Malformations of the fetal kidneys and urinary system are common and easily visualized
and diagnosed on ultrasound. This article presents the typical sonographic findings
of these abnormalities during the various stages of pregnancy. Because malformations
of the urogenital tract often have an association with genetic diseases/ciliopathies,
these are also discussed. To complete the article, we provide a brief overview of
the normal anatomy of the kidneys and urinary system during the various stages of
pregnancy.
Normal Anatomy of the Fetal Kidneys and Urinary System on Ultrasound
Normal Anatomy of the Fetal Kidneys and Urinary System on Ultrasound
The description of the normal anatomy of the fetal urogenital tract follows the points
during pregnancy at which an ultrasound examination is usually performed: in the first
trimester (gestational week 11–13 + 5 days), the screening examination (GW 22), and
in the third trimester (GW 28–32). In principle, both the fetal kidneys and the filled
urinary bladder can be visualized at each of these points in time [1]
[2]
[3]
[4]
[5]
[6]
[7]
[8].
Normal Anatomy of the Kidneys and Urinary System in the First Trimester
Normal Anatomy of the Kidneys and Urinary System in the First Trimester
During this ultrasound examination it is usually possible to visualize the fluid-filled
urinary bladder. This scan is performed either on a longitudinal plane (sagittal or
coronal) or, more usually, by taking a transverse view between the two umbilical arteries.
Using a high-resolution ultrasound device and from GW 12 onwards, the kidneys can
be visualized as relatively echogenic structures lateral to the spine with a hypoechoic
renal pelvis (transverse view, sagittal longitudinal view, and coronal longitudinal
view) ([Fig. 1]). During the first trimester, the amniotic fluid depends primarily on the mother
and is therefore not an indicator of fetal renal function [4]
[5]
[6] ([Fig. 1]).
Fig. 1 Illustration of a normal urogenital tract in the first trimester (GW 13 + 2): a Frontal view of both kidneys. b Transverse view of both kidneys. It is easier to make out the area near the transducer.
c Illustration of the filled urinary bladder in transverse view. d Illustration of the filled urinary bladder in transverse view between the umbilical
arteries shown in color. e Longitudinal view of the filled urinary bladder.
Normal Anatomy of the Kidneys and Urinary System in the Second Trimester
Normal Anatomy of the Kidneys and Urinary System in the Second Trimester
At this point, the fetal kidneys with the renal pelvis and pelvicalyceal system can
be visualized in all three planes (transverse, longitudinal sagittal). At this time
the renal arteries can also be visualized using color Doppler ultrasound. However,
this does not conclusively prove that kidneys are present, as in the absence of kidneys
the image usually focuses on the adrenal arteries.
Under normal conditions, the ureter cannot be visualized.
The urinary bladder can be visualized both in the longitudinal plane (sagittal, coronal)
or, even better, in the transverse plane between the two umbilical arteries. A normal
urethra can sometimes be visualized using a very high-resolution probe, but this is
not standard ([Fig. 2]).
Fig. 2 Illustration of the normal urogenital tract in the 2nd trimester (GW 22 + 2): a Frontal view of the two kidneys lateral to the aorta. b Parasagittal longitudinal view showing the kidney on the psoas muscle. c Transverse view in the dorsoanterior position, showing both kidneys lateral to the
spine. d Transverse view (back is right), also showing both kidneys. e Illustration of the urinary bladder in transverse view in B-mode. f Illustration of the filled urinary bladder between the umbilical arteries.
The amniotic fluid gives an indication of kidney function from about the 16th to the 18th gestational week, and certainly after GW 20. As a rule, the urinary bladder fills
during an ultrasound examination [1]
[8]
[9].
Normal Anatomy of the Kidneys and Urinary System in the Third Trimester
Normal Anatomy of the Kidneys and Urinary System in the Third Trimester
The basic presentation of the kidneys in the third trimester is similar to that of
the second trimester. Because more fluid is produced, the renal pelvis and pelvicalyceal
system often appear more fluid-filled. They often appear circular and hypoechoic;
it is important not to confuse them with renal cysts ([Fig. 3]).
Fig. 3 Illustration of normal kidneys in the 3 rd trimester (GW 30 + 4): a Longitudinal view. The normal renal pelvic-calyceal system can be easily visualized.
b Transverse view. Due to acoustic shadowing from the spine, it is usually only possible
to see the kidney that is close to the transducer (as shown in the image). c Frontal view of both kidneys.
