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Duplicated Collecting System
Introduction
Duplication of the renal collecting system, also termed duplex kidney, is one of the few renal anomalies more common in females. Duplex kidneys have an upper pole and a lower pole, called moieties, each drained by a ureter. Approximately one-third of the duplex kidney is drained by the upper pole ureter, with two-thirds drained by the lower pole ureter.
Many individuals with duplex kidneys are asymptomatic and have no impairment of renal function. Those who come to attention prenatally or postnatally usually have a complication related to abnormal implantation of one or both ureters. The upper pole ureter may form a ureterocele within the bladder that causes obstruction, resulting in little or no function of the upper pole moiety. The lower pole ureter may have a short intravesical segment that results in vesicoureteral reflux (VUR). Duplex systems that have these complications also have ultrasound (US) findings that facilitate their prenatal detection. Clinically significant duplication of the collecting system may manifest prenatally with upper pole hydronephrosis and dilatation of the upper pole ureter, lower pole hydronephrosis, or a ureterocele within the bladder.
Disorder
Definition
Duplication of the renal collecting system is a condition in which a kidney is divided into separate upper and lower pole moieties, each drained by a ureter. If the duplication is complete , the upper and lower pole ureters drain separately. If the duplication is partial, the ureters fuse to form a bifid ureter before emptying into the bladder. Complete duplication is often associated with a ureterocele, which is a cystic dilatation of the distal end of the ureter, usually within the bladder but occasionally at other sites along the genitourinary tract.
Prevalence and Epidemiology
The prevalence of duplicated collecting systems is difficult to estimate accurately because most cases are not identified either prenatally or in childhood. Prenatally detected renal duplication occurs in approximately 1 : 3000 to 1 : 5000 births, based on data from the 1990s. In autopsy studies, the prevalence of a duplicated system (including partial duplication) is approximately 1 : 125.
Renal duplication is twice as common in females, and 80% of cases complicated by a ureterocele occur in females. The duplication is bilateral in 15% to 20%. Unilateral cases are more likely to affect the left kidney. Duplication is generally considered to be a sporadic anomaly, although an increased incidence has been described in some families, suggesting autosomal dominant inheritance with incomplete penetrance and variable expressivity.
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