Maternal ultrasonography is used to determine the gestational age of the fetus, fetal wellbeing in high-risk pregnancies and in families with a history of congenital anomalies, and in many cases simply as a screening tool. In 1% of pregnancies, a structural fetal anomaly is detected. '"4 Approximately one half of these anomalies involve the central nervous system, while 20% are genitourinary, 15% gastrointestinal, and 8% cardiopulmonary in origin. The probability of recognizing a structural anomaly by prenatal ultrasound depends on the experience and skill of the sonographer and usually is better late in gestation when the fetus is larger and an anomaly is easier to image.
Genitourinary anomalies are present in between 1 in 500 and 1 in 1000 pregnancies.5 Improvements in ultrasound equipment and greater experience have resulted in increasing accuracy of sonography in identifying these lesions. Prenatal ultrasonography allows the identification of urological abnormalities that otherwise would be unrecognized until later in life when symptoms of pyelonephritis, stone disease, or renal colic develop. Anomalies of the urinary tract detectable by prenatal ultrasonography are limited to obstructive lesions, conditions that mimic obstruction, cystic disease, and renal agenesis (Table). Obstructive anomalies usually occur in males. The timing and type of evaluation necessary in the newborn period depend on the nature of the abnormality visualized on ultrasound.
DEVELOPMENT OF THE KIDNEY AND RENAL FUNCTION
The human kidney is derived from the ureteral bud and the metanephric blastema. During the fifth week of gestation, the ureteral bud arises from the mesonephric (wolffian) duct and penetrates the metanephric blastema, which is an area of undifferentiated mesenchyme on the nephrogenic ridge. The ureteral bud undergoes a series of approximately 15 generations of divisions and by 20 weeks gestation forms the entire collecting system, ie, the ureter, renal pelvis, calyces, papillary ducts, and collecting tubules. Under the inductive influence of the ureteral bud, nephron differentiation begins during the seventh week. By 20 weeks, when the collecting system is completely developed, approximately one third of the nephrons are present. Nephrogenesis continues ata nearly exponential rate and is complete by 36 weeks.
Genitourinary Anomalies Detectable by Prenatal Ultrasonography
Throughout normal gestation, the placenta functions as the fetal hemodialyzer, and the fetal kidneys play a minor role in the maintenance of fetal salt and water hemostasis. Formation of urine begins between the fifth and ninth week of gestation. The rate of urine production increases throughout gestation and at term, volumes may approach 26 to 28 mL/hour6 (Figure 1 ). Normally fetal urine is hypotonic. 7 The glomerular filtration rate (GFR) has been measured at 6 mL/min/1.73m2 at 28 weeks gestation, increasing to approximately 25 mL/min/1. 73m2 at term, and thereafter triples by 3 months of age. The main factors responsible for this rise in GFR include an increase in the capillary surface area available for filtration, changes in intrarenal vascular resistance, and redistribution of renal blood flow to the cortical nephrons, in which the majority of nephrons are located.8 It is apparent that a congenital obstructive lesion of the urinary tract may have a deleterious effect on renal function.
SONOGRAPHY OF THE FETAL URINARY TRACT
In a normal fetus, the bladder is visualized as early as 14 weeks gestation. Although the kidneys also may be seen at 14 weeks, they should always be visualized by 18 weeks. The presence of a filled bladder provides evidence of renal function. Conversely, nonvisualization of the urinary bladder, particularly in association with oligohydramnios, suggests that renal function is poor. Standards for normal fetal renal size are established,9 and kidney circumference remains constant at approximately one fourth of the abdominal circumference throughout gestation. Normally, the fetal ureter is not seen. Fetal sex may be determined early in gestation and requires the unequivocal visualization of the penis or scrotum or both, or of the labia majora. In a recent study, 40% of fetuses under 24 weeks gestation were definitely identified as to sex with misdiagnosis occurring in only 3%.10
Assessment of the amniotic fluid is important as well. During the first trimester, amniotic fluid represents a transudate of maternal plasma. Beyond 18 weeks, however, nearly all of the amniotic fluid is the result of voided urine. Thus, with high-grade bladder outlet obstruction or bilateral renal agenesis, the volume of amniotic fluid is severely diminished (oligohydramnios or anhydramnios). In the presence of prolonged oligohydramnios, fetal lung development is impaired, and pulmonary hypoplasia results, which is fatal. Thus, the identification of an obstructive uropathy in association with oligohydramnios often predicts a poor outcome.
