Discussion

Duplication of the renal collecting system, also known as duplex kidney, is a condition in which the kidney has two pyelocaliceal systems, dividing it into an upper and lower pole each drained by a ureter [1,2]. When duplication is complete, the two ureters drain into the bladder separately. When it is incomplete or partial, the ureters fuse to become a bifid ureter before entering the bladder. The anatomical relationship of both ureters as they enter the bladder is so consistent that it has been termed the Weigerr-Meyer rule: the lower pole ureter is the analogue of the single-system ureter and enters the bladder in its normal position in the trigone. The upper pole ureter is ectopic, inserting inferior and medially to the orthotopic ureteral orifice [1,3]. In cases of complete duplication, the upper pole ectopic ureter can end in a ureterocele, which corresponds to a cystic dilation of the submucosal segment of the intravesical ureter and is associated with a narrowing of its orifice. The lower pole ureter has an intravesical portion that is shorter than normal, which may result in vesicoureteral reflux. When duplication is incomplete, the implantation of the bifid ureter into the bladder also predisposes to vesicoureteral reflux [3].

The true incidence of duplicated renal collecting system in the population has been the subject of continued conjecture, with a variable incidence ranging from 1 in 17 to 1 in 303 according to different sources (radiological, surgical, urological, or autopsy) [4]. Prenatal diagnosis has improved in recent years, with this anomaly being identified in approximately 1 in 1250 ultrasounds performed during the first and second trimesters, a figure that increases to 1 in 875 pregnancies when the third trimester examination is also included [5]. Ultrasound has good diagnostic accuracy in identifying duplex collecting systems prenatally, with approximately 90% of diagnoses confirmed at birth [6]. Most prenatal series report 20% of cases affected bilaterally, and a female predominance (2:1 female-to-male ratio) [2,4,7-9]. Although it is usually sporadic, autosomal dominant inheritance attributed to heterozygous TNXB gene mutations has been described in a family with members presenting with vesicoureteral reflux and/or duplex collecting systems [10].

The most important event in the embryonic development of the urinary tract and the kidney is the interaction between the mesonephric duct and the metanephric mesenchyme [3]. The ureteric bud arises from the primitive mesonephric (Wolffian) duct and migrates to meet the metanephros, inducing the formation of nephrons. In turn, the metanephros signals the ureteric bud to bifurcate to form the renal collecting system. With complete duplication, an extra ureteric bud arises from the mesonephric duct and fuses with the metanephric mesenchyme at a separate site from the original bud. Incomplete or partial duplication occurs when a single ureteric bud divides before fusing with the metanephric mesenchyme.

Ureters are not easily visible on antenatal ultrasound since the ureteral walls are very thin and difficult to distinguish from surrounding structures. They may be visualized as an echolucent tubular structure extending from the renal pelvis towards the bladder, sometimes with regular contractions called peristalsis. In a study reviewing nearly 100 normal pregnancies, the ureters were visible in approximately 60% of fetuses [12]. The ureter examination was performed in the coronal plane at the level of the lumbar vertebrae, then the transducer was tilted anteriorly to view the kidneys, bladder and pelvic vessels. The ureters were sought emerging from the renal pelvis, crossing over the iliac arteries and entering the bladder. Doppler imaging was used to identify the iliac arteries. Peristalsis assisted in visualizing the ureters but was not always present [12]. This important sonographic clue can be appreciated if the ureter is sufficiently filled with urine, though patient cooperation and prolonged observation may be required.

The most characteristic finding of a duplex collecting system on prenatal ultrasound is the presence of two separate non-communicating renal pelves with an intervening band of renal tissue. Imaging each kidney in at least two planes, including sagittal or coronal views, can capture both pelves and both renal arteries in the same image [1,6,7,11]. Eccentric location of a dilated single renal pelvis high or low in the kidney should lead to careful evaluation of the renal parenchyma, the ureter exiting the pelvis, and the bladder. Key features of a duplicated ureter on ultrasound includes two parallel tubular structures running from the kidney to the bladder with absence of Doppler flow and visible peristalsis during real-time scan. In many cases, the diagnosis of duplicated collecting system is only made indirectly after visualization of a ureterocele and/or dilatation of one or both renal pelves.  In a metanalysis reviewing nearly 300 fetuses diagnosed with duplex collecting system, 70% had a ureterocele, 60% had dilation of one renal pelvis and 15% had dilation of both renal pelvises [6]. Dilatation of the upper pole moiety is frequently more severe and accompanied by ureteral dilatation, usually secondary to a ureterocele which is visualized as a thin-walled anechoic cyst within the bladder. Most ureteroceles (80%) are associated with a double urinary collecting system, however they can also occur in a single system with urinary tract dilation or in a multicystic dysplastic kidney [8]. Rarely, the ureterocele can protrude into the lower urinary bladder, causing mechanical obstruction of the urethra and anhydramnios. Lower pole ureteral dilatation suggests vesicoureteral reflux, which should be further evaluated after birth. Fetal magnetic resonance imaging can be used as an adjunct in detecting the existence of two separate pelvicalyceal systems [13].

