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Chapter 39 ■ The Fetal Urogenital Tract 1367
A B
FIGURE 39-16. Hyperechogenic kidneys. A and B, Transverse and longitudinal scans of a 24-week fetus shows increased cortical
echogenicity of both kidneys (calipers), more than expected when compared with liver (L) and increased corticomedullary differentiation.
The kidneys are normal in size, and AFV is normal. Ultrasound at 32 weeks (not shown) showed similar findings. Postnatal ultrasound
(not shown) on day 8 confirmed normal-sized kidneys with increased cortical echogenicity, and there were several tiny cortical cysts.
Subsequent ultrasound scans showed increasing number of cysts. The child, now 3 years old, has normal renal function and blood pressure. Parents have normal kidneys. Genetic testing for ARPKD and TCF2 was negative. Thus far, there is no definitive diagnosis for the
polycystic kidney disease.
Hyperechogenic kidneys
Normal sizeSmall kidney
Obstructive
dysplasia
FIGURE 39-17. Algorithm for evaluation of hyperechogenic kidneys. (Modified from Twining P: Genitourinary mal-
formations. In Nyberg DA, McGahan JP, Pretorius DH, Pilu G, editors: Diagnostic imaging of fetal anomalies. Philadelphia, 2003, Lippincott–
Williams & Wilkins.)
7
Normal variant
or obstructive
dysplasia
hyperechogenic kidneys that are normal in size with
preservation of medullary pyramids, and that are associated with normal AFV, have a favorable outcome and
may represent a normal variant.
61,82
Enlarged kidneys
Autosomal recessive
and autosomal dominant
polycystic kidney disease
Renal Neoplasm
Congenital mesoblastic nephroma is the most
common renal neoplasm in the fetus and newborn.
Enlarged kidneys with
associated anomalies
Trisomy 13
Meckel-Gruber syndrome
Beckwith-Wiedemann syndrome
Perlman syndrome
It is a benign hamartoma composed of mesenchymal
Simple Renal Cysts
Simple renal cysts have been reported in the fetus as early
as 14 to 16 weeks’ gestation.
83
Ultrasound usually shows
a small, solitary, unilocular cyst near the periphery of the
kidney. It should be differentiated from a cyst arising
from structures close to the kidney, such as a duplication
or mesenteric cyst. Most simple cysts resolve by 20 to 24
weeks’ gestation. However, one study documented a
simple renal cyst seen at 14 weeks that developed into a
MCDK at 18 weeks.
83
Therefore, if simple cysts are seen
in either the first or the second trimester, follow-up scans
are indicated.
tissue (spindle cells), as opposed to the epithelial tissue
of Wilms’ tumor. Wilms’ tumor is a malignant lesion
that is extremely rare in the fetus. Sonographically,
mesoblastic nephroma is indistinguishable from Wilms’
tumor. Mesoblastic nephroma is usually seen as a
moderately echogenic, solid mass completely replacing
the kidney or localized to part of the kidney
(Fig. 39-18). The mass may demonstrate increased vascularity and cystic components. Polyhydramnios is a
frequent association
84
and may lead to preterm labor
and preterm birth. Perinatal complications are likely,
including acute fetal distress, neonatal hypertension, and
hypercalcemia.
84
84
85
1368 PART IV ■ Obstetric Sonography
FIGURE 39-18. Congenital mesoblastic nephroma.
Longitudinal scan of a 35-week fetus shows a large, heterogeneous
solid mass (calipers) replacing most of the right kidney, except for
the upper pole (arrow).
FIGURE 39-20. Adrenal neuroblastoma. Longitudinal
scan shows a large solid mass (cursors) adjacent to the upper pole
of the left kidney (K). (Courtesy John R Mernagh, MD, McMaster
University Medical Center, Hamilton, Ontario.)
FIGURE 39-19. Normal adrenal gland. Longitudinal
scan of a 31-week fetus demonstrates the
gland (arrow) at the superior border of the kidney (K).
Y- or V-shaped adrenal
Adrenal Mass
At the end of the first trimester, the normal adrenal
glands appear as pyramid-shaped hypoechoic structures
at the superior aspect of the hyperechoic kidneys. They
are quite prominent, approximately half the size of the
kidney. The size of the adrenal gland increases with
gestation, but relatively less than the kidney. During the
second and third trimesters, corticomedullary differentiation is apparent, with a hyperechoic medulla and a
hypoechoic cortex. On longitudinal sonogram, the adrenals are seen as V- or Y-shaped structures superior to the
kidneys (Fig. 39-19).
