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Renal malformations account for 30% to 50% of end-stage renal disease in children. Approximately one-half of such cases are associated with lower urinary tract abnormalities. A spectrum of phenotypes exists and has been termed CAKUT (congenital abnormalities of the kidney and urinary tract). This chapter focuses on the epidemiology of these abnormalities and their etiologies, clinical manifestations, and aspects of clinical management.


Kidney malformations are classified at the clinical level by gross and microscopic anatomic features. One commonly used classification system is as follows:

  • Renal agenesis—congenital absence of the kidney and ureter

  • Simple renal hypoplasia—small kidney with a reduced number of nephrons and normal renal architecture

  • Renal dysplasia—presence of malformed kidney tissue elements

  • Renal dysplasia/hypoplasia (hypodysplasia)—small, dysplastic kidney

Characteristic microscopic features of the dysplastic kidney (Fig. 465-1) include abnormal differentiation of mesenchymal and epithelial elements; a decreased number of nephrons; loss of corticomedullary differentiation; and the presence of dysplastic elements, including cartilage and bone. Dysplastic kidneys range in size from large distended kidneys with multiple large cysts to small kidneys, with or without cysts. A small dysplastic kidney without macroscopic cysts is often classified clinically as a hypoplastic/dysplastic kidney, since pathologic examination, which provides a means to distinguish between simple hypoplasia and dysplasia, is not commonly performed during life. The multicystic dysplastic kidney (MCDK) is an extreme form of renal dysplasia.

Figure 465-1

Microscopic features of human renal dysplasia. A: Multicystic dysplastic kidney characterized by numerous cysts (arrow) distorting the renal architecture. B: Dysplastic renal tissue demonstrating lack of recognizable nephron elements, dilated tubules, large amounts of stromal tissue, and primitive ducts (arrows) characterized by epithelial tubules with fibromuscular collars.


The majority of renal malformations are detected during the antenatal period by fetal ultrasound and account for 20% to 30% of all anomalies detected. The incidence of renal and urinary tract malformations is 0.3 to 1.6 per 1000 live-born and stillborn infants. Renal agenesis and dysplasia are 1.3- to 1.9-fold more common in males than in females. Fifty percent of individuals with a kidney malformation have a lower urinary tract anomaly. Of these, 25% have vesicoureteral reflux, 11% have ureteropelvic junction obstruction, and 11% are affected by ureterovesical junction obstruction. Renal malformations, other than mild antenatal pelviectasis, are associated with malformation of nonrenal tissues in about 30% of cases. There are over 100 syndromes associated with renal and urinary tract malformations. The most frequent syndromes are cited in Table 465-1. Bilateral renal agenesis occurs in 1 in 3000 to 10,000 births, while unilateral renal agenesis is estimated to have a prevalence of 1 in 1000. The incidence of unilateral dysplasia is 1 in 3000 to 5000 births (1 in ...

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