What are the definition, epidemiology, clinical presentation, diagnostic work‑up, and management options for a duplicated renal collecting system?

Duplicated Renal Collecting System: Comprehensive Overview

Definition and Epidemiology

A duplicated (duplex) collecting system is a congenital anomaly where two separate pelvicalyceal systems drain a single kidney, occurring in approximately 1-7% of children undergoing imaging for UTI or prenatal hydronephrosis. 1 This represents one of the most common congenital urinary tract anomalies. 2

The condition results from premature division of the ureteric bud during embryological development, creating either:

• Complete duplication: Two separate ureters drain into the bladder independently 2
• Incomplete duplication: Two collecting systems join before reaching the bladder (bifid system) 2

Anatomical Principles: The Meyer-Weigert Rule

The upper pole moiety ureter inserts inferomedially into the bladder (prone to obstruction and ectopic insertion), while the lower pole moiety ureter inserts superolaterally (prone to vesicoureteral reflux). 3 This predictable drainage pattern guides diagnostic thinking, though rare exceptions exist. 3

Clinical Presentation

Pediatric Patients

• Recurrent urinary tract infections (most common presentation) 4
• Enuresis or continuous urinary incontinence (suggests ectopic ureter insertion below the sphincter mechanism) 4
• Prenatal hydronephrosis detected on antenatal ultrasound 5
• Febrile UTI in infancy 5

Adult Patients

• Recurrent pyelonephritis 4
• Flank pain with urinary symptoms 4
• Incidental finding on imaging (asymptomatic cases) 6
• Hematuria 6

Critical pitfall: Duplicated systems may present with differential hydronephrosis affecting only one pole rather than the entire kidney, which can be missed if the sonographer incompletely visualizes the kidney. 4

Diagnostic Work-Up

Initial Imaging: Renal Ultrasound

Renal ultrasound is the primary imaging modality for identifying duplex systems and assessing for associated pathologies including hydronephrosis, ureterocele, or parenchymal abnormalities. 1

Key sonographic findings to identify:

• Separation of the renal sinus echo complex (two distinct collecting systems) 5
• Differential hydronephrosis between upper and lower poles 4
• Dilated ureter(s) extending from the kidney 5
• Ureterocele (cystic dilation at the ureteral insertion) 5
• Bladder wall thickening (suggests outlet obstruction or reflux) 5, 7

Both kidneys must be imaged to identify unilateral versus bilateral disease processes. 5 The bladder should always be included in the examination. 5

Important caveat: Medullary pyramids may mimic hydronephrosis, especially in young patients, and dehydration may mask true obstruction. 5

When to Obtain VCUG (Voiding Cystourethrography)

VCUG should be obtained in patients with duplex kidneys and moderate-to-severe hydronephrosis (SFU grade 3-4 or APRPD >15mm), bladder wall thickening, or recurrent UTIs to evaluate for vesicoureteral reflux. 5, 1

VUR accounts for 30% of urinary tract abnormalities in infants with antenatal hydronephrosis. 5 The lower pole moiety is most commonly affected by reflux due to its lateral insertion. 3

Timing considerations:

• For moderate-to-severe neonatal hydronephrosis: Perform VCUG at 1 month of age 5
• For recurrent UTIs: Obtain after acute infection resolves 1

Advanced Functional Imaging

Tc-99m MAG3 renal scan should be obtained for severe grade 3-4 hydronephrosis to assess split renal function and drainage, particularly when surgical intervention is being considered. 5

MAG3 provides:

• Differential renal function (each moiety and total kidney) 5
• Drainage assessment with furosemide washout (T1/2 >20 minutes suggests obstruction) 5
• Baseline function for monitoring over time 5

Indications for surgical intervention based on MAG3:

• T1/2 >20 minutes (obstructed drainage) 5
• Differential renal function <40% (severely impaired moiety) 5
• >5% decline in function on consecutive scans 5

MR Urography

MR urography provides detailed morphologic and functional information without radiation exposure, particularly valuable for complex anatomy requiring surgical planning, though it may require sedation in young children. 5, 1

MRU is especially useful for:

• Atypical urinary tract anatomy (certain duplicated systems or renal dysgenesis) 5
• Assessing degree of obstruction 5
• Pre-operative planning 1

Limitation: MRU systematically underestimates split renal function compared to MAG3 in kidneys with severely diminished function or severe hydronephrosis. 5

Associated Pathologies and Complications

Upper Pole Moiety Problems (Obstruction)