Abnormalities of the adrenal gland include hemorrhage, cyst, hypertrophy, and tumor. The differential
diagnosis for fetal suprarenal masses includes adrenal
neuroblastoma, adrenal hemorrhage or adrenal cyst,
intra-abdominal pulmonary sequestration, enteric duplication cysts, and renal masses, including mesoblastic
nephroma, upper-pole cystic dysplasia, or hydronephrosis in a duplex kidney.
86
Neuroblastoma is the most common abdominal
malignancy in neonates, and the adrenal gland is the
most common primary site. On prenatal sonography,
the retroperitoneal mass can be cystic, solid, or of mixed
echogenicity
87,88
(Fig. 39-20). Most reported cases of
neuroblastoma have been identified in the third trimester. Metastases (liver, placenta) and hydrops have been
reported.
characterization of the tumor and extent of disease.
89
Fetal MRI is useful for detailed anatomic
90
There may be maternal symptoms of hypertension,
tachycardia, or preeclampsia, which result from elevated
catecholamines and correlate with a more advanced stage
of disease.
ally have a favorable outcome, and surgical resection is
usually curative.
recommended for small tumors (<
for cystic forms; this strategy may avoid surgery in some
neonates whose tumors regress spontaneously.
91
Prenatally detected neuroblastomas gener-
92
A short period of close observation is
3 cm) and particularly
88
Adrenal hemorrhage, which is much more common
in the neonate, can have a sonographic appearance
similar to that of an adrenal or renal neoplasm. Color
Doppler ultrasound may be helpful in differentiation.
93
The key to the diagnosis of adrenal hemorrhage is
evolution of the lesion over time; serial sonograms
demonstrate a change in echogenicity (from solid to
cystic) and a decrease in size of the mass.
94,95
Because
neuroblastomas may also regress, however, it is important to obtain postnatal follow-up in fetuses with presumed adrenal hemorrhage.
FIGURE 39-21. Measurement of renal pelvis. Trans-
ULTRASOUND GRADING SYSTEM OF HYDRONEPHROSIS
verse scan of the abdomen in a 21-week fetus shows prominent
renal pelves (cursors). The anteroposterior diameter measures
6 mm on the left side (L) and 4 mm on the right side (R).
Upper Urinary Tract Dilation
Dilation of the upper urinary tract accounts for approximately 50% of all prenatally detected renal abnormali-
96
It may be unilateral or bilateral and is more
ties.
common in males than females.
97
studies in unselected populations have reported a prevalence of 0.7% to 3.9% in the second trimester.
Hydronephrosis refers to abnormal dilation of the renal
pelvis and calyces. The term pyelectasis implies a milder
form of hydronephrosis, with dilation of the renal pelvis
only. Dilation of the urinary tract can be obstructive or
nonobstructive.
Hydronephrosis
Several prospective
98-100
Chapter 39 ■ The Fetal Urogenital Tract 1369
Grade 0
Grade 1
Grade 2
Grade 3
Grade 4
Measurement of the anteroposterior (intra-) renal pelvic
diameter (RPD) on a transverse scan of the fetal abdomen
is the simplest and most common technique used to
evaluate and classify renal pelvic dilation (Fig. 39-21).
The Society for Fetal Urology (SFU) proposed a different classification system based on the degree of renal
pelvic dilation (mild, moderate, marked), calyceal dilation, and parenchymal atrophy; with five grades (0-4) of
increasing severity
The size of the renal pelvis increases throughout gesta-
tion and there are published nomograms for RPD.
101,102
(Fig. 39-22).
5,103
However, controversy surrounds the definition and clinical importance of mild renal pelvic dilation, and different threshold values of RPD have been used for the
antenatal diagnosis of hydronephrosis. In general, the
cutoff values for RPD vary between 4 mm and 5 mm in
the second trimester and between 7 and 10 mm in the
third trimester.
