Principles of Prenatal Diagnosis, Intervention and Neonatal Evaluation1

Renal abnormalities represent approximately 17% of all prenatally detected anomalies. In order to place prenatal diagnosis of fetal urinary anomalies in the proper context, a brief review of the chronology of critical events that take place during the development of the fetal urinary tract is in order.^{1, 2}

While embryonic development in the first 10 weeks is devoted to organogenesis, the rest of the pregnancy is devoted to growth, maturation and the development of function. The ureteric bud, a diverticulum that arises from the lower portion of the mesonephric ducts, appears at the 5th week of gestation and interacts with the metanephric blastema (the sacral portion of the intermediate mesoderm) to induce nephrogenesis. This process continues throughout gestation and is completed by 36 weeks.

Fetal urine composition is an indicator of renal function. During fetal life, nutrients are supplied to the fetus by the mother through the placental circulation and fetal homeostasis is assured without intervention of the fetal kidneys. Fetal urine, therefore, depends exclusively on fetal renal function and reflects renal potential.⁵ The pioneering work of Harrison’s group in the 1980s in the evaluation and treatment of fetuses with obstructive uropathy focused on the establishment of prognostic parameters to select the best candidates for inutero intervention.

Although fetal kidneys can initially be imaged starting at 9 weeks of gestation with transvaginal probes and about 80% can be identified by 11 to 12 weeks, most studies are performed in the second trimester (20–22 weeks) as part of the routine screening for fetal anomalies. By this time, the kidneys are visible during a transabdominal examination in >90% of the patients (Fig. 1.2 and Fig. 1.3).

Early in gestation fetal kidneys are uniformly echogenic; as gestation progresses, corticomedullary differentiation becomes visible and by 18 to 22 weeks the renal pelves and calyceal patterns can be identified. Fetal urine production begins at about 13 weeks of gestation, at which time the fetal bladder becomes apparent. Absence of the bladder at 15 weeks of gestation is considered abnormal.¹⁴

Since many genetic syndromes can be diagnosed in the first trimester using biochemical, cytogenetic and molecular techniques, the addition of accurate sonographic data will facilitate counseling, particularly with respect to termination of pregnancy.¹⁵ A number of charts of fetal renal size are available (Tables 1.1 and 1.2).^16,17 Prenatal identification and measurement of the kidney size early in gestation is particularly relevant in the diagnosis of unilateral functioning kidneys and in the presence of obstructive uropathy; compensatory renal hypertrophy (length >95% upper limit) has been demonstrated in association with unilateral renal agenesis and posterior urethral valves by sonography performed between 13 and 22 weeks of gestation.¹⁷

Magnetic resonance imaging (MRI) may be ideally suited for prenatal diagnosis since it lacks ionizing radiation, provides excellent soft tissue contrast, and the images can be studied in various orientations. No adverse effects of the magnetic field have been reported in the developing fetus.^19-21

Pelviectasis is one of the most common findings on prenatal sonography. Mild fetal pelviectasis has been reported to have an association with aneuploidy, in particular Down’s syndrome,²³ and postnatal genitourinary pathology. Chromosomal abnormalities are more likely 8when extrarenal malformations are also detected. The finding of a dilated renal pelvis in the fetus should prompt a careful and thorough investigation for markers of aneuploidy or other structural abnormalities. Pelviectasis detected in the second trimester should be reevaluated for progression in the third trimester.

Characteristics on ultrasound that are associated with true pathology include progression of pelviectasis throughout gestation, AP diameter greater than 7mm in the 3rd trimester, increased cortical echogenicity with or without cysts, lower tract dilatation, and caliectasis.^26,29 A recent study³⁰ has also looked at the resistive index (RI) of fetal interlobar arteries since the RI is found to be increased with obstruction and is normal in cases of physiologic dilatation. These authors recommend further evaluation for a pathologic cause of obstruction if the RI is elevated.