• Ureterocele (cystic dilation at bladder insertion) 5
• Ectopic ureter insertion (may insert into urethra, vagina, or seminal vesicles) 4, 8
• Ureteropelvic junction obstruction 5
• Primary obstructing megaureter (5-10% of antenatal hydronephrosis) 5

Lower Pole Moiety Problems (Reflux)

• Vesicoureteral reflux (most common lower pole complication) 5, 1
• Recurrent pyelonephritis 1
• Renal scarring 5

Bilateral and Systemic Complications

• Renal scarring (detected in 73-95% of cases with recurrent UTI) 5
• Progressive renal dysfunction (requires urgent intervention) 7
• Hypertension (from scarred renal tissue) 1

Management Strategies

Asymptomatic Duplicated Systems

An asymptomatic adult with a duplicated collecting system and no history of urinary tract issues requires no intervention, only surveillance imaging. 6

Surveillance protocol:

• Annual renal ultrasound to assess for parenchymal changes, hydronephrosis, stones, or masses 1, 6
• Annual urinalysis to screen for infection, hematuria, and proteinuria 6
• Urine culture only if urinalysis suggests infection (routine screening promotes antibiotic resistance) 6

Recurrent UTIs with Documented Reflux

For children under 5 years with documented VUR in a duplex system and recurrent UTIs, initiate continuous antibiotic prophylaxis. 1 This recommendation has high-strength evidence. 1

Critical step: Evaluate thoroughly for bladder and bowel dysfunction (BBD) in all toilet-trained children, as this doubles the risk of UTI recurrence and must be addressed before treating VUR. 1 Management of BBD may be as impactful as antibiotic prophylaxis. 1

Surgical Indications

Offer surgical reimplantation or endoscopic correction for:

• Frequent breakthrough infections despite prophylaxis 1
• Persistent high-grade reflux (grades IV-V) 1
• Children >1 year with high-grade reflux and abnormal renal parenchyma 1
• Deteriorating renal function (>5% decline on consecutive MAG3 scans) 5
• Obstructed drainage (T1/2 >20 minutes with differential function <40%) 5

Important consideration: Endoscopic injection success rate is significantly lower for duplicate systems (50%) compared to single systems (73%). 1 For lower-grade reflux (I-III), endoscopic correction may be attempted first, but surgical reimplantation provides superior outcomes for higher grades. 1

For non-functional moieties with recurrent complications, partial nephrectomy and ureterectomy may be performed. 8

Red Flags Requiring Urgent Intervention

Immediate parenteral antibiotic treatment is required for any febrile breakthrough infection. 1

Urgent imaging (within 24-48 hours) is indicated for:

• Febrile UTI with inadequate response to antibiotics (exclude obstruction or abscess) 6
• Bladder wall thickening with dilated posterior urethra in male infants (suggests posterior urethral valves) 7
• New hydronephrosis on surveillance ultrasound 6
• Hematuria (requires upper tract imaging and cystoscopy) 6

Critical Pitfalls to Avoid

1. Assuming hydronephrosis always indicates obstruction – it may represent reflux, bladder distention, or postobstructive dilation requiring bladder decompression and repeat imaging 1, 6

2. Missing ectopic ureteral insertion – absence of hydronephrosis does not exclude ureteral pathology in duplex systems 1, 6

3. Incomplete kidney visualization – if only one ureter is identified and the kidney is incompletely visualized, obstruction of one duplicated structure may be missed 4

4. Treating asymptomatic bacteriuria – never treat in non-pregnant patients as it promotes antibiotic resistance 6

5. Inadequate hydration before functional studies – dehydration may mask obstruction on imaging 1

6. Performing ultrasound during acute UTI – may increase false-positive rate for hydronephrosis due to temporary dilation from infection 5

7. Ignoring bladder and bowel dysfunction – must be addressed before treating VUR, as it doubles recurrence risk 1

Long-Term Monitoring and Prognosis

Most primary megaureters (5-10% of antenatal hydronephrosis) will resolve spontaneously. 5 However, patients with existing renal impairment require a more aggressive multidisciplinary approach. 1

Follow-up protocol for managed cases:

• Repeat ultrasound at 1-6 months after initial neonatal scan to re-evaluate after bladder catheterization 5
• Annual renal ultrasound for long-term surveillance 1
• Serial MAG3 scans if obstruction suspected or to monitor differential function 5
• Annual urinalysis with culture if indicated 1

Prophylactic antibiotics and urgent urologic follow-up are essential, as untreated duplicated systems with complications can lead to irreversible renal damage. 4