97,98,100,104-108
In our study of 328 fetuses,
the 95th centile value for RPD was 4.4 mm at 20 weeks,
5.1 mm at 23 weeks, and 6.6 mm at 33 weeks—similar
FIGURE 39-22. Society for Fetal Urology grading
system. This classification is based on the appearance of the
renal pelvis, calyces (calices), and renal parenchyma. Grade 0: no
hydronephrosis; intact central renal complex. Grade 1: only
dilated renal pelvis; there is some fluid in the renal pelvis. Grade
2: dilated renal pelvis and a few calices are visible. Grade 3: all
the calices are dilated. Grade 4: further dilation of renal pelvis
and calices, with thin renal parenchyma. (Modified from Baskin
LS: Prenatal hydronephrosis. In Baskin LS, Kogan B, Duckett J,
editors: Handbook of pediatric urology. Philadelphia, 1997,
Lippincott-Raven.)
values to the published nomograms.
5,103
We consider the
following measurements as abnormal:
• RPD of 5 mm or greater at 18 to 23 weeks
• RPD of 7 mm or greater in the third trimester
Although decreasing the cutoff value to 4 mm in
midgestation can increase the sensitivity for the detection
of renal pathology, it can lead to a high false-positive rate,
perhaps generating unnecessary parental anxiety.
109
On
1370 PART IV ■ Obstetric Sonography
the other hand, it should be recognized that a normalappearing renal pelvis at midtrimester ultrasound does
not exclude obstruction.
hydration status, maternal pyelectasis, and size of the fetal
bladder may affect the RPD measurement.
more, RPD can vary greatly over 2 hours of ultrasound
observation.
115
These findings suggest caution when con-
110
Factors such as maternal
111-114
Further-
sidering the implications of renal collecting system dilation based on a single RPD measurement. Calyceal
dilation is an important finding and is always pathologic,
independent of the pelvic size.
100,109
The detection of fetal pyelectasis is important for two
reasons: aneuploidy and postnatal uropathy. The significance of pyelectasis as a marker for aneuploidy is
discussed in Chapter 31. Pyelectasis is usually an isolated
finding, but a detailed ultrasound examination should be
performed to detect other urinary tract pathologic processes and nonrenal anomalies. Renal pelvic dilation
may be transient or physiologic, but it may be the first
manifestation of a urinary tract abnormality, including
ureteropelvic or vesicoureteral junction obstruction,
VUR, duplex kidney, and urethral obstruction. In a
meta-analysis of 17 studies (104,572 patients screened),
Lee et al.
subjects) as a predictor of postnatal outcome.
116
examined antenatal hydronephrosis (1308
116
Hydro-
nephrosis was classified into mild, moderate, or severe
(based on RPD of <
7 mm, 7-10 mm, and >10 mm,
respectively, if ultrasound was performed in the second
trimester; <
9 mm, 9-15 mm, and >15 mm if in the
third trimester). The risk of any postnatal pathology was
12% for mild, 45% for moderate, and 88% for severe
hydronephrosis (Table 39-3). There was a significant
increase in the risk of postnatal pathology with increasing degree of hydronephrosis.
A prospective study by Sairam et al.
100
reported on
the natural history of hydronephrosis diagnosed in 227
fetuses on midtrimester ultrasound in an unselected
population. They demonstrated that 96% of the fetuses
with mild hydronephrosis (RPD >
4 mm and <7 mm at
18-23 weeks) experienced resolution in either the third
trimester or the early neonatal period; none required
postnatal surgery. However, approximately one in three
fetuses with moderate/severe hydronephrosis (RPD
7 mm or presence of caliectasis at 18-23 weeks) required
>
postnatal surgery. The overall third-trimester resolution
rate was 67% and postnatal resolution rate was 21%.
100
Different definition of third-trimester hydronephrosis
(RPD >
7 mm instead of >10 mm) and preselected study
populations weighted by more severe cases of hydronephrosis likely account for the lower third-trimester resolution rates (~30%) in other studies.
by Sidhu et al.
118
combined data from seven studies
109,117
A meta-analysis
and reported on the findings of serial postnatal renal
ultrasonography in children with isolated antenatal
hydronephrosis.
118
There was resolution, improvement,
or stabilization of hydronephrosis in 98% of patients
with SFU grades 1 and 2 and stabilization of hydronephrosis in 51% of those with SFU grades 3 and 4. These
results suggest that mild hydronephrosis is a relatively
benign condition.