Vesicoureteral reflux (VUR), defined as abnormal retrograde flow of urine from the bladder into the upper urinary tracts, can be primary or secondary. Primary VUR can result from one or more of the following: a deficiency in the muscular backing of the intramural ureter, lateral insertion of the ureter into the bladder with a foreshortened submucosal tunnel so that a flap valve effect is lost when the bladder is full, an abnormal trigone, and an overly-enlarged ureteral orifice.³⁵ Secondary reflux can be caused by elevated intravesical pressures as seen in patients with bladder outlet obstruction (posterior urethral valves, urethral atresia), or neurogenic bladders. VUR can also be associated with duplex systems, prune-belly syndrome, multicystic dysplastic kidneys (MCDK), or ureteropelvic junction obstruction. The incidence of postnatally-detected VUR associated with prenatal hydronephrosis ranges from 10 to 40%.^36-39

Ureteropelvic junction (UPJ) obstruction, defined as obstruction between the renal pelvis and the ureter, is the most common cause of prenatal hydronephrosis⁴⁰ with an overall incidence (detected by prenatal sonography) of 1 in 500 live births.⁴¹ UPJ obstruction is more common in males and is unilateral in 70% of the cases. Bilateral UPJ obstruction is usually asymmetric. The majority of UPJ obstructions appear to be caused by functional rather than anatomical factors.

Evaluation of obstructed segments by electron microscopy has shown abnormal arrangement of smooth muscle cell layers and replacement of muscle by abundant collagen.⁴² Less common causes of UPJ obstruction include fibrous adhesions, kinks in the ureter, ureteral valves, and aberrant vessels.⁴³ Multicystic kidneys are believed to represent the far end of the spectrum of UPJ obstruction; these kidneys become dysplastic as opposed to hydronephrotic because of the severity of the obstruction.⁴² Associated urologic anomalies include vesicoureteral reflux, horseshoe kidney, contralateral renal agenesis, meatal stenosis and hypospadias.⁴⁰ On prenatal ultrasound, UPJ obstruction appears as a dilated renal pelvis with normal ureter and bladder (Fig. 1.6). The degree of renal pelviectasis or pelvicalyectasis is variable. In severe cases, the calyces are severely blunted and there is thinning of the renal parenchyma. The amount of amniotic fluid is usually normal if the contralateral kidney is not involved. It is often difficult to prenatally determine the exact etiology of hydronephrosis, since one cannot rule out other causes such as vesicoureteral reflux until after birth. Serial scans during pregnancy are useful to monitor the progress of pelvic dilatation. Oligohydramnios, in association with bilateral involvement and increased echogenicity, indicates a poor prognosis.

Ureterovesical junction obstruction (UVJ) refers to an anatomic or functional obstruction at the point of entry of the ureter into the bladder.12 UVJ obstruction is bilateral in 13% of the cases and predominantly affects males.⁴⁴ These obstructions have been attributed to partial ureteral atresia or an abnormal angle of implantation of the ureter into the bladder.^45,46 In many instances a functional obstruction is present, due to an abnormal arrangement of circular and longitudinal smooth muscle fibers and increased deposition of collagen in the distal ureter resulting in lack of effective peristalsis. UVJ obstruction is associated with duplicated renal systems and ureteroceles. Sonographically, the ureter is dilated and appears as a serpiginous, anechoic, cystic structure in the abdomen and behind the urinary bladder.⁴⁰ The bladder usually appears unremarkable except in cases with intravesical ureteroceles. Amniotic fluid volume is normal. It is often hard to distinguish UVJ obstruction in utero from other causes of hydroureters such as vesicoureteral reflux, prune-belly syndrome, or posterior urethral valves, and further diagnostic studies are required after birth.

