Duplicated Ureter or Duplex Kidney
Definition of Duplex Kidney and Ureteral Duplication
The duplex kidney is a anatomic variant of the ureter and pyelocaliceal system with a large variability [variants of the duplex kidney]:
Duplex Kidney with Duplicated Ureters:
Duplex kidney with complete duplicated ureters, renal pelvis and two orifices of the ureters.
Ureter fissus or bifid ureter:
Duplex kidney with incomplete ureteral duplication and a common orifice of the ureter.
Bifid renal pelvis:
Duplex kidney with duplication of the renal pelvis and a common ureter.
Different variants of the duplex kidney: duplicated ureter (left), ureter fissus (middle) and bifid renal pelvis (right).
Inverted Y ureteral duplication:
Two separate ureteral buds merge on the way to the metanephric blastema, resulting in a common renal unit. Very rare.
Three independent ureteral orifices with three complete ureters or incomplete ureteral triplication with two orifices of the ureters (see ureter fissus).
Epidemiology of the Duplex Kidney
Prevalence 1:150 (0.67%)
Etiology of the Duplex Kidney
Two ureteral buds:
The reason for a complete ureteral duplication are two ureteral buds on the mesonephric duct (Wolffian duct). Two ureteral buds result in two separate ureters and a duplex kidney in a common renal capsula. The ureteral buds rotate by 180 degrees while being incorporated into the urogenital sinus: the latero-cranial orifice drains the lower pole of the kidney, and the medio-caudal orifice drains the upper pole of the kidney. This relation is called Weigert-Meyer rule.
Division of the ureteric bud:
Incomplete ureteral duplications (bifid ureter, bifid pelvis) are caused by a division of a single ureteric bud on its way to the metanephric blastema.
Pyelocaliceal system of the duplex kidney:
In comparison to a normal kidney with 8–9 calices, the duplex kidney consists of 11–12 calices. The upper pole system is smaller with an average of 3.7 calices.
Pathogenesis of a duplex kidney:
Duplication of the ureter is often associated with vesicoureteral reflux or hydronephrosis and leads to a susceptibility to urinary tract infections. An increased susceptibility to infections may also result in a bifid ureter by pendulum urine (rare).
Signs and Symptoms of a Duplex Kidney
- Associated malformations: vesicoureteral reflux to the lower pole system (40%), ureteropelvic junction obstruction of the lower pole system, ectopic ureteral orifice of the upper pole system, ureterocele of the upper pole ureter, hypoplastic and dysplastic renal parenchyma most common of the upper pole system.
- Without associated malformations, a duplex kidney presents usually without symptoms.
- Associated malformations may lead to urinary tract infections, fever or abdominal tumor
Without associated malformations, a normal finding in renal ultrasound is often diagnosed. A duplex kidney may be recognized if two separated renal pelves are separated by renal parenchyma. It is easier to detect a duplex kidney with ureteroceles or hydronephrosis, most often the upper pole system is affected (fig. ultrasound of a duplex kidney with hydronephrosis of the upper pole system).
Ultrasound imaging of a duplex kidney with hydronephrosis of the upper pole system due to a ureteral stone.
A duplex kidney is often an incidental finding during the diagnostic work-up for other diseases [fig. duplex kidney in IVU]. If the upper pole system has only poor kidney function, imaging with urography may miss the upper portion of the duplex kidney. A non-contrasting upper portion should be suspected if the number of calyces is reduced or by a larger distance between the renal system to the spine. Sometimes, a ureterocele may be delineated in the bladder.
Left-sided duplex kidney in intravenous urography: both parts of the left-sided duplex kidney present without hydronephrosis. The complaints of the patient were not caused by the urinary tract. Note the abdominal mass due to an abdominal aortic aneurysm, which displaces the left psoas shadow. With kind permission of Dr. G. Antes, Kempten.
Voiding cystourethrogram is indicated for recurrent urinary tract infections or dilatation of pyelocaliceal system. A possible ureterocele is detectable in the early filling phase. Reflux into the lower renal system is detectable in 50%.
Cystoscopy and retrograde pyelography is necessary depending on complaints.
Renal scintigraphy enables the separate determination of renal function of the upper and lower part of the duplex kidney for further treatment planning.
Treatment of the Duplex Kidney
Duplex kidney or ureteral duplication is a normal variant and thus not subject to treatment. The accompanying malformations and symptoms guide the therapy (vesicoureteral reflux, ureterocele, ectopic ureter, recurrent urinary tract infections or urinary tract obstruction).
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