It is important to identify those cases of prenatal pyelectasis most at risk of postnatal renal pathology, using the
RPD and the SFU grading system. When a maximum
prenatal RPD of 15 mm is used as a threshold, it predicts
postnatal obstruction and the need for surgery in 80% of
119
fetuses.
hood of postnatal uropathy and urologic surgery.
In utero progression also increases the likeli-
108,120,121
There is a greater likelihood for bilateral renal pelvic
TABLE 39-3. RISK OF POSTNATAL PATHOLOGY BY DEGREE OF
ANTENATAL HYDRONEPHROSIS
Degree of Antenatal Hydronephrosis, % (95% CI)
PATHOLOGY MILD MODERATE SEVERE TREND (P)
Any pathology 11.9 (4.5-28.0) 45.1 (25.3-66.6) 88.3 (53.7-98.0)
UPJ 4.9 (2.0-11.9) 17.0 (7.6-33.9) 54.3 (21.7-83.6)
VUR 4.4 (1.5-12.1) 14.0 (7.1-25.9) 8.5 (4.7-15.0) .10
PUV 0.2 (0.0-1.4) 0.9 (0.2-2.9) 5.3 (1.2-21.0)
Ureteral obstruction 1.2 (0.2-8.0) 9.8 (6.3-14.9) 5.3 (1.4-18.2) .025
Other* 1.2 (0.3-4.0) 3.4 (0.5-19.4) 14.9 (3.6-44.9) .002
Modified from Lee RS, Cendron M, Kinnanmon DD, et al. Antenatal hydronephrosis as a predictor of postnatal outcome: a meta-analysis. Pediatrics 2006;118:586-593.
Hydronephrosis was classified based on anteroposterior diameter of renal pelvis:
Second trimester: mild <
Third trimester: mild <9 mm, moderate 9-15 mm, severe >15 mm.
CI, Confidence interval. UPJ, ureteropelvic junction obstruction; VUR, vesicoureteral reflux; PUV, posterior urethral valve.
* Includes prune belly syndrome, VATER syndrome, and unclassified.
7 mm, moderate 7-10 mm, severe >10 mm.
<.001
<.001
<.001
Chapter 39 ■ The Fetal Urogenital Tract 1371
dilation to progress (26%) compared with unilateral renal
pelvic dilation (3%).
abnormal RPD in the third trimester to be the best ultrasound criterion to predict postnatal uropathy.
122
Some investigators have found an
108
Although
serial follow-up scans every 3 to 4 weeks are not necessary,
a repeat ultrasound in the third trimester may be useful.
The majority of infants identified prenatally as
having renal dilation are asymptomatic at birth. Pediatric
nephrologists and urologists vary greatly in their
management of antenatally diagnosed hydronephrosis,
because an evidence-based protocol is lacking.
123
There are no uniformly accepted guidelines regarding
antibiotic prophylaxis, postnatal workup, and surgery.
However, a postnatal renal ultrasound is the first examination of choice. In the neonate, the relative state of
dehydration and physiologic oliguria in the first 24 to 48
hours of life can result in underestimation of the degree
of hydronephrosis and a false-negative renal ultrasound.
124
Therefore, ultrasound should not be performed before
72 hours after delivery, unless severe bilateral hydronephrosis or severe hydronephrosis in a solitary kidney may
require early intervention. Because a normal ultrasound
does not exclude VUR, some authors have recommended
routine voiding cystourethrogram regardless of postnatal
ultrasound findings.
would perform voiding cystourethrogram only when
postnatal renal ultrasound is abnormal.
107,125,126
However, other authors
127,128
It has been
shown that screening with two successive renal ultrasound examinations performed at day 5 and 1 month has
high sensitivity and negative predictive value for prediction of significant uropathy.
129
The indication for renal
scintigraphy depends on the particular clinical situation.
The preferred isotope scan in the infant is the mercaptoacetyl triglycine (MAG3) renal scan. It provides a dynamic
study of the urinary tract, assessment of drainage, and
estimation of differential renal function. The static renal
scan using dimercaptosuccinic acid (DMSA, succimer) is
indicated for detection of focal parenchymal abnormalities. In renal failure and bilateral dilation, it helps to
distinguish between two equally affected kidneys or
markedly asymmetrical renal function.