Posterior urethral valves (PUV), found exclusively in males, result form thin mucosal folds in the posterior urethra that obstruct the flow of urine at the bladder outlet. This condition is the most common cause of lower urinary tract obstruction in male infants and has a reported incidence between 1 in 8000 ⁴⁷ and 1 in 25,000 live male births.⁴⁸ Patients with PUV develop renal and bladder dysfunction. The mortality rate in patients with PUV in earlier series was 50%, however, over the last two decades it has dropped to < 5% ^49,50 and pulmonary hypoplasia is now the leading cause of death.⁵¹ Although the mortality rate has declined, approximately one-third of patients will develop chronic renal insufficiency.^49,52 Differential diagnosis includes urethral atresia and prune-belly syndrome. Approximately 10 to 65% of PUV will be detected on prenatal ultrasound.^52,53 Urologic anomalies resulting from PUV are usually not detected prior to 24 weeks gestation.⁵⁴ On ultrasound, characteristic findings include bilateral hydronephrosis, hydroureter, a bladder that stays full or empties incompletely, thickened bladder wall, dilated posterior urethra and various degrees of oligohydramnios (Figs 1.7 to 1.9). Amniotic fluid volume is preser-ved in the first trimester, since prior to 16 weeks of gestation the fluid is not primarily of renal origin. In most large series, oligohydramnios is present in less than half of the cases of suspected urethral valves. A persistently dilated bladder with bilateral hydroureters is very suggestive of PUV and must be followed closely throughout pregnancy. With severe obstruction, progressive worsening of oligohydramnios and increased renal echogenicity with loss of corticomedullary differentiation is seen.

Renal dysplasia is often present in patients with posterior urethral valves and is characterized by increased renal echogenicity, loss of corticomedullary differentiation and subcortical cysts. The obstructed upper urinary tract may spontaneously decompress giving rise to urinary ascites or perinephric urinoma. PUV can be differentiated from prune-belly syndrome by the sonographic characteristics of the bladder, which is thick-walled and tense with PUV and floppy and thin-walled in prune-belly syndrome (Figs 1.10 and 1.11). Urethral atresia may be difficult to differentiate from PUV on prenatal ultrasound.⁵⁵

Shunt placement has a 45% complication rate, including problems with the shunts themselves such as migration or obstruction, problems from violation of the maternofetal barrier including premature labor and chorioamnionitis, and from the presence of placenta previa.⁵⁶ Given the difficulty in placing the shunts, multiple attempts are usually needed and this increases the risk of complications. Overall perinatal mortality after vesicoamniotic shunting approaches 47%,⁵⁵ and 40% of the survivors have end stage renal disease.⁵⁶ Intervention that is reserved only for those fetuses with good prognosis (see prenatal evaluation of fetal renal function above) appears to yield the best outcomes.⁵⁵ New approaches include fetal cystoscopy in an antegrade fashion which allows for accurate diagnosis and disruption of the valves via YAG laser, hydroablation, placement of transurethral stents, or mechanical disruption with guide wires.^55,57 These technically difficult procedures are still in the early stages of development.

Congenital megaureters are dilated ureters with or without associated pelviectasis or calyectasis and are classified as either primary or secondary. Megaureters may be refluxing or nonrefluxing and obstructive or non-obstructive. Primary megaureters refer to obstructive, non- refluxing megaureters resulting from an abnormal ureterovesical junction, thus the term ureterovesical junction (UVJ) obstruction (see above). Secondary megaureters result from high grade reflux, bladder outlet obstructions (PUV), neuropathic bladder, or systemic disorders such as prune-belly syndrome (Fig. 1.12).

Normally, ureters are not visualized in utero, thus, visualization of the one or both ureters is abnormal. On sonography, megaureters are characteristically serpeginous, anechoic, tubular structures that fill the fetal abdomen and retrovesical space.⁵⁸ The ureters can be traced from its origin in the renal pelvis to its insertion in the bladder. In patients with primary megaureters, the volume of amniotic fluid is normal. The bladder is normal in appearance and there is varying degrees of hydronephrosis. The distinction between refluxing and non-refluxing megaureters is impossible by sonography alone, though the presence of an enlarged bladder will suggest the former. Dilated ureters must also be differentiated from bowel obstruction and mesenteric or adnexal masses.

