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Prof. Dr. Ali Avanoğlu
Vesicoureteral Reflux (VUR) • By age and special situations

Duplex Collecting System (“Double Kidney”), Ureterocele, and Vesicoureteral Reflux

In a duplex collecting system (duplication) — popularly called a “double kidney” — two separate urine channels run from one kidney down to the bladder, and this arrangement brings vesicoureteral reflux (VUR) along with it in a particular pattern: the lower half's channel causes reflux, while the upper half's channel is obstructed by a ureterocele or an ectopic opening. On this page we explain the anatomy, the outlook for resolution, and the treatment options.

Who this is forFamilies of children found on ultrasound to have a duplex collecting system, ureterocele, or ectopic ureter, and who also have reflux
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ByProf. Dr. Ali AvanoğluUpdated
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01 · What is a duplex collecting system (“double kidney”)?02 · The Weigert-Meyer rule: what does each half do?03 · Diagnosis: ultrasound, VCUG, and DMSA read together04 · Outlook for resolution: why is it lower?05 · Surgical options: which one, and when?
In brief5 madde
  • What is a duplex collecting system (“double kidney”)?: A duplex collecting system is the splitting, during development, of the kidney's urine-collecting basin and channel system into two; it is popularly called a “double…
  • The Weigert-Meyer rule: what does each half do?: In complete duplication, where the two channels open into the bladder is not random; according to the pattern known as the Weigert-Meyer rule, the ureter of the…
  • Diagnosis: ultrasound, VCUG, and DMSA read together: In a duplex system, ultrasound most often gives the first clue: dilation in the upper half of the kidney, the appearance of two separate collecting basins, a…
  • Outlook for resolution: why is it lower?: Spontaneous resolution in lower-pole reflux associated with complete duplication is reported to be lower than for the same grade in a single system, because the…
  • Surgical options: which one, and when?: In a duplex system, treatment is chosen by weighing the lower pole's reflux, the upper pole's obstruction, and the upper pole's function together; there is no single…
01

What is a duplex collecting system (“double kidney”)?

A duplex collecting system is the splitting, during development, of the kidney's urine-collecting basin and channel system into two; it is popularly called a “double kidney,” but there are not actually two separate kidneys — there is a single kidney with two collecting portions.

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  • It occurs in about 1% of the population and is most often an incidental, trouble-free finding. If the two channels join before reaching the bladder, this is called “partial (incomplete)” duplication; if they open into the bladder through two separate openings, it is called “complete” duplication. Reflux and obstruction problems mainly arise with complete duplication; in partial duplication, urine passing back and forth between the two channels at their point of union (“yo-yo reflux”) rarely causes symptoms.
  • A duplex system is noticed on antenatal ultrasound through kidney enlargement or a cyst-like appearance in the bladder (ureterocele), and afterward through a febrile urinary tract infection or, in girls, constant wetness. This finding is also important for diagnosis because it changes both the cause of the reflux and the path of treatment.
02

The Weigert-Meyer rule: what does each half do?

In complete duplication, where the two channels open into the bladder is not random; according to the pattern known as the Weigert-Meyer rule, the ureter of the kidney's lower half (lower pole) opens into the bladder higher up and more laterally, while the ureter of the upper half (upper pole) opens lower down and more medially.

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  • This arrangement has a clinical consequence in two directions. Because the lower-pole ureter's opening stays higher and more lateral, its tunnel through the bladder wall is short and its valve function is weak; reflux therefore occurs mostly in the lower pole. The upper-pole ureter, on the other hand, can open lower down in the bladder, at the bladder neck, or outside the bladder entirely (the urethra or vagina in girls; the prostatic urethra in boys); this is called an ectopic ureter. If the upper-pole ureter's opening within the bladder balloons outward to form a cystic sac, this is called a ureterocele; a ureterocele both obstructs the upper pole and can disrupt the tunnel of the neighboring lower-pole opening, increasing reflux.
  • Because of this obstruction, upper-pole tissue is often underdeveloped (dysplasia) and shows low function on DMSA (renal scan); the lower pole, meanwhile, carries a risk of scarring (permanent kidney damage) from reflux and infection. Having two different problems together in the same kidney makes it hard to fit treatment into a single mold.
03

Diagnosis: ultrasound, VCUG, and DMSA read together

In a duplex system, ultrasound most often gives the first clue: dilation in the upper half of the kidney, the appearance of two separate collecting basins, a thin-walled sac within the bladder (ureterocele), or a dilated ureter behind the bladder.

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  • VCUG (voiding cystourethrogram) shows which pole the reflux involves and its grade; in lower-pole reflux, the typical appearance is a “drooping lily” sign, where only the lower portion of the kidney fills and the number of calyces is reduced. A ureterocele appears on VCUG as a filling defect within the bladder; with an ectopic ureter, reflux can also occur into a channel that opens outside the bladder. DMSA measures the function and scarring of each pole separately; whether upper-pole function is preserved is the most decisive piece of data for the surgical decision.
  • In a girl, constant, drop-by-drop wetness — by day and night — despite completed toilet training raises suspicion of an ectopic ureter; this differs from the usual pattern of urinary incontinence (enuresis) and should be investigated with imaging.
04

Outlook for resolution: why is it lower?