In summary, we propose the following management
protocol for hydronephrosis detected in the second trimester with regard to repeat antenatal ultrasound, postnatal ultrasound, and antibiotics prophylaxis after
delivery. In fetuses with mild renal dilatation (RPD
7 mm), parents are counseled that it is a common
<
finding and may be physiologic or transient, and that the
risk of postnatal uropathy is small. The important
message is that prediction of outcome after a single scan
is difficult, and postnatal renal ultrasound is necessary.
Antibiotic prophylaxis is initiated after delivery to
prevent UTIs. In addition to postnatal renal ultrasound
and antibiotic prophylaxis, a repeat ultrasound in the
third trimester is recommended in fetuses with greater
degrees of renal pelvic dilation.
The effect of urinary obstruction on subsequent renal
development depends on the time of onset and severity
of the obstruction. The fetal urinary tract responds differently to chronic obstruction than the adult urinary
tract. In adults with chronic urethral obstruction, the
pelvicalyceal system is usually greatly dilated; in the fetus
there may be a relative lack of pelvicalyceal dilation and
possible development of macroscopic renal cysts. Experimental work in lambs has shown that ureteral obstruction originating in the last half of gestation causes simple
hydronephrosis and parenchymal atrophy.
if ureteral obstruction originates during the first half of
gestation, renal dysplasia and sometimes cyst formation
will occur
130
(Fig. 39-23). Therefore, in fetuses with
58
However,
A
FIGURE 39-23. Urinary tract obstruction produces a varied response from the kidneys. A, Normal
kidney. B, Pelvicaliectasis, with or without parenchymal atrophy. C, Renal cystic dysplasia, with parenchymal cysts. D, The dysplastic
kidney may cease to function (lack of pelvicaliectasis). E and F, Alternatively, the kidney may show increased echogenicity, with no visible
cysts but with pelvicaliectasis (E) or without pelvicaliectasis (F). In these cases, dysplasia is probably, but not invariably, present.
B
C
D
E
F
1372 PART IV ■ Obstetric Sonography
FIGURE 39-24. Ureteropelvic junction obstruction.
Longitudinal scan shows moderate caliectasis and extremely
dilated renal pelvis (P), with thinning of the cortex (arrows).
dilation of the renal pelvis, assessment of renal parenchymal echogenicity, thickness, and cysts are important,
because thinned parenchyma, echogenic parenchyma,
and cysts suggest more severe and long-standing obstruction with likely loss of renal function on the affected side.
Ureteropelvic Junction Obstruction
Obstruction at the ureteropelvic junction is the most
common cause of nonphysiologic neonatal hydronephrosis, with an incidence of 1 in 2000 live births.
131
Most cases of UPJ obstruction are functional (caused by
a muscular abnormality) rather than the result of fixed
anatomic lesions such as fibrous adhesions, kinks, valves,
or aberrant vessels. UPJ obstruction is more common in
males and is often unilateral. In 10% to 30% of cases, it
is bilateral.
On sonography, a dilated renal pelvis with or without
caliectasis is identified. The ureter and the bladder are
not dilated. Severe chronic obstruction leads to effacement of the calyces and thinning of the renal cortex
(SFU grade 4) (Fig. 39-24). Rarely, the renal pelvis may
be extremely dilated, presenting as a large, unilocular
cystic mass. Rupture of the collecting system results in
the development of a perirenal urinoma (Fig. 39-25;
Video 39-1). This “pop-off” mechanism may protect the
obstructed kidney from further prenatal damage and
may diminish the degree of hydronephrosis. The affected
kidney should be carefully assessed for renal dysplasia,
however, because the probability of a nonfunctional dysplastic ipsilateral kidney is about 80%.
132
The AFV is usually normal but may be increased
paradoxically. When unilateral hydronephrosis is accompanied by oligohydramnios, a search for contralateral
renal pathology is warranted (e.g., renal agenesis, multicystic renal dysplasia). UPJ obstruction may be associated with VUR and extrarenal abnormalities, including
anorectal anomalies, congenital heart disease, VATER
FIGURE 39-25. Perirenal urinoma. Transverse scan of
fetal abdomen at 22 weeks’ gestation shows a large perirenal
urinoma (*) displacing and compressing the kidney, which demonstrates dilated calyces and pelvis (P) caused by ureteropelvic
junction obstruction.
syndrome, and esophageal atresia. When the contralateral kidney is normal, the prenatal detection of unilateral UPJ obstruction should not alter obstetric
management. In cases of unilateral UPJ obstruction
diagnosed before 24 weeks, severe dilation (RPD
15 mm) is predictive of impaired postnatal renal func-
>
tion in the affected kidney.
