Q. The paediatrician asks for your advice. He has taken over the care of a 13-month-old girl who had a renal anomaly detected antenatally. USS has a normal kidney on one side, but on the other side there is a 7 cm long kidney composed of large cysts and no normal tissue. A DMSA scan has shown no function in the abnormal kidney. What is the most likely problem affecting this kidney?
A. Multicystic dysplastic kidney (MCDK).
Q. This is exactly what the paediatrician is concerned about. He would like to know how you would manage the child and your justification for the plan.
A. Usually these involute, in which case very little further needs to be done. There has been a previous concern about increased risk of hypertension and malignancy in these abnormal kidneys. These problems usually represent episodic case reports rather than a significant risk to the patient. A minority of patients (19%) may have contralateral VUR, but this is rarely associated with scarring. The patient is probably more at risk of reduced renal function and hypertension from having a solitary functioning kidney. Rarely nephrectomy may be necessary if the kidney is causing symptoms from mass effects. So a patient with a MSDK should have follow-up of their renal function, perhaps with checks at 1, 2, 5 and 10 years.
Duplex kidney
Q. Describe the embryology of a duplex kidney and the Meyer-Weigert rule.
A. The ureteric bud originates from the lower mesonephric duct, grows cranially and meets the metanephros at 32 days. By reciprocal induction, they generate a kidney. Urine production begins at 10 weeks. If an extra ureteric bud is generated it will also head to the metanephros and so develop into a second ureter and a duplex kidney will form. The ureteric bud that meets the more cranial metanephrosis will generate the upper moity ureter. This ureteric bud will have arisen from a more caudal position on the mesonephric duct than the other ureteric bud. When this part of the mesonephric duct is subsumed into the developing bladder, the more caudal ureteric bud will maintain its lower position.
The resultant relationship with the upper moiety ureter inserting lower and more medially into the bladder is the Meyer-Weigert rule (in comparison the lower renal moiety ureter inserts higher and more laterally into the bladder).
Q. You see a 3-month-old girl in clinic with her parents. There was a confident prenatal diagnosis of a duplex kidney on one side. What is a duplex kidney and is it common?
A. It is a kidney with a double collecting system and two separate ureters entering the bladder separately. It is said to occur in 1 in 125 births.
Q. Where does the ureter of the upper moiety insert relative to the lower moiety ureter?
A. The upper moiety ureter inserts inferiorly and more medially in comparison to the lower moiety ureter.
Q. What are the complications associated with a duplex kidney?
A. In general complications are related to the insertion of the ureter into the bladder.
The upper moiety ureter may insert into the bladder normally, or its insertion may be related to a ureterocele, or its insertion may be ectopic.
A ureterocele is a swelling associated with the insertion of the ureter into the bladder.
It is usually a cause of obstruction at the VUJ. A ureterocele may be complicated by infection, prolapse and bladder outlet obstruction.
Ectopic ureter insertion in girls may be into the urethra or vagina, and if below the external sphincter will cause continuous incontinence. The more ectopic the ureter, the more dysplastic the kidney it drains, and the more difficult it can be to spot radiologically. In boys the ectopic insertion may be into the prostate, ejaculatory duct or vas; the insertion of the ectopic ureter is always above the external urethral sphincter in boys so incontinence will not result.
The lower moiety ureter can insert normally or be associated with vesico-ureteric reflux.