Renal dysplasia refers to abnormal differentiation and organization of the renal parenchyma. The epidemiology is not known, since many patients with mild dysplasia are asymptomatic. Most cases appear to be sporadic.
A diagnosis of renal dysplasia can only be established based on histologic ﬁndings. Primitive-appearing ducts are generally seen surrounded by smooth muscle collars and embedded in a ﬁbrous matrix. Primitive-appearing tubules are present and appear comma or S-shaped, suggesting a developmental arrest during nephrogenesis. Glomeruli may be discernable but are poorly devel- oped. Cystic dilation of glomeruli and ducts may also be seen. Finally, cartilaginous metaplasia is often but not invariably present. These dysplastic ﬁndings may be diffuse or focal.
On gross examination, a dysplastic kidney may appear enlarged, hypoplastic or normal sized. If the contralateral unit is not dysplastic, it may nonetheless exhibit other abnormalities, such as hypoplasia, ectopia, ureteral stenosis, or ureterocele.
The causes of renal dysplasia are poorly understood and likely multifactorial. In some cases, it appears that early obstruction of the ureteric bud interferes with normal branching morphogenesis and induction of nephron formation. Such a mechanism would explain the association between renal dysplasia and conditions that cause congenital outﬂow obstruction, such as pos- terior urethral valves (see Plate 2-34) and bladder or cloacal exstrophy (see Plate 2-30).
Some patients, however, exhibit renal dysplasia in the absence of an outﬂow obstruction. In these cases, there are likely intrinsic defects in the signaling cascades that mediate the interaction between the ureteric bud and metanephric mesenchyme. The responsible abnormalities, however, remain poorly understood and are likely vast in number, given the wide range of different genetic syndromes that feature renal dysplasia as a component.
The clinical implications of renal dysplasia depend on its extent. Those with diffuse bilateral dysplasia produce little urine in utero, resulting in oligohydramnios and the Potter sequence (see Plate 2-8). In contrast, those with segmental unilateral disease may remain asymptomatic through adulthood.
MULTICYSTIC DYSPLASTIC KIDNEY DISEASE
Multicystic dysplastic kidney (MCDK) disease is an extreme form of cystic renal dysplasia. It is the most common cause of cystic kidney disease in children, with an estimated incidence of 1 : 3600 to 1 : 4300.
In most instances, nearly all renal tissue is replaced with cysts of varying sizes. Between the cysts are narrow septa consisting of dysplastic stroma and ﬁbrous tissue. The kidneys may be small, normal, or enlarged, but they do not retain a normal reniform shape. A developed pelvicalyceal system is not present.
In about one ﬁfth of cases, MCDK is bilateral, causing oligohydramnios and neonatal demise second- ary to pulmonary hypoplasia. In the remaining cases, the contralateral kidney generally appears normal, but it is often subject to vesicoureteral reﬂux or ureteropelvic junction obstruction.
The pathogenesis of MCDK remains uncertain. As with renal dysplasia in general, congenital obstruction to urine outﬂow is thought to play a key role; however, early obstruction of the fetal ureter in animal experiments has so far failed to recreate the MCDK phenotype.
In most cases, the diagnosis of MCDK is ﬁrst suspected through prenatal ultrasound. It may be difﬁcult, however, to distinguish MCDK from congenital hydronephrosis, which is more common. In general, however, an MCDK features cysts of various sizes in a haphazard arrangement, whereas hydronephrotic kidneys contain a large central cystic region, representing the dilated renal pelvis, surrounded by smaller cystic regions, representing the dilated calices. In addition, the cysts of a MCDK generally do not appear continuous, unlike the dilated calices seen in hydronephrosis. If MCDK and hydronephrosis cannot be differentiated based on ultrasound, a postnatal renal scan may be helpful. An MCDK will show almost no radioisotope uptake, whereas a hydronephrotic kidney will exhibit some remaining function.
Once MCDK has been diagnosed, the contralateral kidney should be carefully assessed, and regular surveillance ultrasound should be performed to monitor the growth or degeneration of the MCDK.
In the past, MCDKs were often removed to prevent the development of Wilms tumor. Further research, however, has found that the risk of Wilms tumor in the MCDK is elevated but not enough to warrant routine removal (approximately 1 : 2000 compared with 1 : 8000 in a normal kidney). As a result, MCKDs are now removed only if their size interferes with the function of adjacent organs or surveillance ultrasound demonstrates changes concerning for neoplasm. In most cases, however, the MCKD undergoes spontaneous involution, and the long-term prognosis depends on the function of the contralateral kidney.