As a rule, a normal ureter cannot be visualized. The urinary bladder is best visualized
in the transverse view between the umbilical arteries. The amniotic fluid is predominantly
an indicator of the fetal renal function ([Fig. 3]).
In both the second and third trimester, fetal urination can sometimes be detected
in the amniotic fluid [2]
[3].
The urinary bladder is best visualized in the transverse view between the umbilical
arteries at any gestational age. Detection of a filled urinary bladder always means
that at least one kidney is present.
Malformations of the Fetal Kidneys and Urinary System
Malformations of the Fetal Kidneys and Urinary System
In order to present them in a systematic way, it makes sense to classify the malformations
according to the respective gestational age.
Malformations in the First Trimester
Malformations in the First Trimester
In principle, it is possible to diagnose bilateral or unilateral renal agenesis or
pelvic kidney ([Fig. 4]) during the first trimester; however, this is technically very challenging. The
presence of a filled urinary bladder always implies the presence of a functioning
kidney.
Fig. 4 Fetus with initial megacystis: a Megacystis at GW 13 + 0. Hypertrophy of the bladder wall is visible. b Same fetus at GW 17. A normal quantity of amniotic fluid is present. c Same fetus at GW 22. Unchanged hypertrophy of the bladder wall and normal amniotic
fluid volume. d+e The hypertrophy of the bladder wall can be seen in particular in the color Doppler,
as the umbilical arteries are located far from the lumen in this image. The finding
remained stable, and a healthy child was born.
We always start an ultrasound examination in the first trimester by imaging the urinary
bladder. This allows us to be sure that at least one fetal kidney is present. This
indirect detection is much easier than direct visualization of the kidneys.
The malformation of the urinary system that is typically detected during the first
trimester is megacystis (lower urinary tract obstruction – LUTO); this is explained
in detail below.
Megacystis/Lower Urinary Tract Obstruction (LUTO)
Megacystis/Lower Urinary Tract Obstruction (LUTO)
Due to a permanent or temporary drainage obstruction, urine is retained in the urinary
bladder, making it appear significantly enlarged. This usually leads to more or less
massive displacement of abdominal organs and significant bulging of the abdominal
wall. Usually, both kidneys appear significantly enlarged and echogenic. The renal
pelvis is also significantly enlarged, and both ureters appear as hypoechoic sections.
A distinction is made between megacystis with a diameter of 7–15 mm, which is often
associated with chromosomal abnormalities, and severe cases of megacystis with a diameter
> 15 mm, which is usually an expression of drainage impairment. The drainage impairment
may be permanent (this occurs in 40 % of LUTO cases, often in female fetuses); this
is referred to as urethral atresia. Typically, the portion of the urethra that is
close to the urinary bladder is distended (keyhole phenomenon) ([Fig. 5], [6]).
Fig. 5 Initial diagnosis of megacystis at GW 17 + 1: a Megacystis in the transverse view with enlargement of the urethra. b Megacystis in the longitudinal view with bulging of the abdominal wall. c 3 D visualization of megacystis; in this technique (minimum mode), fluids are displayed
in black. d Frontal view showing both hyperechoic and dysplastically altered kidneys. e Transverse view showing megacystis and both hyperechoic and dysplastic kidneys. There
is still a normal amount of amniotic fluid, which argues against urethral aplasia.
Fig. 6 Illustration of two fetuses with pelvic kidney in the first trimester: a Transvaginal ultrasound at GW 12 + 5. Sagittal longitudinal view, the kidney is very
deep. b Frontal view of the same fetus, the kidneys can be seen in the pelvis. c Transabdominal ultrasound in a fetus at GW 14 + 5, the orthotopic kidney can be seen,
and a second kidney can be seen which is located much deeper, in the pelvis above
the urinary bladder.
As a result of pronounced drainage impairment involving megalourethra, megacystis,
and megaureter, there is urinary retention and dysplastic remodeling of the kidneys.
Pronounced megacystis can also lead to diaphragmatic elevation and pulmonary hypoplasia.
Renal failure that develops in untreated cases can also cause pulmonary hypoplasia
due to developing anhydramnios and due to the “exhalation” of pulmonary fluid into
the amniotic fluid. Pulmonary hypoplasia can arise from very different causes, and
its severity is virtually impossible to predict prenatally.
