Duplex renal collecting systems are common congenital anomalies of the upper urinary tract. In most cases they are incidental findings and not associated with additional pathologies. They demonstrate, however, higher incidences of hydroureteronephrosis, ureteroceles, and ectopic ureters. The most comprehensive morphologic and functional evaluation of duplex systems can be achieved using magnetic resonance urography. Functional magnetic resonance urography allows better separation of the renal poles, thus more accurate calculation of the differential renal functions compared with renal scintigraphy. Magnetic resonance urography is the study of choice when upper urinary tract anatomy is complex or when functional evaluation is needed.
Key points
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Duplex renal collecting systems are common congenital anomalies of the upper urinary tract.
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Complicated duplex renal systems may require further evaluation using cross-sectional imaging and/or functional imaging after an ultrasound (US) and voiding cystourethrography (VCUG).
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Functional magnetic resonance urography (fMRU) provides comprehensive morphologic and functional evaluation of duplex kidneys.
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Magnetic resonance urography (MRU) can demonstrate barely or nonfunctioning renal poles as well as ectopic ureters.
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fMRU allows better delineation of each pole of a duplex kidney, leading to improved calculation of differential renal function.
Duplex renal collecting systems are one of the most common congenital anomalies of the urinary tract. The exact prevalence of this anomaly is difficult to ascertain because most patients are asymptomatic, and the abnormality is frequently detected incidentally on imaging studies performed for other reasons. In general, it is estimated that ureteral duplication occurs in 1 in 125 (0.8%) of the autopsy population. Hartman and Hodson reported a higher incidence of 2% to 4% in a clinical series of patients with urinary symptoms. In a large study of 700 children presenting with urinary tract infection, ureteral duplication was identified in 8% of the patients. The right and left kidneys are affected equally. Bilateral duplication occurs in approximately 20% to 40% of the affected individuals. Girls are affected 2 times more often than boys. Duplication may be transmitted as an autosomal dominant trait with incomplete penetrance, with an incidence of approximately 8% among members of affected families.
Terminology
A standard set of terminologies is used in describing duplex systems based on the recommendation by the Committee on Terminology, Nomenclature and Classification of the Section on Urology of the American Academy of Pediatrics.
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Duplex kidney refers to a single renal parenchymal unit that is drained by 2 pyelocaliceal (pelvicalyceal) systems.
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Upper poles or lower poles represent 1 of the components of a duplex kidney.
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Duplex system refers to a kidney that has 2 pyelocaliceal systems and is associated with a single or bifid ureter (partial/incomplete ureteric duplication) or, in cases of a complete duplication, 2 ureters (double ureters) that drain separately into the urinary bladder.
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Bifid system refers to 2 pyelocaliceal systems that join at the ureteropelvic junction (bifid pelvis) or 2 ureters that join before draining into the urinary bladder (bifid ureters).
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Double ureters are 2 independent ureters that drain separate pyelocaliceal systems and open separately intravesical or extravesical.
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Upper pole ureter (UPU) and lower pole ureter (LPU) drain the upper pole and lower pole, respectively, of a duplex kidney.
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Upper pole or lower pole orifice refers to the orifice in the bladder associated with the ureter draining the upper pole or lower pole.
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Ectopic ureter refers to either a medially (caudal) ectopic ureter, where the ureter (most often upper pole) inserts medial and distal (inferior) to the normal position of the trigone, or a laterally (cranial) ectopic ureter, where the ureteral orifice is situated lateral to the normal position. Ureteral ectopy can be intravesical or extravesical.
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Ureterocele refers to cystic dilation of the intravesical submucosal distal ureter. Ureteroceles contained entirely within the bladder are classified as intravesical ureteroceles, whereas ectopic (extravesical) ureteroceles contain a portion permanently situated at the bladder neck or in the urethra. A single-system ureterocele is associated with a kidney with only 1 ureter. A duplex system ureterocele is associated with the upper pole of a kidney with a complete ureteral duplication.
Terminology
A standard set of terminologies is used in describing duplex systems based on the recommendation by the Committee on Terminology, Nomenclature and Classification of the Section on Urology of the American Academy of Pediatrics.
- •
Duplex kidney refers to a single renal parenchymal unit that is drained by 2 pyelocaliceal (pelvicalyceal) systems.