Following the prenatal diagnosis of a duplicated collecting system, a detailed ultrasound is indicated to evaluate for associated anomalies. Amniocentesis should be offered, though if the duplicated collecting system is isolated, the probability of an abnormal chromosomal microarray analysis is comparable to that of the general population [14,15]. A repeat ultrasound examination in the third trimester is recommended to assess for hydronephrosis and monitor the amniotic fluid volume [15,16]. If the diagnosis of a duplex collecting system is made prenatally, neonatal evaluation typically begins with an initial ultrasound 3 to 5 days after birth, followed by voiding cystourethrogram to assess for vesicoureteral reflux and radionuclide renogram to assess kidney function [6]. A major benefit of prenatal diagnosis is the use of prophylactic antibiotics to prevent urinary tract infection [7]. In the absence of obstruction or vesicoureteral reflux, many individuals with a duplex collecting system will be asymptomatic with no renal compromise throughout their lives. Others are diagnosed incidentally during evaluation for hypertension or renal failure. Significant pathology is usually due to the presence of vesicoureteric reflux, which usually involves only the lower pole ureter in 90% of cases, or obstruction, occurring in the upper renal moiety in 85% of cases [2].

The differential diagnosis includes findings frequently found in the duplicated collecting system, such as ureterocele and vesicoureteral reflux, in addition to other pathologies such as ureteropelvic junction obstruction, congenital megaureter, multicystic dysplastic kidney, and solitary renal cyst [1,2,6].

References

1. Chao TT-M and Dashe JS. Duplicated Collecting System. In: Copel JA, ed. Obstetric Imaging. Fetal diagnosis and care, 3rd ed. Elsevier, Philadelphia, PA, 2026; pag 408-411.e1.
2. Whitten SM, Wilcox DT. Duplex systems. Prenat Diagn. 2001 Nov;21(11):952-957.
3. Didier RA, Chow JS, Kwatra NS, et al. The duplicated collecting system of the urinary tract: embryology, imaging appearances and clinical considerations. Pediatr Radiol. 2017 Oct;47(11):1526-1538.
4. Privett JT, Jeans WD, Roylance J. The incidence and importance of renal duplication. Clin Radiol. 1976 Oct;27(4):521-530.
5. Ficara A, Syngelaki A, Hammami A, et al. Value of routine ultrasound examination at 35-37 weeks' gestation in diagnosis of fetal abnormalities. Ultrasound Obstet Gynecol. 2020 Jan;55(1):75-80.
6. Bascietto F, Khalil A, Rizzo G, et al. Prenatal imaging features and postnatal outcomes of isolated fetal duplex renal collecting system: A systematic review and meta-analysis. Prenat Diagn. 2020 Mar;40(4):424-431.
7. Adiego B, Martinez-Ten P, Perez-Pedregosa J, et al. Antenatally diagnosed renal duplex anomalies: sonographic features and long-term postnatal outcome. J Ultrasound Med. 2011 Jun;30(6):809-815.
8. Turkyilmaz G, Cetin B, Sivrikoz T, et al. Antenatally detected ureterocele: Associated anomalies and postnatal prognosis. Taiwan J Obstet Gynecol. 2019 Jul;58(4):531-535.
9. Whitten SM, McHoney M, Wilcox DT, et al. Accuracy of antenatal fetal ultrasound in the diagnosis of duplex kidneys. Ultrasound Obstet Gynecol. 2003 Apr;21(4):342-346.
10. Gbadegesin RA, Brophy PD, Adeyemo A, et al. TNXB mutations can cause vesicoureteral reflux. J Am Soc Nephrol. 2013 Jul;24(8):1313-1322.
11. Abuhamad AZ, Horton CE Jr, Horton SH, Evans AT. Renal duplication anomalies in the fetus: clues for prenatal diagnosis. Ultrasound Obstet Gynecol. 1996 Mar;7(3):174-177.
12. Karasu AFG, Yuksel A, Kutuk MS, et al. Sonographic depiction of fetal ureters. J Matern Fetal Neonatal Med. 2016;29(14):2378-81.
13. Ji H, Dong SZ. Prenatal diagnosis of renal duplication by magnetic resonance imaging. J Matern Fetal Neonatal Med. 2020 Jul;33(14):2342-2347.
14. Singer A, Maya I, Frumkin A, et al. Is fetal isolated double renal collecting system an indication for chromosomal microarray? J Matern Fetal Neonatal Med. 2021 Mar;34(5):696-700.
15. Khatib N, Bronshtein M, Bachar G, et al. Fetal Renal Duplicated Collecting System at 14-16 Weeks of Gestation. J Clin Med. 2023 Nov 16;12(22):7124.
16. Society for Maternal-Fetal Medicine (SMFM); Hopkins LM. Duplicated collecting system. Am J Obstet Gynecol. 2021 Nov;225(5):B12-B13.