Introduction
Background
Duplicated collecting systems (also known as duplex collecting systems) can be defined as renal units containing 2 pyelocaliceal systems that are 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.
Duplex collecting systems can be unilateral or bilateral and can be associated with a variety of congenital genitourinary tract abnormalities. Most patients are asymptomatic, with genitourinary tract abnormalities being 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. Ureteropelvic obstruction is more common when a duplex kidney exists and can be inherited in an autosomal dominant pattern.1
See also the following related eMedicine topic:
Ureteral Duplication, Ureteral Ectopia, and Ureterocele
Pathophysiology
During embryogenesis, if a single ureteral bud bifurcates before bifurcation of the ampulla, a duplex kidney results, with a bifid pelvis or a bifid ureter. If 2 ureteral buds arise from the Wolffian duct, a duplex kidney with complete ureteral duplication results. The ureteral bud that is associated with the future lower pole separates first from the Wolffian duct, and the orifice progresses superiorly and laterally as a result of the urogenital sinus's growth. The common excretory duct, with the remaining ureter still attached, is taken up into the urogenital sinus. The orifice of the ureter that drains the upper pole opens medial and inferior to the orifice that drains the lower pole.
A duplex kidney may be drained by a single ureter or by 2 ureters. In the latter case, the ureters can unite to form a single ureter or they can drain separately. Usually, the lower pole system is dominant; a large renal pelvis drains the lower pole through many calices. The upper pole pyelocaliceal 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 intravesical (V-shaped) or, commonly, extravesical (Y-shaped). The ureter usually empties into the bladder.
Rarely, a single proximal ureter divides distally to form an inverted Y. Usually, 2 ureteral orifices are observed on the same side; rarely, one of the ureters can be ectopic. In infrequent cases, the middle third of the ureter is duplicated, with a single proximal third and a single distal third. Double ureters remain completely separated, and in approximately 85% of patients, the upper pole ureter drains below and medial to the lower pole ureter (Weigert-Meyer rule).
Certain potential abnormalities result from or are related to duplex 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, resulting from maldevelopment of the valve mechanism
See also the following related eMedicine topics:
Hydronephrosis and Hydroureter
Ureterocele
Frequency
United States
The overall incidence of duplex collecting systems appears to be the same in the United States as it is internationally.
International
Complete duplication of ureters occurs in 0.2% of live births; the chance of occurrence is 12% in first-degree relatives of persons with this condition. Bilateral duplication occurs in 15-40% of individuals with complete duplication. The prevalence of partial duplication found on urograms is 0.6%.2
Mortality/Morbidity
Most cases of duplex collecting system are detected incidentally; however, the incidence of ureteropelvic obstruction is increased with duplex kidney, hydronephrosis, and pyelonephritis in patients with complete ureteral duplication.1,3
Race
No race predilection has been recorded.
Sex
No sex predilection is found among patients with bifid collecting systems and partial ureteric duplication. The presence of double ureters appears to be twice as common in females. Duplex kidney with uterus didelphys has been reported in identical twins.
Age
A duplex collecting system is a developmental anomaly, and patient age at presentation varies according to the type of abnormality. Patients with duplex kidney are usually asymptomatic, which means that, unless complications arise, duplex kidney is detected incidentally on imaging studies performed for other reasons.
- Because of the widespread use of antenatal ultrasonography, duplex collecting systems are being diagnosed in utero.
- An ectopic ureter observed in a patient with complete ureteral duplication can present in the early stage, especially in female patients. This is because the ectopic ureter may insert below the sphincter or outside of the urinary tract (eg, vagina).
- Duplex ureter complicated by transitional cell carcinoma occurs in the elderly population.4
See also the following related topics in eMedicine:
Transitional Cell Carcinoma
Transitional Cell Carcinoma, Renal
Anatomy
When a single ureteral bud bifurcates before the ampulla bifurcates, a duplex kidney with a bifid renal pelvis or bifid ureter results.5 If 2 ureteral buds arise from the Wolffian duct, a duplex kidney with complete ureteral duplication ensues. The Committee on Terminology, Nomenclature, and Classification of the Section on Urology of the American Academy of Pediatrics suggests the use of the following terms in reference to duplex collecting systems6 :
- Duplex kidney - The duplex kidney has a single renal parenchyma that is drained by 2 pyelocaliceal systems.
- Upper or lower pole - The poles represent one component of a duplex kidney.
- Duplex system - The kidney has 2 pyelocaliceal systems and is associated with a single ureter or with a bifid ureter (a partial duplication) or, in the case of a complete duplication, with 2 ureters (double ureters) that drain separately into the urinary bladder.
