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The normal urinary collecting system develops from a single ureteric bud arising from the mesonephric (Wolffian) duct. The ureteric bud migrates to meet the primitive kidney (metanephros), where it develops into the collecting system. Abnormalities in the signaling pathways are thought to result in a variety of collecting system variants, including two ureteric buds arising from the mesonephric duct (complete duplication) or one bud that bifurcates during development (incomplete duplication).
The term duplicated (or duplex) collecting system is defined as one kidney with two separate, noncommunicating renal pelves. In complete duplication, each pelvis has an associated ureter that inserts independently into the bladder. In incomplete duplication, two pelves are present but share a single (bifid) ureter that inserts into the bladder but bifurcates at some point before reaching the two pelves.
Ultrasound Findings
A single kidney measuring slightly larger than normal (above the 95th percentile for gestational age) with two renal pelves is seen longitudinally.
One renal pelvis is seen in the lower pole of the kidney and the second in the upper pole (Figure). Although a bifid ureter or two ureters are present, both are difficult to visualize unless dilated. A duplicated collecting system is estimated to be bilateral in 30% of cases.
The bladder should be inspected for a ureterocele, which appears as a cystic structure at the bladder wall. Of note, if the bladder is full, the ureterocele may be compressed and therefore not seen.
In complete duplication, the ureter draining the upper pole moiety typically has an ectopic insertion site into the bladder located inferior and medial to the normal bladder insertion site. If a ureterocele is present, it is usually associated with the ectopic insertion site.
Associated Abnormalities
In complete duplication, the upper pole moiety is more prone to abnormalities than the lower pole moiety. The abnormality is usually hydronephrosis because of an obstructive process. The lower pole moiety typically has a normal appearance, although vesicoureteral reflux can occur.
A duplicated collecting system is usually an isolated finding and only rarely seen in aneuploidy or genetic syndromes. A detailed evaluation of the fetal anatomy is indicated if a duplicated system is seen, although additional anomalies are uncommon.
Differential Diagnosis
The fetus with a duplicated collecting system typically has a normal-sized bladder. When hydronephrosis is the initial finding, the differential diagnoses include both physiological or transient hydronephrosis and other congenital anomalies of the kidney and urinary tract. Assessment of bladder size and number of pelves using a longitudinal view is recommended. The findings of a normally sized kidney with a single pelvis rule out a duplicated system as the cause of hydronephrosis. All other causes of hydronephrosis, including ureteropelvic junction obstruction and vesicoureteral reflux, should be considered. An abnormally sized kidney may also be present with urethral atresia, Prune-Belly syndrome, and posterior urethral valves. A ureterocele can occasionally be seen in single collecting systems; therefore, a longitudinal view of the kidney is helpful in differentiating between single and duplex systems.
Finally, an enlarged kidney that lacks a second pelvis can be seen in the presence of a renal tumor. Overall, tumors of the kidney are rare, with mesoblastic nephroma the most commonly detected. This tumor arises exclusively in the third trimester of pregnancy and demonstrates rapid growth.
If a duplicated collecting system is seen as an isolated anomaly and the family history is unremarkable, the chance of a genetic syndrome is low. A genetic component of this anomaly is suspected because duplicated systems seem to have an autosomal dominant inheritance in some families.
If no further abnormalities are noted on ultrasound, no genetic evaluation beyond standard aneuploidy screening is typically recommended.
Pregnancy and Delivery Management
An ultrasound examination during the third trimester of pregnancy is recommended to assess for hydronephrosis and monitor the amniotic fluid volume. Otherwise, standard obstetrical care and delivery are warranted.
Prognosis
If neither renal pelvis demonstrates hydronephrosis prenatally, the duplicated system is considered a normal variant, and the prognosis is excellent. However, if hydronephrosis is present, postnatal evaluation is indicated. Obstruction, reflux, incontinence, and infection can occur, which can lead to renal scarring or dysplasia if left untreated.
Summary
A duplicated collecting system in the fetus is a relatively common finding that is typically isolated in nature and not significantly associated with chromosomal abnormalities or genetic syndromes. On ultrasound, a longitudinal view of the kidney is usually necessary to appreciate this anomaly, which is defined by two noncommunicating renal pelves in a single kidney. A duplicated collecting system detected antenatally should be followed up with an ultrasound examination in the third trimester to assess for hydronephrosis in one or both pelves. If hydronephrosis does occur, postnatal follow-up is indicated.
References
Uetani N.
Bertozzi K.
Chagnon M.J.
Hendriks W.
Tremblay M.L.
Bouchard M.
Maturation of ureter-bladder connection in mice is controlled by LAR family receptor protein tyrosine phosphatases.
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