Renal aplasia is the predominant cause of congenital solitary kidneys.

BACKGROUND: Congenital solitary kidneys, which are susceptible to renal failure, have been considered mostly due to unilateral renal agenesis and partly due to renal aplasia. Risk of familial recurrence and of other associated anomalies is known to be much higher in renal agenesis than in renal aplasia. However, differential diagnosis between the two renal anomalies is difficult, and renal agenesis has been found much less frequently in ultrasound screening studies of fetuses than in autopsy studies.
METHODS: In order to investigate the nature and incidence of the congenital solitary kidney, the present study performed ultrasound screening of the kidneys in 4000 newborn babies. A diagnosis of renal agenesis was made when ultrasound identified no renal parenchyma and renoscintigraphy showed no renal function, and renal aplasia when there was a renal parenchyma without any function.
RESULTS: Primary screening detected 52 babies suspected of having small kidneys and one baby with a multicystic dysplastic kidney, but no baby with renal agenesis. Forty-seven of the 53 babies underwent a second ultrasound scanning at one month of age. Three small kidneys in three babies further decreased in size, had no function and were diagnosed as renal aplasia (which has an incidence rate of one in 1300). Follow-up ultrasound studies showed further regression in all three, which became very hard to distinguish by one year of age.
CONCLUSIONS: The present study showed that ultrasound in the neonatal period could identify the aplastic kidney, which had a reniform shape, not rudimentary, during the newborn period, and regressed rapidly thereafter. These findings indicate that most renal agenesis diagnosed clinically thus far might more correctly be renal aplasia.