Spontaneous resolution in lower-pole reflux associated with complete duplication is reported to be lower than for the same grade in a single system, because the problem is not just the shortness of the tunnel but the position of the opening — and growth does not change that position.

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  • Even so, resolution can be seen in the first years with low-grade (I–II) lower-pole reflux; for this reason, surveillance and, if needed, antibiotic prophylaxis (CAP) remain the first option in a young child without infection and with preserved kidney function. With high-grade, dilated lower-pole reflux, or when a ureterocele is also present, the outlook for resolution decreases markedly and a treatment plan is put in place earlier.
  • Guidelines list duplex system, ureterocele, and ectopic ureter under the heading of “anatomical anomaly” in reflux treatment, and recognize this condition as one of the factors that steers the case toward corrective treatment.
05

Surgical options: which one, and when?

In a duplex system, treatment is chosen by weighing the lower pole's reflux, the upper pole's obstruction, and the upper pole's function together; there is no single correct method — there is a path shaped by the child's anatomy.

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  • Endoscopic injection (Deflux) can also be used in a duplex system, but success is reported to be lower than in a single system and repeat treatment is needed more often; it is a reasonable first step for low-to-moderate grade lower-pole reflux when the family prefers a same-day procedure. If a ureterocele is present, the first step is usually puncturing it with a cystoscope (endoscopic incision); this resolves the obstruction immediately, but new reflux can develop into the upper pole afterward, and a second procedure is needed in some cases. When a lasting correction is needed, because the two ureters run within a single common sheath, they are reimplanted together (common-sheath ureteral reimplantation); alternatively, the lower-pole ureter can be joined to the upper-pole ureter, reducing them to a single channel (uretero-ureterostomy or pyelo-ureterostomy). If the upper pole is non-functioning and is a source of infection or wetness, removal of the upper pole and its ureter (upper-pole heminephrectomy; open or laparoscopic) is an option.
  • In our practice, these decisions are made through the joint evaluation of the two pediatric urologists, reading pole function on DMSA, reflux grade, infection history, and the child's age together; because endoscopic procedures and reimplantation are performed by the same team, the physician does not change between steps.
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Frequently asked questions
Is a double kidney a disease — does my child count as having only one kidney?
No. A duplex collecting system is a developmental variant; there is a single kidney with two portions, and the kidney on the other side is separately present as usual. In most children it causes no problem at all; problems arise only when reflux, a ureterocele, or an ectopic opening is also present.
Does lower-pole reflux resolve on its own?
It can resolve at low grade, but the chance is lower than in a single system, because the position of the opening does not change with growth. Surveillance remains the first option in a young child without infection; if the grade is high or a ureterocele is present, a treatment plan is put in place earlier.
Does a ureterocele always require surgery?
A ureterocele that causes obstruction and infection usually needs intervention; the first step is often a small procedure such as puncturing (incision) with a cystoscope. If reflux develops afterward, or if the upper pole is non-functioning, a second stage can be planned. Small, trouble-free ureteroceles can be followed with surveillance.
My daughter has constant wetness even though toilet training is complete; could this be related?
Yes. Drop-by-drop wetness that continues day and night despite toilet training raises suspicion of an ectopic ureter; the upper-pole ureter may be opening outside the bladder. This differs from the usual pattern of urinary incontinence and should be investigated with ultrasound and, if needed, MR urography.
←Previous • By age and special situationsSecondary VUR: Neurogenic Bladder and Posterior Urethral Valves
Related pagesFull index →
Diagnosis and evaluationKidney Enlargement Before Birth (Antenatal Hydronephrosis) and VURHearing the words ‘kidney enlargement’ or ‘kidney dilation’ on a pregnancy ultrasound worries families; yet most antenatal hydronephrosis is transient and resolves on its own after birth. Even so, roughly 10–20% of these infants are found to have vesicoureteral reflux (VUR). This page explains what the enlargement means and which tests are done, and when, after birth.BasicsWhat Causes VUR? Primary and Secondary RefluxVesicoureteral reflux (VUR) results from the tunnel through which the ureter passes within the bladder wall failing to act properly as a valve. In most children, this tunnel is short from birth (primary VUR); in a smaller group, high pressure inside the bladder overwhelms the valve instead (secondary VUR). This distinction determines where treatment starts.Treatment pathwaysUreteral Reimplantation (Anti-Reflux Surgery): Cohen, Lich-GregoirUreteral reimplantation, used in vesicoureteral reflux (VUR), reconnects the ureter to the bladder through a new, longer tunnel. This operation, which has the highest success rate among reflux treatments, eliminates reflux with roughly 95–98% success in the literature. This page explains the principle behind the surgery, the techniques used, possible complications, and what to expect in hospital.Treatment pathwaysEndoscopic Injection (Deflux / Subureteral Bulking) TreatmentEndoscopic injection is a treatment that aims to correct vesicoureteral reflux (VUR) through a same-day procedure with no incision. A cystoscope is used to enter the bladder, and a bulking agent (Deflux) is injected beneath the ureteral opening to strengthen the valve function. This page explains how the procedure is performed, success rates by grade, and its limitations.
Vesicoureteral Reflux (VUR) topic clustersMain treatment page →
Basics 6Diagnosis and evaluation 6By age and special situations 5Bladder and bowel 5Treatment pathways 10Kidney health and infection 5After treatment and follow-up 6
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