Renal aplasia is the predominant cause of congenital solitary kidneys. Kidney International (2002) 61, 1840–1844; doi:10.1046/j.1523-1755.2002.00322.

Backround: 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.

- Genetic analysis of adrenal absence: agenesis and aplasia. Trends in Endocrinology & Metabolism. Volume 16 , Issue 10 , December 2005, Pages 458-468.

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A familial syndrome of isolated "aplasia" of the anterior pituitary. Diagnostic studies and treatment in the neonatal period. J Pediatr. 1974 Jan;84(1):79-84.

A male newborn infant developed hypoglycemia, collapsed, and convulsed at eight hours of age. The diagnosis of pituitary "aplasia" was suspected, because of a previously affected female sibling, and treatment with glucocorticoids was instituted. Diagnostic studies revealed a deficiency of thyrotropin, growth hormone, and prolactin. He is now six months of age and is thriving on replacement therapy. Analysis of previous reports of this entity indicates that isolated "aplasia" of the anterior pituitary is a genetic syndrome with an autosomal recessive mode of transmission. The course in this patient suggests that this disorder, if diagnosed, is amenable to therapy.