- •
Upper poles or lower poles represent 1 of the components of a duplex kidney.
- •
Duplex system refers to a kidney that has 2 pyelocaliceal systems and is associated with a single or bifid ureter (partial/incomplete ureteric duplication) or, in cases of a complete duplication, 2 ureters (double ureters) that drain separately into the urinary bladder.
- •
Bifid system refers to 2 pyelocaliceal systems that join at the ureteropelvic junction (bifid pelvis) or 2 ureters that join before draining into the urinary bladder (bifid ureters).
- •
Double ureters are 2 independent ureters that drain separate pyelocaliceal systems and open separately intravesical or extravesical.
- •
Upper pole ureter (UPU) and lower pole ureter (LPU) drain the upper pole and lower pole, respectively, of a duplex kidney.
- •
Upper pole or lower pole orifice refers to the orifice in the bladder associated with the ureter draining the upper pole or lower pole.
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Ectopic ureter refers to either a medially (caudal) ectopic ureter, where the ureter (most often upper pole) inserts medial and distal (inferior) to the normal position of the trigone, or a laterally (cranial) ectopic ureter, where the ureteral orifice is situated lateral to the normal position. Ureteral ectopy can be intravesical or extravesical.
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Ureterocele refers to cystic dilation of the intravesical submucosal distal ureter. Ureteroceles contained entirely within the bladder are classified as intravesical ureteroceles, whereas ectopic (extravesical) ureteroceles contain a portion permanently situated at the bladder neck or in the urethra. A single-system ureterocele is associated with a kidney with only 1 ureter. A duplex system ureterocele is associated with the upper pole of a kidney with a complete ureteral duplication.
Embryology
Basic understanding of the embryology of urinary tract development is important to describe duplicated renal collecting systems and associated abnormalities. The ureter forms from the ureteral bud, which arises as a diverticulum from a ventral bend or elbow in the mesonephric (wolffian) duct, at the end of the 4th week of gestation. This is the point at which the mesonephric duct bends to enter the cloaca. The ventral cloaca develops into the urogenital sinus and ultimately the urinary bladder and urethra. The portion of the mesonephric duct between the cloaca and the origin of the ureteral bud forms the common excretory duct. This duct gradually expands to be incorporated onto the urogenital sinus and forms a portion of the trigone and the underlying detrusor musculature. The growing ureteral bud penetrates the metanephric blastemal ridge, the kidney progenitor, late in the 5th week. The ureteral bud subsequently forms the ureter, renal pelvis, calyces, papillary ducts, and collecting tubules. The metanephros differentiates into the more proximal portions of the nephron. The process of branching of the ureteric bud is complete by approximately 14 weeks, but new nephrons are produced throughout gestation.
Incomplete ureteral duplication results from a ureteral bud that bifurcates shortly after its origin from the mesonephric duct. If the division occurs after the ureteral bud penetrates into the metanephric blastema (5th week of gestation), a bifid pelvis results. If the division occurs before the 5th week of gestation, varying degrees of ureteral duplication result with fusion distally as a single ureter entering the urinary bladder.
A duplex kidney with complete double ureters requires 2 ureteral buds arising from the mesonephric duct. The bud that arises more cranial on the mesonephric duct extends to the upper pole of the kidney, and the caudally positioned bud drains the lower pole. During the process of incorporation of the buds onto the urogenital sinus, the lower pole orifice makes first contact with the urogenital sinus, rendering it affected more by the rotation and migration forces affecting the ureters and mesonephric duct. As a consequence, its final position is lateral and cranial to the ultimate position of the upper pole orifice. This relationship of the orifices of the UPU and LPU is described in the Weigert-Meyer rule, which states that the UPU is medial and caudal in relation to the lateral and cranially positioned lower pole orifice. Exceptions to this rule occur but are rare.
