Duplex kidney
Key points
Approximately 1% incidence
Mackie Stephens Theory:
If ureteric budding occurs at an ectopic site, renal tissue may become dysplastic, and the ureteric orifice will be ectopic
Mackie-Stephens Relationship: In a duplex system with dysplastic and normal tissue, normal tissue drains to a normally sited ureteric orifice, while dysplastic tissue drains to an ectopic ureteric orifice
Ureteric Bud Signalling:
GDNF-RET pathway involved
Genetic Predisposition:
Possible autosomal dominant inheritance with varying penetrance
Types of Duplex Systems:
Partial Duplex: Bifurcation of the ureteric bud – most common
Complete Duplex: Two ureteric buds form; Weigert-Meyer rule applies where the upper moiety ureteric orifice drains below the lower
40% of cases are bilateral
Upper moeity is often obstructed (ureterocoeles are often associated with it)
Lower moeity is often refluxing - may have Drooping Lily Sign on MCUG - Lower pole reflux, with the upper pole calyx positioned far from the midline
Complications:
Lower pole is prone to reflux, and if PUJO occurs, it typically affects the lower pole
Upper pole obstruction can be due to ureterocele or VUJ
Ectopic Infra-sphincteric Ureter:
In girls, may not cause wetting if there is vaginal pooling or if lying horizontally at night
Surgical Considerations:
No need to treat uncomplicated cases
If complicated (infected, obstructed or ectopic ureter casuing incontinence) - perform DMSA
Uretero-ureterostomy can be performed in most cases if needed - may risk blood supply of healthy ureter though
If complicated moeity has poor function, Heminephroureterectomy is an option - however the remaining distal ectopic ureter stump can reflux, become infected, especially in females who are sexually active, or it may fill with menstrual fluid
Radical ureteric excision should not be routinely performed as both ureters share a common blood supply, risking damage to the normal ureter
References
Essentials of Pediatric Urology, 3rd edition, 2022, Chapter 8 Duplication Anomalies, Ureteroceles and Ectopic Ureters
Mackie GG, Stephens FD. Duplex kidneys: a correlation of renal dysplasia with position of the ureteral orifice. J Urol. 1975 Aug;114(2):274-80. doi: 10.1016/s0022-5347(17)67007-1. PMID: 1171997.