133
When there is bilateral
UPJ obstruction, the prognosis depends on the severity
and duration of obstruction and on the AFV. Serial
ultrasound evaluations are necessary to assess the AFV,
progression of hydronephrosis, and development of
renal dysplasia. Early delivery is rarely indicated, except
when there is progressive bilateral obstruction with
severe oligohydramnios. Prophylactic antibiotic therapy
is initiated soon after birth, and neonatal urologic
workup is necessary.
Vesicoureteral Junction Obstruction
(Primary Nonrefluxing Megaureter)
Vesicoureteral junction obstruction is caused by a structural anomaly of the distal segment of the ureter, with
localized dysfunction or obstruction. It is more common
in males and is bilateral in up to 25% of cases.
ing anomalies of the urinary tract (VUR, UPJ obstruction, multicystic dysplasia) are frequently present.
Megaureters are classified into three types according to
their morphologic appearance. Type I megaureter dilation involves only the distal ureter, with a normalappearing upper tract. Type II extends to both ureter
and pelvis. Type III is associated with severe hydronephrosis and ureteric tortuosity.
On sonography, the affected kidney may demonstrate
dilation of the ureter and renal pelvis. A slightly dilated
134
Coexist-
Chapter 39 ■ The Fetal Urogenital Tract 1373
A B
FIGURE 39-26. Duplication anomalies. A, Longitudinal scan shows two separate collecting systems. The hydronephrotic, upper-
pole renal pelvis (U) is continuous with the dilated ureter (arrow). The lower-pole collecting system (L) is dilated because of reflux.
B, Longitudinal scan of the pelvis shows the ureterocele (Ur) separated from the lumen of the bladder (B) by a thin, curvilinear wall of
the ureterocele.
ureter may be difficult to recognize or may be mistaken
for bowel, although bowel contents are usually more
echogenic than urine. Identification of peristalsis in a
fluid-filled tubular structure does not confirm bowel
because it is often seen with hydroureter as well. To be
certain, the serpiginous cystic segments must be traced
to the renal pelvis and bladder. In addition, the ureter
generally comes into close contact with the spine, but
the small bowel does not. The ureteral dilation can be
severe, so size alone does not preclude megaureter as a
possible diagnosis. On prenatal sonography, nonobstructive causes such as VUR usually cannot be differentiated
from primary megaureter or vesicoureteral junction
obstruction.
Prophylactic antibiotic therapy is commenced soon
after birth. Postnatal investigations are necessary to
exclude VUR, duplication anomalies, and bladder outlet
obstruction. Most cases of primary megaureter either
resolve spontaneously or improve when managed conservatively.
(SFU classification) and megaureter type are significant
predictors of the spontaneous resolution rate.
135,136
The presenting hydronephrosis grade
135,137
In
children with grade IV or V hydronephrosis or a retrovesical ureteral diameter of greater than 1 cm, the condition may persist or resolve slowly and may require
surgery.
Duplication Anomalies
Unlike most urinary tract disorders, duplication of the
renal collecting system is more common in females. Two
ureteral buds arise from the mesonephric duct to grow
into the metanephric blastema. The duplex kidney contains two separate pelvicalyceal systems, with either complete or partial duplication of the ureters. When complete
duplication exists, the upper-pole ureter may end in
the bladder (where it often forms an ureterocele) or
DETECTION OF A URETEROCELE
• Thin-walled cystlike structure in bladder.
• Overlooked if bladder is empty.
• Full bladder can compress ureterocele.
• May cause bladder outlet obstruction.
ectopically, usually into the vagina or urethra. Classically, the upper-pole moiety obstructs, whereas the
lower-pole moiety refluxes. Sonographic findings useful
for prenatal diagnosis include identification of two separate noncommunicating renal pelves, hydronephrosis
in the upper or lower pole, ipsilateral dilated ureter,
and ureterocele.
138-140
The most common sonographic
appearance is hydronephrosis of the upper-pole moiety,
often associated with a dilated ureter and a ureterocele
within the bladder
138,139
(Fig. 39-26). If the dilated ureter
appears to insert at a level below the bladder base, an
ectopic ureter should be considered. The lower-pole
moiety may also appear hydronephrotic because of VUR.