Duplication of the collecting system is the most common renal abnormality and is seen in up to 4% of the population. In this condition, there are two distinct pelvicalyceal systems within the kidney with complete or partial duplication of the ureter. The condition is unilateral 70% of the time and is more common in females. The overall incidence of prenatally-diagnosed duplex systems ranges from 1 in 70 to 1 in 500.¹ Hydronephrosis, renal cysts, polycystic kidneys and ureteropelvic junction obstruction need to be considered in the differential diagnoses.

In duplex systems, the upper pole moiety is often obstructed due to an ectopic insertion of the ureter into the bladder neck, urethra or vagina, while the lower pole moiety refluxes because of a cephalad and lateral insertion of the ureter into the bladder.⁴⁰ Several sonographic characteristics are useful in the diagnosis of duplex systems. These include separate and non-communicating renal pelves, hydronephrosis limited to one pole of the kidney, ipsilateral ureteral dilatation, and cystic structures within the bladder consistent with ureteroceles (Fig. 1.13).⁵⁹ Asymmetric hydronephrosis of the kidney can be easily confused with ureteropelvic junction obstruction.⁴⁰ Of special note is one report of a duplicated system diagnosed with prenatal MRI because the sonographic images were nondiagnostic.⁶⁰

This condition results from failure in the embryonic development of the metanephros. When bilateral, renal agenesis may be diagnosed as early as 12 weeks gestation ⁶¹ from the absence of paravertebral reniform masses in both the transverse and parasagittal planes, associated with anhydramnios (Fig. 1.14). Bilateral renal agenesis occurs infrequently with an incidence of approximately 1 in 4000 births. Although first recognized in 1671 by Wolfstrigel, this condition was extensively described by Potter in the 1940s and 1950s. The anomaly has a male predominance⁶² and the incidence appears to be influenced neither by maternal age nor by any specific complication of pregnancy or concurrent maternal disease. There is a genetic predisposition to this syndrome with a high level of penetrance, and an autosomal recessive pattern of inheritance has been suggested.⁶³

The kidneys and renal arteries are usually completely absent, although dysgenetic vestiges may be noted at times.⁶⁴ The bladder trigone, if present, is poorly- formed as a result of failure of incorporation of mesonephric duct structures into the base of the bladder. The bladder is hypoplastic and may be absent.

The pelvic kidney lies within the confines of the bony pelvis and represents the most prevalent form of renal ectopia. Pelvic kidneys are located opposite the sacrum and below the aortic bifurcation. The presence of pelvic kidneys has been reported in 1/2100 to 3000 autopsies. Ectopic kidneys, on the other hand, are kidneys that never reached their proper position within the renal fossa. Solitary ectopic kidneys are seen in 1/22,000 autopsies.⁶⁹ The exact mechanism for failure of renal ascent is unknown. Factors that might prevent normal migration of the kidney include maldevelopment of the ureteral bud,⁷⁰ defective metanephric tissue that fails to induce ascent,⁷¹ genetic abnormalities, and maternal illnesses or teratogenic factors.⁷² Another theory is that a vascular barrier may exist secondary to persistent fetal blood supply.⁷³ During sonographic examination, failure to identify the kidneys in the proper position should prompt a more careful examination of other sites. Although the diagnosis is usually possible with a transabdominal study, on occasion transvaginal sonography is helpful. Color Doppler assessment may identify the blood supply to an ectopic or pelvic kidney.