Figure 1. Changes in GFR and urine output during fetal development and infancy (reprinted with permission8).
Because visualization of the fetal kidneys may be marginal until 18 to 20 weeks gestation, and since fetal urine output does not contribute significantly to amniotic fluid during the first trimester, it is not uncommon for a newborn with obstructive uropathy to have a normal fetal ultrasound during the first trimester. In order to be certain that renal development is normal, an ultrasound at or beyond 20 weeks gestation is necessary.
An obstructive anomaly is recognized by demonstrating a dilated renal pelvis or calyces, ureter, or bladder. In one recent report of 39 patients, if the fetal renal pelvis was 10 mm or more in diameter, in nearly all cases an obstructive anomaly was found. In contrast, if the renal pelvis was less than 10 mm, no urinary tract abnormalities were subsequently identified. Unfortunately, follow-up was unavailable in one third of the latter group of patients. 5 Although hydronephrosis is the most common urological anomaly detected by prenatal ultrasonography, a multicystic kidney or distended loop of bowel may be mistakenly identified as hydronephrosis. Furthermore, dilatation of the renal collecting system may occur in the absence of obstruction. This is termed "physiologic hydronephrosis" and may result from the high fetal urine production, reflux of urine from the bladder up to the kidney, or simply from a dysmorphic urinary tract.
MANAGEMENT OF THE FETUS WITH SUSPECTED HYDRONEPHROSIS
When a genitourinary anomaly is discovered prenatally, it is essential that the obstetrician, pediatrician, and pediatric urologist work together to maximize the chances for a successful outcome. The knowledge that the fetus has a potential tenal abnormality is extremely anxiety-provoking to the parents, and a team approach is helpful in providing not only optimal medical care but emotional support as well.
In the fetus with bilateral hydronephrosis and a distended bladder, the most important prognostic feature is the volume of amniotic fluid. If there is a normal volume of amniotic fluid, then renal function is sufficient to allow normal pulmonary development. Usually the renal cortex is visualized and may be demonstrated to be normal, whereas in other cases macroscopic renal cysts may be seen, which are strongly suggestive of dysplasia. The fetus should be monitored every few weeks to ascertain that the volume of amniotic fluid remains normal. If oligohydramnios develops, the cause must be determined. Early delivery is not advised except for the rare cases when the amniotic fluid volume severely diminishes, which could conceivably have an adverse effect on pulmonary development. In the fetus with unilateral hydronephrosis, plans should be made to evaluate the infant following delivery, and early delivery should not be performed.
Following the initial report of fetal surgery in 1981 for obstructive uropathy11 there was tremendous interest in decompression of thé fetal urinary tract. This should never be performed for unilateral hydronephrosis unless the renal dilatation is massive and causing dystocia, which is extremely rare. In most cases, intervention has been used for presumed posterior urethral valves, characterized by bilateral hydroureteronephrosis and a distended bladder, with placement of a one-way catheter shunt between the fetal bladder and the amniotic sac. In other cases single or multiple aspirations of the fetal bladder or kidney have been performed. Recently the International Fetal Surgery Registry reported 73 cases in which a vesicoamniotic shunt was placed, and survival was 40% Kith in those with oligohydramnios and normal amniotic fluid.12 However, in 45% of the cases the urologie diagnosis was unknown and the sonographic criteria for oligohydramnios were not defined. Although the authors did not conclude that such intervention was effective, they believed that it should be studied further.
The reason that intervention rarely seems effective is that early severe obstructive uropathy results in renal dysplasia, which is irreversible. Thus, the fetus with urethral valves and associated anhydramnios identified at 18 to 20 weeks gestation has little or no expectation of recoverable renal function even if pulmonary development could be satisfactory. In a review of 28 reported cases with bilateral hydronephrosis, oligohydramnios, and an interventional procedure, in no case was the intervention shown to improve renal function, although in one or two cases it may have allowed more normal pulmonary development to occur.11 Furthermore, the complication rate has been approximately 40%. including failure of the shunt to function properly, migration of the shunt, perforation of bowel, induction of premature labor, and maternal chorioamnionitis.n Thus, intervention should be considered only in the rare case.
In the fetus with bilateral hydronephrosis or unilateral hydronephrosis in a solitary kidney, the baby should be delivered in a tertiary care center to allow an expeditious evaluation. If the fetus has a normal kidney, then it is unnecessary to alter plans for delivery.