However, identification of two separate collecting
systems, or the nondilated lower pole of a duplex kidney,
may be difficult because of its small size and displacement by the dilated upper renal pelvis and ureter.
A careful evaluation of the urinary bladder is necessary
to detect the ureterocele, which is seen as a thin-walled,
cystlike structure within the bladder (Fig. 39-26). The
diagnosis is easy when the bladder is partially full but
can be overlooked if the bladder is empty or only minimally distended. A full bladder can compress the ureterocele, resulting in nonvisualization. If ureteroceles
become sufficiently large, they may also obstruct the
contralateral ureteric orifice or cause bladder outlet
obstruction. Antibiotic prophylaxis is initiated at birth
1374 PART IV ■ Obstetric Sonography
when upper-pole hydronephrosis is detected antenatally
in a duplex kidney.
Vesicoureteral Reflux
Vesicoureteral reflux (VUR) can be primary (incompetent valve mechanism at the ureterovesical junction)
or secondary (due to an obstruction in the urinary
tract and high detrusor pressures). The main prenatal
sonographic finding is hydronephrosis, which may
be unilateral or bilateral. The ureter may be dilated.
Intermittent dilation of the collecting system favors
VUR. Fluctuation or variation in the RPD (changing
by more than 3 mm) during the course of an obstetric
sonogram was strongly associated with high-grade VUR
(grades IV-V).
renal abnormalities, including UPJ obstruction, duplex
kidney, MCDK, and unilateral renal agenesis.
141
VUR may be associated with other
125
The reported prevalence of VUR in children with
prenatally detected hydronephrosis varies widely because
different RPD cutoff values are used for inclusion and
different protocols for postnatal investigations (voiding
cystourethrogram either in all cases or only after an
abnormal postnatal renal ultrasound). A recent systematic review of 18 studies showed a mean prevalence of
15% for postnatal primary VUR after prenatally detected
hydronephrosis.
142
A normal postnatal ultrasound does not exclude VUR.
However, if two successive renal ultrasound examinations (at day 5 and at 1 month) were normal (RPD
7 mm), voiding cystourethrography showed abnor-
<
malities in only 6.7% of patients.
143
Neonatal reflux is
more common in male infants. It is often of low grade,
with a high rate of spontaneous resolution by 2 years of
144
However, in those children with high-grade VUR,
age.
spontaneous resolution is rare. There is a significant correlation between high-grade VUR and findings of either
renal dysplasia on ultrasound or renal damage scars on
DMSA renal scan.
144,145
Most neonates with VUR are
managed conservatively with antibiotic prophylaxis.
Lower Urinary Tract
(Urethral) Obstruction
Fetal megacystis has been reported as early as 10 to
14 weeks’ gestation when the longitudinal bladder
diameter is 7 mm or more
is 0.3% at 11 to 15 weeks.
of 145 fetuses with early megacystis, chromosomal
abnormalities were detected in 21%.
146
(Fig. 39-27). The incidence
147
In the largest case series
148
In the chromo-
somally normal group, severe megacystis (bladder length
15 mm) was invariably associated with progressive
>
obstructive uropathy.
148
However, if the bladder length
was 7 to 15 mm, there was spontaneous resolution of
the megacystis by 20 weeks in 90% of cases. Therefore,
follow-up ultrasound is necessary to interpret correctly
the importance of megacystis detected in the first
FIGURE 39-27. Megacystis in first trimester. Trans-
abdominal sagittal image of a 12-week fetus shows a distended
thick-walled bladder (arrow),
is no hydronephrosis.
measuring 13 mm in length. There
CAUSES OF FETAL MEGACYSTIS
Posterior urethral valves
Urethral atresia/stricture
Prune belly syndrome
Megalourethra
Cloacal malformation
Megacystis-microcolon–intestinal hypoperistalsis
syndrome
trimester. The role of early vesicocentesis and shunt
placement needs further investigation.
149
In fetuses with
severe megacystis, survival to the neonatal period was
poor even with intervention, and the survivors are at risk
of developing renal failure.
147,150
Posterior urethral valves are the most common cause
of lower urinary tract obstruction, followed by urethral
atresia or stricture. Posterior urethral valves are seen
exclusively in males and may cause total, intermittent,
or partial obstruction, with variable prognosis. Most
cases are sporadic, occurring in 1 in 5000 male births;
and the recurrence risk is small.