Multicystic dysplastic kidney (MCDK) represents a severe form of non-genetic dysplasia and is a different entity from polycystic kidney disease. MCDKs are characterized by tense, non-communicating cysts with non-medial location of the largest cyst and absence of functioning renal parenchyma, usually in association with atresia or hypoplasia of the ureter and the renal artery. These kidneys have the appearance of a “bunch of grapes” and lack a reniform configuration. The condition is 20usually unilateral (76%) with the left side being more commonly involved.⁷⁴ Contralateral renal abnormalities are commonly associated with MCDK, particularly vesicoureteral reflux, ureteropelvic junction obstruction, megaureter and dysplasia. MCDK is second only to hydronephrosis as a cause of abdominal masses in the neonate.⁷⁵

With bilateral involvement, oligohydramnios or anhydramnios and absence of the urinary bladder are noted. Differential diagnosis includes hydronephrosis and other intra-abdominal cystic masses.

ARPKD has been referred as the “infantile” form of polycystic kidney disease. This description is somewhat misleading since ARPKD may manifest only at adolescence or in young adulthood, although with significantly less frequency. ARPKD has been reported to affect 1 of every 40,000 live births.⁷⁶ Antenatally, ARPKD is characterized on sonography by symmetrical, bilateral enlargement of the kidneys with numerous corticomedullary cysts and marked parenchymal echogenicity. This echogenicity can be detected as early as 12 weeks but is more often evident by 22 to 24 weeks of gestation and results from sound waves reflecting off an extensive numbers of dilated tubules (Figs 1.17 and 1.18). Oligohydramnios may not be initially present, but may develop as the pregnancy progresses and carries a poor prognosis. Because of the variability in presentation, prenatal diagnosis is not always possible. When there is a high index of suspicion for this disease, genetic diagnosis is possible as early as 11 to 12 weeks of gestation using chorionic villous sampling since the locus of the mutation is well known.

Congenital mesoblastic nephroma (CMN) is the most common (50%) renal neoplasm in patients younger than 6 months and constitutes 3 to 6% ⁷⁷ of the renal tumors found in the first 15 years of life. This benign neoplasm arises from the renal mesenchyme and is usually associated with polyhydramnios, the cause of which is not known. It has been suggested that a mass effect that interferes with swallowing and gastrointestinal uptake of amniotic fluid, or that hypercalcemia resulting from increased prostaglandin production by the tumor results in increased fetal urine output. Prenatal diagnosis of CMN with sonography in the third trimester of pregnancy has been reported in less than 30 cases.²² Classically, CMN appears as a hypoechoic mass with an echogenic rim, the “ring sign”, or as a homogenous or heterogeneous mass without a rim; the latter presentation is impossible to differentiate from congenital Wilms’ tumor. Extrarenal masses arising form the liver or the adrenal gland (neuroblastoma) are included in the differential diagnosis. MRI has shown promise in difficult cases because it allows discrimination of the outline of the tumor from the kidney and adrenal gland.

The bladder can be demonstrated in 80% of fetuses by 11 weeks gestation and in up to 90% by 13 weeks. The bladder fills and empties regularly during sonographic examination. Failure to identify the bladder should prompt a detailed fetal scan with particular emphasis on the kidneys. If the kidneys are present, there are no abdominal wall defects, and the bladder is not in an extra-abdominal location (as in bladder and cloacal extrophy), the issue is usually poor fetal growth associated with oligohydramnios. Absence of the bladder is also seen in patients with 23generalized neuropathy with associated contractures and abnormal fetal movements.

The number of neonates referred for uroradiological evaluation has risen dramatically since the advent of prenatal renal imaging. Most referrals are for prenatally-detected hydronephrosis, but other conditions such as complicated duplications, MCDK, megaureter, renal dysplasia and ectopia are also readily-detected by sonography and require neonatal screening. Another group includes infants with syndromes or multiple anomalies that involve the genitourinary tract such as the VATER or CHARGE associations. Sonographic evaluation is also performed in babies with severe hypospadias and in conditions associated with increased risk of renal malignancy such as the Beckwith- Wiedemann syndrome, aniridia and hemihypertrophy.