MANAGEMENT OF THE NEWBORN WITH SUSPECTED OBSTRUCTIVE UROPATHY
At birth, the abdomen is examined to detect the presence of a mass, which is often "Secondary to either a multicystic kidney or ureteropelvic junction (UPJ) obstruction. In addition, the newborn is evaluated for anomalies involving other organ systems. Renal function should be monitored with periodic serum creatinines, particularly if the baby has bilateral hydronephrosis. At birth, the serum creatinine reflects maternal renal function. However, by I week of age, the creatinine should decrease to 0.3 to 0.4 mg/ dL. The exception is the premature infant, in which the creatinine may not decrease until the child reaches 34 to 35 weeks conceptional age, because of the immaturity of renal function prior to that time.
The radiologic evaluation is performed to delineate the abnormality responsible for changes on prenatal ultrasound. Serial abdominal sonograms, a voiding cystourethrogram (VCUG), renal scan, and an excretory urogram (IVP) provide the diagnosis.
A renal and bladder ultrasound should be obtained first. If the fetus has bilateral hydronephrosis, the sonogram should be obtained shortly after birth, whereas with unilateral hydronephrosis, the evaluation may be delayed for a few weeks. It is important that the bladder be imaged by ultrasound as well, to detect a dilated posterior urethra (urethral valves), thickening of the bladder wall, inadequate bladder emptying, or ureteral dilatation.
Figure 2. Neonate with bilateral hydronephrosis discovered on prenatal ultrasonography. The infant was delivered at term. (A) Large right-sided abdominal mass apparent. (B) Preoperative IVP demonstrating the large right abdominal mass and left hydronephrosis. Bilateral UPJ obstruction was found and a bilateral pyeloplasty was performed. (C) Postoperative IVP demonstrating excellent function bilaterally (reprinted with permission8).
Next, a VCUG should be performed. This test may indicate posterior urethral valves, a bladder diverticulum, or vesicoureteral reflux. Even if the neonatal ultrasound is normal, a VCUG should be performed, because reflux may be the cause of fetal hydronephrosis.
If the ultrasound and VCUG are normal, then only a follow-up ultrasound in six to eight weeks is necessary. However, in most cases it is necessary to evaluate the upper urinary tracts further. In the newborn, the best method of evaluation is to perform a renal scan (diuretic renogram) using technetium DTPA or glucoheptonate (GHA). Technetium DTPA is filtered by the kidney, whereas GHA is filtered and also secreted by the renal tubules. The renal scan has several advantages over the IVP in the newborn. First, because of the relative immaturity of renal function in the newborn, visualization of the collecting system by an IVP may not be optimal. Second, a substantial amount of bowel gas frequently obscures the renal cortical outline and detail of the collecting system. Third, the renal scan provides an excellent method of objectively assessing how well each kidney is functioning. Differential renal function is computed by measuring the uptake over each kidney during the first two to three minutes before the radionuclide enters the collecting system.
In some cases of a UPJ obstruction, even though an IVP demonstrates poor visualization, the scan may demonstrate that the GFR in an obstructed kidney is quite good compared to the normal kidney. Furthermore, in the multicystic kidney, the renal scan is necessary to confirm that the kidney is nonfunctional. Fourth, the presence of renal obstruction may be detected by injecting furosemide when the collecting system is full, which stimulates washout of the radionuclide from the renal pelvis. If no obstruction is present, then one half of the radionuclide is cleared from the renal pelvis within 15 minutes, termed the "half-time." If there is significant obstruction, the half-time will be longer than 20 minutes. In some cases, the half-time is in the indeterminate 15 to 20 minute range. If vesicoureteral reflux is present, a catheter must be inserted into the bladder at the beginning of the study. There are multiple factors that can affect the results of the diuretic renogram, including hydration of the patient, when the furosemide is administered, and the "regions of interest" that are drawn by the radiologist to delineate the renal cortical outline. 14 Thus, the results of the study require careful interpretation.
Figure 3. Obstructing ectopic ureter. (A) Transverse section at 30 weeks gestation, dilated ureter (large arrows) and lower pole, right kidney (small arrows). (B) Longitudinal view demonstrating dilated ureter in abdomen (arrows). (C) Postnatal IVP 15-minute film. Drooping lily deformity, right kidney with deviation of lower pole ureter. The obstructed upper pole segment is nonvisualized and causes displacement of the lower pole of the kidney and ureter. The left kidney is normal. A right upper pole heminephrectomy was performed, with removal of the dysplastic segment (reprinted with permission8).
