RENAL DYSPLASIA
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 posterior 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.
