As shown in Plate 2-1, the ureteric buds appear toward the caudal ends of the mesonephric ducts at 5 weeks of gestation. Each bud grows into its adjacent mass of metanephric mesenchyme, the precursor of the kidney, to form a ureter, a pelvicalyceal system, and collecting ducts.
Ureteral duplication results from abnormalities of the ureteric bud. It is one of the most common congenital malformations of the urinary tract, with an incidence of approximately 1 in 125. Duplication is more often unilateral than bilateral, and more often incomplete than complete. There does not appear to be any predilection for a particular side.
Complete Ureteral Duplication
In complete ureteral duplication, the kidney is drained by two distinct renal pelves, each of which leads to a ureter with its own insertion into the bladder. This anomaly occurs when a single mesonephric duct sprouts two ureteric buds, each of which induces a separate portion of the adjacent metanephric mesenchyme. The more cranial of the two ureteric buds becomes the collecting system of the upper pole, while the more caudal of the ureteric buds becomes the collecting system of the lower pole. Because of the manner in which the mesonephric ducts exstrophy into the bladder; however, the upper pole ureter terminates at an oriﬁce located inferior and medial to that of the lower pole ureter. In many cases, the upper pole ureter has an ectopic site of termination, reﬂecting an especially cranial position of the ureteric bud from which it originated. The consistent pattern of ureteral crossing seen in a duplicated system, where the ureter serving the upper pole terminates inferior to the ureter serving the lower pole, is known as the Weigert-Meyer law.
The upper-pole collecting system tends to serve about one third of the renal parenchyma. The upper pole ureter generally has a long course within the bladder wall, frequently has an ectopic site of termination, and is also prone to ureterocele. Because of these factors, upper pole obstruction and hydroureteronephrosis is common.
Meanwhile, the lower pole ureter tends to have a short course within the bladder wall because of its superior and lateral site of termination. As a result, it is prone to vesicoureteral reﬂux (VUR, see Plate 2-21), which can cause lower pole hydronephrosis if severe.
Complete ureteral duplication may be detected using various imaging techniques, including intravenous pyelography, ultrasound, CT, and renal scanning. A classic ﬁnding is known as the “drooping lily” sign. It consists of downward and lateral rotation of the lower pole segment by an obstructed, hydronephrotic, poorly functioning (and thus nonopaciﬁed) upper pole segment. If the lower pole reﬂux is marked enough, the drooping lily sign may be seen on voiding cystourethrogram (VCUG).
Several different patterns of incomplete ureteral duplication may be seen, reﬂecting various embryologic anomalies. Unlike completely duplicated ureters, incompletely duplicated ureters rarely cause obstruction, VUR, or other sequelae. As a result, they are generally asymptomatic and discovered as incidental ﬁndings.
A bifid ureter, or “Y ureter,” occurs when one ureteric bud arises from the mesonephric duct but divides before entering the metanephric mesenchyme. As a result, the upper and lower poles are drained by different renal pelves, but the associated ureters fuse before the ureterovesical junction.
The site of ureteral fusion can occur at any distance from the bladder. In some cases, the ureters do not fuse until the ureterovesical junction, where they form a short common ureteral trunk.
A blind-ending ureter is a rarer kind of incomplete duplication. As with a biﬁd ureter, this anomaly reﬂects early division of a single ureteric bud; however, in this case, one of the ureteric bud branches fails to induce a portion of metanephric mesenchyme. As a result, a biﬁd ureter is seen, but one branch does not drain any renal parenchyma and is thus blind-ending. For unknown reasons, blind-ending ureters occur three times more often in women and are more often found on the right side.
The blind-ending ureteral branch typically terminates adjacent to the distal or middle section of the normal branch. It contains all of the normal layers of the ureteral wall, and it terminates either as a dilated bulb or as an atretic stalk.
Because the blind-ending ureter does not drain renal parenchyma, it cannot be seen on excretory studies, such as intravenous pyelogram or contrast-enhanced CT. As a result, retrograde studies are generally required to establish the diagnosis.
An “inverted Y” ureter is the rarest form of incomplete duplication. It occurs when two ureteric buds appear on the mesonephric duct but fuse before reaching the metanephric mesenchyme. As a result, a single renal pelvis is seen, but the ureter divides as it approaches the bladder, such that two distinct ureteric oriﬁces are seen on the affected side. One of the ureteral branches often has an ectopic oriﬁce or forms a ureterocele, resulting in obstruction.