The first step consists of confirming all antenatally-detected abnormalities after birth since this initial sonographic study will direct subsequent radiological evaluation. In studying pelviectasis or hydronephrosis, sonograms performed in the first 24 to 72 hours of life may yield false-negative results due to the relatively low physiologic urine output in the neonate in the first few days of life. The study should either be deferred for 5–7 days, or, if the initial study is normal, it should be repeated at 1 to 2 weeks. Immediate investigation is indicated in anuric or septic infants; again, the results should be confirmed by a repeat study one week later. In general, any infant with pelviectasis should also have a repeat ultrasound at 6 weeks of age. Although pelviectasis may be transient, dilatation of the calyx or its infundibular portion in the neonate is never normal. Physiologic distention of the fetal and neonatal urinary bladder may result in transient dilatation of the ureters or hydronephrosis which resolves after bladder emptying. The absolute significance of mild antenatal pelviectasis (<10 mm AP diameter) and its natural course throughout gestation has yet to be clearly defined. Retrospective studies have shown that approximately 9% of fetal hydronephrosis progresses and only 4% of fetuses with hydronephrosis have functionally-significant urologic lesions. Careful monitoring of patients with bilateral obstruction, low amniotic index, or both is advocated.

Vesicoureteral reflux will be present in 20 to 40% of neonates (with history of antenatal hydronephrosis) who are evaluated after birth. In more than half the reflux is bilateral.^37,39 Although ultrasound has come a long way in detecting prenatal abnormalities, it is poor at diagnosing VUR in utero. This is why some have advocated obtaining a postnatal VCUG in all babies with antenatally-detected hydronephrosis, irrespective of the postnatal ultrasound findings.²⁵

Prognosis for children with VUR is good, given that in most cases it can be treated with observation and antibiotics. Even with high grades of VUR, operative correction is successful close to 98% of the time.

Prognosis in children with UPJ obstruction is eminently good since severe, bilateral obstruction is rare. Majority of infants with incidentally discovered hydronephrosis are asymptomatic, have no palpable abdominal mass and normal renal function. Spontaneous improvement or resolution of hydronephrosis is frequent. In most fetuses with grade I hydronephrosis (pelvic AP diameter <10 mm.), hydronephrosis resolves before the first postnatal examination and these patients will not need pyeloplasty. The threshold value associated with an increased likelihood of the need for pyeloplasty is a pelvic AP diameter of 9 to 12 mm.^26,80 Besides the degree of pelvicalyceal dilatation, other indicators that might prompt surgical intervention include the presence of cortical thinning (which is relatively easy to detect in postnatal sonograms) and increased echogenicity with loss of corticomedullary differentiation 27(which is suggestive of associated parenchymal dysplasia). However, sonographic criteria to discriminate between cortical thinning secondary to compression and the one due to irreversible loss of renal parenchyma are lacking. Worsening hydronephrosis in serial sonographic studies, progressive parenchymal thinning, and an obstructive pattern on diuretic renography with decrease in ipsilateral differential function are accepted criteria for surgical correction of UPJ obstruction. In general, the trend in radiographic and clinical data with serial evaluations is more helpful than relying on isolated imaging studies in making the decision of surgery vs. observation. It is reasonable, at least initially, to repeat sonography and diuretic renograms at 3 month intervals. The majority of children with a minimal or moderate degree of hydronephrosis can be treated conservatively without fear of progressive loss of renal function.⁸¹ When surgery is indicated, dismembered pyeloplasty carries high success rates and is well tolerated by most children.