The most common cause of hydronephrosis in the newborn is a UPJ obstruction (Figure 2). Usually this is secondary to an intrinsic obstruction resulting in severe narrowing at the junction between the ureter and renal pelvis. In 20% of cases an accessory renal artery supplying the lower pole of the kidney is present, which has been implicated in its pathogenesis. In 15% to 20% of cases the obstruction is bilateral. The degree of renal pelvic and calyceal dilatation and alteration in renal function depends on the severity of the obstruction and the compliance of the renal pelvis. The diuretic renogram is extremely helpful in assessing the function of the obstructed kidney. We have had numerous cases in which a hydronephrotic kidney functions nearly as well as its counterpart in spite of documented obstruction. The condition is managed by excision of the stenotic area and anastomosing the ureter to the renal pelvis, termed "dismembered pyeloplasty." With a solitary kidney, bilateral UPJ obstruction, or if the obstructed kidney is contributing less than 35% to 40% of overall renal function, this procedure is performed shortly after the diagnosis is made, occasionally before the baby leaves the hospital. On the other hand, with unilateral obstruction and reasonable kidney function, we generally delay the repair until 2 to 3 months of age to avoid the slightly increased anesthetic risks that exist in the newborn15 and to allow bonding between the parents and their baby.10
If hydronephrosis and a dilated ureter are identified, the etiology may be an ectopic ureterocele, ectopic ureter, primary obstructive megaureter, vesicoureteral reflux, or a nonobstructive megaureter. A ureterocele is a cystic dilatation of the distal end of the ureter and is obstructive. The ectopic ureterocele and ectopic ureter occur most frequently in girls and usually are associated with complete duplication of the collection system, with obstruction of the upper pole. These conditions are bilateral in 10% to 15% of patients. The upper pole moiety usually is dysplastic and nonfunctional, and heminephrectomy is performed (Figure 3). A megaureter may result from an aperistaltic segment of the distal ureter that does not allow normal propulsion of urine. This may be severe, resulting in obstruction and functional renal impairment, necessitating ureteral reimplantation. In other cases the obstruction may be minimal and not require any therapy (Figure 4). A megaureter also may result from severe vesicoureteral reflux.
Figure 4. Three-month-old boy with bilateral hydronephrosis and mildly distended bladder demonstrated by prenatal ultrasonography. (A) IVP at 2 months demonstrates left megaureter and hydronephrosis with blunted calyces. The right collecting system is normal. (B) Tc-DTPA diuretic renogram performed. 22 minute image, at time of furosemide administration. Left kidney on right side of image (same study). (C) 37 minute image |same study). Half-time following furosemide administration less than 15 minutes (normal). Differential renal function: left 58%, right 42%.
An entity that may be confused with UPJ obstruction is the multi cystic dysplastic kidney. Although this is the most common cause of an abdominal mass in the neonate, many smaller multicystic kidneys are being detected incidentally by prenatal ultrasound. These kidneys are completely replaced by renal dysplasia and multiple cysts of varying sizes; renal scan demonstrates nonfunction. A contralateral renal anomaly is present in 25%. This is not an inherited condition. There are reports of Wilms' tumor or renal cell carcinoma developing in multicystic kidneys.17 Accordingly, these are generally removed electively at 6 months of age, unless the kidney is massive, in which case earlier nephrectomy may be necessary. Other forms of renal cystic disease include infantile polycystic kidney disease, which is characterized by tiny cysts involving the collecting tubules and is autosomal recessive, and adult polycystic kidney disease, which is autosomal dominant.
With bilateral hydronephrosis and a distended bladder, the most common conditions are posterior urethral valves and prune belly syndrome. Valves are abnormal mucosal folds in the prostatic urethra that result in obstruction of the bladder outlet. Severity of functional renal impairment depends on the severity of the obstruction. In the newborn, treatment includes transurethral endoscopic ablation of the valves or cutaneous vesicostomy. Although prune belly syndrome has an identical appearance to urethral valves on prenatal ultrasonography, the phenotypic appearance of the wrinkled abdomen is characteristic. This is caused by a generalized mesenchymal abnormality involving the urinary tract with a distended bladder, bilateral hydroureteronephrosis, and bilateral cryptorchidism. In contrast to urethral valves, however, significant obstruction usually is not present. An immediate urological evaluation is necessary to determine whether surgical treatment is required.
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Genitourinary Anomalies Detectable by Prenatal Ultrasonography