A normal kidney contains approximately 600,000 to 1,400,000 nephrons. Renal hypoplasia occurs when a kidney possesses such a signiﬁcant reduction in its nephron endowment that it weighs less than half of the standardized mean. Renal hypoplasia can occur in association with otherwise normal renal development; in combination with renal dysplasia (i.e., hypodysplasia), in which case the renal parenchyma appears undifferentiated and disorga- nized; or as a secondary effect of conditions that cause renal atrophy or involution, such as chronic reﬂux nephropathy secondary to vesicoureteral reﬂux.
The incidence of hypoplasia without dysplasia is not known, but it is likely far rarer than hypodysplasia or atrophic renal hypoplasia. Two major forms of pure hypoplasia have been identiﬁed in the literature. The ﬁrst, known as oligomeganephronia, is a mostly sporadic condition in which both kidneys possess a reduced number of nephrons, and the individual nephrons appear hypertrophic. The number of renal lobes is also reduced, with sometimes only one or two calices seen in each kidney. The second kind of hypoplasia, known as simple hypoplasia, is a much rarer, poorly described condition in which there is a reduced number of nephrons in one or both kidneys, but the individual neph- rons do not appear hypertrophic.
Hypoplasia likely represents either a premature arrest of nephrogenesis or partial failure of the normal interactions between the metanephric mesenchyme and branching ureteric bud. In either case, genetic factors appear to play a signiﬁcant role, and most of the current knowledge about the pathogenesis of renal hypoplasia comes from the study of genetic syndromes that feature it as a component. The renal coloboma syndrome, for example, features both optic nerve coloboma and renal hypoplasia, and it results from mutations in the PAX2 gene, which encodes a protein that promotes branching and survival of the ureteric bud.
In addition to genetic factors, the intrauterine milieu and other environmental factors also appear to play a role. Both uteroplacental insufﬁciency and maternal malnutrition, for example, are known to cause intra-uterine growth restriction and a reduced nephron endowment. Likewise, maternal vitamin A deﬁciency is associated with renal hypoplasia because it prevents normal production of the RET receptor, an essential component in normal nephrogenesis.
No matter the cause of hypoplasia, the small nephron population is often unable to provide a normal level of ﬁltration function. Although the resulting renal insuf- ﬁciency is initially offset by glomerular hyperﬁltration, which increases the functional output of each nephron, this seemingly adaptive mechanism causes podocyte injury and can ultimately result in focal segmental glomerulosclerosis (FSGS, see Plate 4-10). As FSGS becomes more advanced, renal function continues to deteriorate, and end-stage renal disease (ESRD) eventually occurs.
PRESENTATION AND DIAGNOSIS
Children with oligomeganephronia, the most common kind of pure renal hypoplasia, often present in the ﬁrst years of life with evidence of renal insufﬁciency and dysfunction, including salt wasting, anorexia, vomiting, polyuria, polydipsia, and failure to thrive. Urine dipstick may reveal proteinuria if there is already a signiﬁcant degree of FSGS, while serum chemistries reveal an elevated creatinine concentration. On ultra-sound, the size of each kidney is less than two standard deviations below the mean size for patient age. Ultra-sound alone, however, often cannot distinguish purely hypoplastic kidneys from those that are scarred and shrunken secondary to chronic reﬂux nephropathy. A renal scan may be useful in this setting, as purely hypo-plastic kidneys generally lack focal areas of dysfunction, whereas kidneys with chronic reﬂux nephropathy exhibit areas of renal scarring that have reduced tracer uptake. Although a deﬁnitive diagnosis could be estab-lished with histopathologic examination of affected tissue, renal biopsy is rarely performed.
The primary goal in the treatment of renal hypoplasia is to delay the onset of ESRD. As with any form of progressive renal insufﬁciency, controlling hypertension is essential because it reduces intraglomerular pressure and slows the progression of glomerulosclerosis. Angiotensin-converting enzyme (ACE) inhibitors are especially useful because they exert a selective vasodilatory effect on efferent arterioles, which further reduces intraglomerular pressure and also decreases proteinuria. Once ESRD occurs, however, renal transplantation becomes the only viable long-term solution.