Despite advances in neonatal care and prenatal detection, the long-term outcome in children with PUV is not clearly defined. Although some authors report no change in outcome in children with PUV detected on prenatal sonography, other studies have indicated a better prognosis if the anomaly was detected antenatally as compared to detection after birth.⁸² Four variables appear to be independent predictors of a poor outcome: oligohydramnios, need for ventilatory support, BUN > 40 mg/dl and bilateral vesicoureteral reflux.⁵² In addition, detection of PUV before 24 weeks of gestation portends a poor prognosis.⁸³ On meta-analysis, unilateral vesicoureteral reflux was found to be prognostic of a better outcome.⁵² Renal dysfunction is the result of chronic obstruction, dysplasia and reflux nephropathy.⁵⁴

Determination of the etiology of ureteral dilatation (obstructive vs refluxing) is only possible after birth. Initial management should involve placing the newborn on prophylactic antibiotics and performing a postnatal ultrasound and VCUG to determine the presence of reflux or obstruction. When high grade obstruction is suspected, a diuretic renogram is indicated. With primary megaureter, the ureter is often equally or more dilated than the collecting system since the ureter tends to dilate from the bottom up. Primary megaureter tends to be a non-progressive disorder. The prognosis following surgical correction, when indicated, is usually good in these children since procedures to reimplant the ureter are highly successful.

The clinical significance of the prenatal diagnosis of duplex systems is unknown. At the present time, there are no criteria for predicting who will remain asymptomatic and who will eventually develop complications. Prophylactic antibiotics are generally prescribed until the workup is complete.^59,60 A VCUG is imperative to rule out associated vesicoureteral reflux and ureteroceles. Upper pole hydronephrosis, particularly in the absence of reflux, should be investigated with a diuretic renal scan to rule out obstruction.

Prognosis of bilateral renal agenesis is understandably poor. Anuria beyond the first 24 hours of life without distention of the bladder should 29suggest renal agenesis. Almost 40% of affected infants are stillborn and most infants who are born alive do not survive beyond the first 24–48 hours because of pulmonary hypoplasia and respiratory failure.⁸⁵ The diagnosis can be confirmed in the neonate by Doppler ultrasonography, renal scintigraphy, and, if necessary, umbilical artery catheterization with aortography.

Pelvic kidneys are no more susceptible to disease than normally-positioned kidneys except for the association with ureteral abnormalities, VUR and the propensity to form renal calculi. Causes of hydronephrosis in these kidneys may be malrotation, crossed-fused ectopia, vesicoureteral reflux, or dysmorphism. Thorough assessment of hydronephrosis should be performed before treatment is initiated due to the fact that nonobstructive dilatation may occur in up to 50% of cases.⁹²

A clear distinction between MCDK and a hydronephrotic kidney is not always possible prior to birth, though an identifiable renal sinus is usually consistent with hydronephrosis. Renal sonography in an infant with MCDK will show a somewhat random distribution of cysts of various sizes without a larger central or medial cyst and without visible communication between cysts. A DMSA renal scan can also be used to demonstrate non-function of the kidney in cases of MCDK, whereas a hydronephrotic kidney will show some function. As was mentioned earlier, contralateral renal abnormalities are commonly associated with MCDK, particularly vesicoureteral reflux, UPJ obstruction, megaureter and dysplasia. VCUG is therefore mandatory in the evaluation of these infants, and a diuretic renal scan is recommended if there is any degree of contralateral hydronephrosis.⁹⁴

ARPKD has a spectrum of severity, with the most severe form presenting early in life. There is an invariable association with congenital hepatic fibrosis. The disease may present at birth and prove fatal in the neonatal period. Children in whom ARPKD manifests later in life develop renal failure and hypertension more slowly than those in whom ARPKD is manifest at birth, and may suffer more from liver disease than from the renal manifestations. Hepatic fibrosis leads to portal hypertension, esophageal varices, and hepatosplenomegaly.

Confirmation of the nature of the mass at birth is performed with ultrasound, CT scan or MRI. Classical CMN has excellent prognosis following complete excision. Prognosis of the tumors with the cellular subtype remains controversial. In this group, in patients younger than 3 months, prognosis is excellent after removal, but close follow up is recommended. In patients older than 3 months at the time of diagnosis and in those younger than 3 months with positive surgical margins, chemotherapy may be considered.