

3. If moderate-to-severe hydronephrosis develops:

   • Decompression should be considered to prevent further renal damage 6
   • Definitive surgical correction may be required, including possible heminephrectomy of the affected pole if function is severely compromised 2

Critical Pitfalls to Avoid

Do not assume this will resolve spontaneously—duplex systems with hydronephrosis require investigation and monitoring. 5, 3

• Incomplete visualization of the kidney during ultrasound may miss obstruction of one duplicated structure; always examine both upper and lower poles separately 3
• Delaying intervention when infection is present can lead to irreversible renal damage and sepsis 6, 3
• Patients with duplicated systems and hydronephrosis are at high risk for recurrent UTIs and should receive prophylactic antibiotics until definitive evaluation is complete 3
• Bilateral involvement or solitary kidney status would significantly alter management urgency 6

Long-Term Monitoring

Regular follow-up is essential to prevent permanent kidney damage. 6, 5

• Serial imaging should monitor for progression of hydronephrosis 1, 6
• Renal function should be tracked with periodic creatinine measurements 6
• A decrease in differential renal function of >5% on consecutive renal scans or T1/2 >20 minutes on diuretic renography indicates need for surgical intervention 1