149
Back pressure causes
a persistently dilated urinary bladder, with a dilated
proximal urethra (keyhole sign) (Fig. 39-28, A). There
may be thickening of the bladder wall (>
2 mm) or a
severely distended thin-walled bladder, bilateral tortuous
hydroureters, and hydronephrosis (Fig. 39-28, B). If
the obstruction is severe and of long-standing, progressive renal parenchymal fibrosis and dysplasia develop,
resulting in severe oligohydramnios, pulmonary hypoplasia, and compression deformities (Potter’s syndrome).
There may be spontaneous bladder rupture with urinary
ascites or a calyceal rupture with perirenal urinoma
151
(Fig. 39-29). If spontaneous decompression occurs, this
Chapter 39 ■ The Fetal Urogenital Tract 1375
A B
FIGURE 39-28. Posterior urethral valves causing urethral level obstruction. A, Dilated urinary bladder (B) and
proximal urethra (*) give the appearance of a keyhole, characteristic of urethral obstruction in a 21-week fetus. B, Coronal scan shows
dilated tortuous ureters (arrows).
FIGURE 39-29. Urinary ascites. Longitudinal scan of a
22-week fetus shows a thick-walled bladder (B) and urinary ascites
(*) caused by spontaneous rupture of severe megacystis.
“safety valve” may protect the kidneys from further
prenatal damage and may diminish the degree of
hydronephrosis.
Urethral atresia causes the most severe form of
obstructive uropathy. The sonographic features include
a greatly distended bladder and anhydramnios after the
first trimester (Fig. 39-30). In the absence of antenatal
treatment, urethral atresia is almost always fatal, because
of associated renal dysplasia and pulmonary hypopla-
152
A small number of survivors have been reported
sia.
after antenatal intervention.
153
Prune belly syndrome is characterized by the classic
triad of absent abdominal musculature, undescended
testes, and urinary tract abnormalities (megacystis,
ureterectasis). Although some authors believe that the
FIGURE 39-30. Urethral atresia. Coronal scan of a
17-week fetus shows a greatly distended bladder (B) that occupies
the entire abdomen. The thorax (arrows) is compressed and bell
shaped because of pulmonary hypoplasia. There is anhydramnios. P, Placenta.
syndrome results from a primary mesodermal defect,
others explain the pathogenesis as a urethral obstruction
malformation complex (the muscular defect is secondary
to distended urinary system).
154
The bladder is typically
very large. The prostatic urethra is dilated, and the
appearance resembles posterior urethral valves. The
ureters tend to be tortuous and dilated. The kidneys
may be normal, hydronephrotic, or dysplastic. Other
abnormalities may be present, including intestinal malrotation, congenital heart disease, and musculoskeletal
deformities. Although not all infants have urethral
obstruction at birth, it has been suggested that transient
in utero obstruction may initiate the sequence responsible for this syndrome.
155
1376 PART IV ■ Obstetric Sonography
PRUNE BELLY SYNDROME
Absent abdominal musculature
Undescended testes
Very large bladder
Dilated prostatic urethra
Ureters tortuous and dilated
Kidneys normal, hydronephrotic, or dysplastic
B
Megalourethra is characterized by a congenital deficiency of the mesodermal tissues of the phallus, with
dilation of the penile urethra and enlargement of the
penis (Fig. 39-31). This condition has been classified
into two types, fusiform and scaphoid urethra, but it
is preferable to consider it as a spectrum rather than
two distinct entities. Urinary stasis in the dilated penile
urethra results in functional obstruction of the urinary
tract. The prenatal sonographic findings include those of
A
B
C
FIGURE 39-31. Megacystis and megalourethra. A, Oblique scan of the fetal pelvis at 21 weeks’ gestation shows a dilated
urinary bladder (B) with thick walls (calipers). B, Transverse view of the perineum shows a dilated penile urethra (arrow) with deficient
mesodermal tissues in the phallus. C, Postmortem photograph of the fetus at 23 weeks’ gestation shows enlarged penis. The penile urethra
was patent (not shown). (A and B courtesy Ants Toi, MD, Department of Medical Imaging; C courtesy Sarah Keating, MD, Department of
Pathology and Laboratory Medicine; Mount Sinai Hospital, Toronto.)
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