A duplex collecting system (or duplicated collecting system) is one of the commonest congenital renal tract abnormalities 4-5. It is characterised by incomplete fusion of upper and lower pole moieties resulting in complete or incomplete duplication of the collecting system.
Duplex collecting systems are seen in 0.7% of the normal adult population and in 2 - 4% of patients investigated for urinary tract symptoms 7.
Most duplicated systems asymptomatic and diagnosed incidentally. However, where symptoms do occur (infection, reflux or obstruction) the patient is likely to have completely duplicated ureters. Occasionally hydronephrosis can be severe enough to result in flank discomfort or even a palpable mass.
Embryologically, duplication occurs when two separate ureteric buds arise from a single Wolffian duct. Interestingly, and explaining the Weigert-Meyer rule, the future lower pole ureter separates from Wolffian duct earlier and thus migrates superiorly and laterally as the urogenital sinus grows.
Duplication can be variable. At one end of the spectrum there is merely duplication of the renal pelvis, draining via a single ureter. At the other extreme, two separate collecting systems drain independently into the bladder or ectopically (see below)
Duplex systems may be unilateral or bilateral and can be associated with a variety of other congenital abnormalities of the urinary tract, e.g. ureterocoele.
As the abnormality is an anatomic alteration, all modalities able to image the renal tract may be able to visualise the typical features.
General features include :
- duplicated ureters extending a variable distance down to the bladder
- obstruction of the upper pole moiety down to the bladder, often with a ureterocoele
- vesico-ureteric reflux into the lower pole moiety, often due to distortion in its insertion by the aforementioned ureterocoele
- ectopic insertion of the upper pole moiety e.g. into the prostatic urethra in males or vaginal vault in females
Additionally, if reflux is significant, evidence of reflux nephropathy may be evident.
Excretory urography (IVP)
Although able to elegantly image both collecting systems, one should be aware that a poorly functioning system may not excrete contrast. In such a situation the functioning lower pole moiety will be inferiorly displaced, taking on the so called Drooping lily appearance1. The differential for such an appearance is that of an upper pole mass or cyst.
Ultrasound, when no obstruction / hydronephrosis is present can be suboptimal in the detection of a duplicated system, and will especially struggle to distinguish between partial and complete duplication. It is certainly able to detect ureterocoeles if present.
It provides excellent anatomic information but does not necessarily differentiate a bifid renal pelvis from a bifid ureter or from 2 complete ureters.
CT is able to delineate essentially all abnormalities, especially when performed during the excretory phase (contrast outlining the collecting systems). In an unobstructed system, the diagnosis can be difficult. A duplicated renal collecting system can be suspected by identifying the so-called faceless kidney.
Renal scintigraphy is significantly impaired in its ability to identify non-obstructed systems, as the spacial resolution is poor. However, it is able to evaluate renal function, and is particularly useful in planing corrective surgery.
Treatment and prognosis
Duplex kidney usually does not require any treatment per se however complications may necessitate intervention. This includes:
- 1. Callahan MJ. The drooping lily sign. Radiology. 2001;219 (1): 226-8. Radiology (full text) - Pubmed citation
- 2. Hayden CK, Swischuk LE, Fawcett HD et-al. Urinary tract infections in childhood: a current imaging approach. Radiographics. 1986;6 (6): 1023-38. Radiographics (abstract) - Pubmed citation
- 3. Tischkowitz MD, Hodgson SV. Fanconi anaemia. J. Med. Genet. 2003;40 (1): 1-10. doi:10.1136/jmg.40.1.1 - Free text at pubmed - Pubmed citation
- 4. Croitoru S, Gross M, Barmeir E. Duplicated ectopic ureter with vaginal insertion: 3D CT urography with i.v. and percutaneous contrast administration. AJR Am J Roentgenol. 2007;189 (5): W272-4. doi:10.2214/AJR.05.1431 - Pubmed citation
- 5. Fernbach SK, Zawin JK, Lebowitz RL. Complete duplication of the ureter with ureteropelvic junction obstruction of the lower pole of the kidney: imaging findings. AJR Am J Roentgenol. 1995;164 (3): 701-4. AJR Am J Roentgenol (abstract) - Pubmed citation
- 6. Bisset GS, Strife JL. The duplex collecting system in girls with urinary tract infection: prevalence and significance. AJR Am J Roentgenol. 1987;148 (3): 497-500. AJR Am J Roentgenol (abstract) - Pubmed citation
- 7. Gay SB, Armistead JP, Weber ME et-al. Left infrarenal region: anatomic variants, pathologic conditions, and diagnostic pitfalls. Radiographics. 1991;11 (4): 549-70. Radiographics (abstract) - Pubmed citation
Synonyms & Alternative Spellings
|Synonyms or Alternative Spelling||Include in Listings?|
|Duplicate collecting system||✗|
|Duplicated collecting system||✗|
|Duplex renal collecting system||✗|
|Duplicted renal collecting system||✗|