Duplex kidney - Incomplete duplication
Duplicated collecting systems can be defined as renal units containing 2 pyelocalyceal systems associated with a single ureter or with double ureters. The 2 ureters empty separately into the bladder or fuse to form a single ureteral orifice.
Duplicated collecting systems can be unilateral or bilateral and occur in 15% of the population.
Duplicated systems can be associated with a variety of congenital genitourinary tract abnormalities.
Presentation: Most patients are asymptomatic and genitourinary tract abnormalities are detected incidentally on imaging studies performed for other reasons. Symptomatic patients usually have complete ureteric duplication in which the ureters are prone to developing obstruction, reflux, and infection.
Pathophysiology : During embryogenesis, if a single ureteral bud bifurcates before bifurcation of the ampulla, a duplex kidney results with bifid pelvis or bifid ureter.
If 2 ureteral buds arise from the Wolffian duct, a duplex kidney results with complete ureteral duplication. The ureteral bud associated with the future lower pole separates first from the Wolffian duct and the orifice progresses superiorly and laterally as a result of growth of the urogenital sinus. The common excretory duct, with the remaining ureter still attached, is taken up in to the urogenital sinus. The orifice of the ureter draining the upper pole opens medial and inferior to the orifice draining the lower pole.
A duplex kidney may be drained by a single ureter or by 2 ureters that unite to form a single ureter or drain separately. Usually, the lower pole system is dominant; a large renal pelvis drains the lower pole through many calices. The upper pole pyelocalyceal system may have only a single calyx and a single infundibulum and drain directly into the ureter.
Bifid ureters draining a duplex kidney join to form a single ureter, which can be extravesical (common; Y shaped) or intravesical (V shaped) and usually empties into the bladder.
In complete ureteral duplication with each segment having its own ureteral orifice in the bladder, the Weigert-Meyer rule applies. This rule states that the ureteral orifice of the upper pole moiety inserts into the bladder medial and inferior to both its normal location and the orifice of the ureter draining the lower renal segment. In these cases, the ureter draining the upper pole moiety frequently ends in a ureterocele, whereas reflux into the lower moiety typically occurs.
In incomplete ureteral duplication, uretero-ureteral reflux is seen , also known as the “yo-yo†reflux/ “saddleâ€/â€seesaw†peristalsis, in which urine moves down the cephalad ureter and refluxes up the lower pole ureter and vice – versa.
Certain potential abnormalities result from or are related to duplicated systems. The following are well-known examples:
- Upper pole hydronephrosis from stenosis of the upper pole ureteral orifice
- Ectopic insertion of the upper pole ureter.
- Ectopic ureterocele of the upper pole ureter
- Reflux involving the lower pole from maldevelopment of the valve mechanism
The committee on Terminology, Nomenclature, and Classification of the Section on Urology of the American Academy of Pediatrics suggests the following terms relevant to duplex collecting systems:
- Duplex kidney: The duplex kidney has a single renal parenchyma drained by 2 pyelocalyceal systems.
- Upper or lower pole: The poles represent one component of a duplex kidney.
- Duplex system: The kidney has 2 pyelocalyceal systems and is associated with a single or bifid ureters (partial duplication) or 2 ureters (double ureters) that drain separately into the urinary bladder (complete duplication).
- Bifid system: Two pyelocalyceal systems join at the ureteropelvic junction (bifid pelvis) or the 2 ureters join before draining into the urinary bladder (bifid ureters).
- Double ureters: Two ureters open separately into the renal pelvis superiorly and drain separately into the bladder or genital tract.
- Upper and lower pole ureters: The upper pole ureter drains the upper pole of a duplex kidney while the lower pole ureter drains the lower pole of a duplex kidney.