- Bifid system - Two pyelocaliceal systems join at the ureteropelvic junction (bifid pelvis), or 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 - Upper and lower pole ureters drain a duplex kidney's upper and lower poles, respectively.
Presentation
Presentation in patients with a duplex collecting system varies according to the type of anomaly. Patients with a duplex kidney are usually asymptomatic. Ureteropelvic junction obstruction is more common when a duplex kidney exists.7 Giant hydronephrosis in a duplex kidney can manifest as an extremely large abdominal and retroperitoneal mass; in rare cases, it can cause hypertension.8,9,10
The incidence of vesico-ureteric reflux, urinary tract infection, and parenchymal scarring is increased in patients with a duplex collecting system; patients can present with pyrexia and dysuria.11 Prolapsed ureterocele associated with a duplex ureter can cause urethral obstruction in males and females.12 Females who have complete duplication of the ureter along with ectopic insertion of the ureter can present with urinary incontinence. Males in such instances are always "dry," since the insertion is proximal to the sphincter.13
The results from a 2007 pediatric study indicated that the incidence of upper tract genitourinary anomalies is significantly higher in girls with congenital adrenal hyperplasia than it is in the general population (21.2% vs 2%-4%).14 These anomalies include vesico-ureteral reflux, hydronephrosis, and a duplex collecting system. This association represents a significant variation that in some patients can be used to guide therapeutic and prophylactic intervention, including surgical intervention and antibiotic prophylaxis to prevent urinary tract infections and preserve renal function.
See also the following related Medscape topic:
CME Therapy Insight: What Nephrologists Need to Know About Primary Vesicoureteral Reflux
Preferred Examination
- Plain radiography makes no major contribution to diagnosis. However, because a patient's duplex kidney is almost always longer than his/her nonduplex kidney, a renal mass may be apparent on a plain radiograph. Hydronephrotic upper or lower pole moiety in a duplex collecting system also can be observed as a renal mass.
- Excretory urographic findings are almost always diagnostic. Difficulty may arise when function is poor or absent in one of the moieties.
- If vesico-ureteral reflux exists, the presence of an ectopic ureter in a nonfunctioning moiety can best be demonstrated using a voiding cystourethrogram.11,15
- Antegrade pyelography is useful in patients with hydronephrosis, to demonstrate the presence of a second ureter and to determine the level of ureteric termination.
- Arteriography is an invasive procedure and is no longer used to diagnose duplex collecting systems. However, it may occasionally be useful in planning nephron-sparing surgery.16
- Ultrasonography is a noninvasive and extremely useful examination, particularly in children. The ultrasonographic appearance of a duplex kidney is specific but not sensitive.17
- Computed tomography (CT) scanning with contrast is superior to ultrasonography and excretory urography in diagnosing the nubbin.18
- Magnetic resonance (MR) urography may be used as the primary diagnostic method for assessing a duplex ectopic ureter, as well as the complications associated with duplex kidneys.19,20 Spatial resolution is a limiting factor.
- Scintigraphy is useful in the assessment of relative renal function and in the detection of renal scars.21
Limitations of Techniques
A patient's duplex kidney is almost always more elongated than his/her nonduplex kidney. The kidney may be enlarged when hydronephrotic and can be associated with rotational anomalies.
- Plain radiographs can demonstrate a renal mass, which is a nonspecific finding.
- On excretory urography, an obstructed, nonfunctioning upper or lower pole may mimic a renal mass.
- Ultrasonographic findings provide excellent anatomic information but do not necessarily differentiate a bifid renal pelvis from a bifid ureter or from 2 complete ureters.
- On a CT scan, a large column of Bertin can mimic a duplex collecting system. However, CT scanning is valuable in the evaluation of an intravesical ureterocele, either orthotopic or ectopic.
- The availability of magnetic resonance imaging (MRI) is limited, the procedure is expensive, and it requires the sedation of patients with claustrophobia. However, MR urography is an extremely useful technique in patients who have the probability of an adverse reaction to radio-opaque contrast media.19
- Scintigraphy can reveal differential functioning. However, if the functioning is markedly depressed, imaging is limited.
Differential Diagnoses
Other Problems to Be Considered
Hypertrophied column of Bertin
Other renal masses22
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References
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Further Reading
Keywords
duplex collecting system, bifid collecting system, duplicate pyelocaliceal system, duplicate ureters, ureteropelvic obstruction, duplex kidney, duplex ureters, ureteral duplication, hydronephrosis, pyelonephritis, partial ureteric duplication, complete ureteric duplication, ectopic ureter