Typically, the ureteric orifice draining the upper pole develops caudal to the normal location and is often ectopic, somewhere along the pathway of the mesonephric system. In boys, ectopic ureteral orifices always terminate into a suprasphincteric structure, usually into the posterior urethra but also into the ejaculatory ducts, vas deferens, seminal vesicle, or epididymis. In girls, ectopic ureters may be suprasphincteric (anywhere between the trigone and striated sphincter) or infrasphincteric—in the lower urethra, at the introitus, or in the vagina. This explains why urinary dribbling, daytime-nighttime wetness, and incontinence are common presentations in girls but not boys. Other uncommon sites for ectopic ureteral insertion include the uterus, Gartner duct cyst, cervix, perineum, and, rarely, rectum. Varying degrees of renal hypoplasia or dysplasia are frequently associated with ectopically inserting ureters. In general, the more ectopic the ureter is, the more dysplastic the associated renal parenchyma. Severe ectopy with an orifice in the genital system is almost always associated with nonfunctioning renal tissue.
Clinical presentation
Most cases of duplex kidneys are diagnosed in asymptomatic children as incidental findings during imaging for other reasons. Other common clinical presentations, however, include urinary tract infection ( Fig. 1 ), flank/abdominal pain from intermittent ureteropelvic junction obstruction (UPJO) ( Figs. 2 and 3 ), urinary incontinence/dribbling from ectopic ureteral insertion ( Figs. 4 and 5 ), intravaginal/extravaginal mass from a prolapsing ureterocele, epididymo -orchitis in boys with ectopic UPU insertion to the epididymis , and, rarely, a bladder outlet obstruction from a ureterocele. The latter may mimic posterior urethral valves in boys.
Ectopic ureter can be associated with significant urinary tract obstruction, rarely with vesicoureteric reflux (VUR), or both. Currently, many duplex renal systems are identified by prenatal US, particularly if one pole is dilated. It is even possible to differentiate between ectopic ureter and uretroceles in utero using US. In these children, the anomaly is confirmed by US after birth and further work-up includes VCUG and sometimes MRU. The impact of prenatal detection of duplex systems remains uncertain, and prenatal diagnosis at present does not distinguish between those who eventually present clinically with complications and those who remain asymptomatic. It is generally thought that prenatal diagnosis may not reduce the need for surgery but does result in earlier, less complex, and more definitive surgery, with a consequent small reduction in long-term bladder dysfunction.
The natural history of prenatally diagnosed duplex kidneys remains uncertain. For simple, nondilated duplex systems that are not complicated by vesicoureteral reflux or obstruction, there is no need for treatment or intervention. Treatment of complex duplex systems depends on the pathology and presence of dilatation in the renal poles. The goal in medical and surgical treatment options is to conserve as much renal function as possible, by correcting the underlying pathology. Consideration of a surgical procedure is indicated based on the presence of associated abnormalities like VUR, ureterocele, ectopic ureter, obstruction, or nonfunctional moiety. Some pathologic cases may evolve unrecognized up to late childhood, such as ectopically draining upper poles with poor function.
Diagnostic imaging
Duplex kidneys are most often noted as incidental findings on imaging for non-urologic indications, or during the work-up of urologic complaints such as urinary tract infection or incontinence, especially in girls. The uncomplicated duplex kidney is most often detected during an US examination and presents with 2 distinct renal hila separated by a bridge of normal renal parenchyma, namely a hypertrophied column of Bertin. This is excellently demonstrated on US by depicting in transverse plane 3 distinct levels (ie, an upper hilum, a mid-kidney area of hilar separation [faceless kidney], and a lower hilum). Unless there is a clinical indication, no further examination is necessary for a noncomplicated duplex system.
Prenatal diagnosis of duplex systems and associated anomalies is frequently made on antenatal US and possibly MR imaging ( Fig. 6 ), particularly if there is dilatation of the collecting system. Suggestive antenatal sonographic and MR imaging features of complicated duplex systems include hydroureteronephrosis of upper pole or lower pole (or both), an apparent cystic structure within 1 pole, intravesical ureteroceles, and, rarely, ectopic ureters. These findings need to be confirmed using postnatal imaging, starting with US. Further evaluation with VCUG may be performed to assess for VUR, particularly in the setting of lower pole renal collecting system dilatation. MRU further provides valuable diagnostic information, particularly in complicated duplex systems. Evaluation of ectopic ureteral insertion can be difficult with US. MRU is able to demonstrate the ectopic extravesical insertion, even in cases where the upper pole parenchyma is small or functions poorly (see Figs. 4 and 5 ). Moreover, MRU provides surgical planning for complicated duplex renal systems and thoroughly assesses the quality of parenchyma and function in complex anatomic cases, particularly during early infancy.
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