Permanent drainage impairments (often with a diameter > 15 mm) do not regress; in
such cases, the full range of prenatal medical options (conservative, intrauterine
therapy, termination of pregnancy due to a medical indication) should be discussed
with the parents.
There is also the possibility of temporary drainage impairment (often with a diameter
7–15 mm) (60 % of LUTO cases, often found in male fetuses), e. g., due to a valve
(urethral valves – often posterior valves, more rarely anterior valves). Spontaneous
remission can occur in this situation, so if megacystis is detected, a follow-up examination
should be performed promptly (after 1 week). With temporary changes, the prognosis
is significantly better; there is even a possibility that the child will be born healthy.
However, LUTO can also be an expression of a more complex disorder of the urogenital
tract (cloacal dystrophy, anal atresia, neural tube defect), even when there is spontaneous
remission; it is therefore important to conscientiously perform a detailed ultrasound
examination.
In all cases, a diagnostic puncture is recommended, as the more moderate cases of
megacystis (< 7 mm) in particular may be associated with chromosomal abnormalities
(e. g., trisomy 13).
Female fetuses with megacystis also have a 3 to 4-fold higher risk of megacystis-microcolon-intestinal
hypoperistalsis syndrome, which is diagnosed by molecular genetics [2]
[3] ([Fig. 5], [6]).
The malformation of the urinary system typically detected in the first trimester is
megacystis/LUTO. This can be a dynamic condition, which is why prompt follow-up is
recommended.
Malformations of the Kidneys and Urinary System in the Second and Third Trimester
Malformations of the Kidneys and Urinary System in the Second and Third Trimester
The second and third trimesters can be summarized together. Due to the increasing
size of the fetus and the increasing production of urine during the course of pregnancy,
malformations often become more prominent with increasing gestational age.
Bilateral Renal Agenesis
From about GW 16–18, this malformation results in a significant reduction in the quantity
of amniotic fluid. After GW 20, there is clear anhydramnios.
At no point is it possible to detect a filled urinary bladder (look for the umbilical
arteries), and no kidneys can be visualized at the typical paravertebral site. The
kidneys are normally located on the psoas muscle. Because the kidneys are absent,
this space is taken up by the adrenal glands which then appear large (hypoechoic);
the intestine also moves into this space. Visualization of the renal arteries is not
helpful here, as the adrenal arteries can usually be visualized.
Sometimes the umbilical cord has the appearance of small puddles of amniotic fluid,
so the use of color Doppler to detect the vessels is helpful here.
As the fetus discharges more and more of its pulmonary fluid into the amniotic fluid,
pulmonary hypoplasia develops. Small lungs typically result in a large heart, often
with overly developed muscle (myocardial hypertrophy) and pericardial effusion [2] ([Fig. 7]).
Fig. 7 Initial diagnosis of a fetus with bilateral renal agenesis at GW 21 + 5. Note the
poor examination conditions due to anhydramnios: a Frontal view with empty renal beds on both sides. b The same view as a) shows the large adrenal gland near the transducer, which is sliding in a caudal
direction. c) Frontal view with the aorta and lack of evidence of renal arteries. d Pronounced pulmonary hypoplasia with a normal sized heart.
If anhydramnios is detected, this means that the conditions for performing the examination
are always poor. First, try to find the urinary bladder (umbilical arteries), then
try to find evidence that the kidneys are absent.
Unilateral Renal Agenesis
Unilateral Renal Agenesis
This is usually an incidental finding in the context of the examination. The urinary
bladder is filled, and the contralateral kidney is usually visible and normal. Only
when searching systematically for the second kidney does it become apparent that this
is missing at the typical location. Here, again, the space is taken up by the adrenal
gland and intestine.
An important differential diagnosis to consider is pelvic kidney, in which the kidney
is typically located in the small pelvis of the fetus, ventral to the aortic bifurcation.
Unilateral renal agenesis has a favorable prognosis, although a possible association
with malformations of the internal genitalia in female fetuses must also be considered.
These often appear very late, so in all cases we recommend a follow-up at GW 28–30
[2] ([Fig. 8], [9]).
Fig. 8 Illustration of various fetuses with unilateral renal agenesis. In each case, an
orthotopic normal kidney and an empty renal bed are found on the contralateral side:
a GW 29 + 1, frontal view with normal kidney, aorta, and empty renal bed on the side
distant from the transducer. b The same fetus with evidence of a normal renal artery supplying the kidney. c GW 23 + 5, sagittal longitudinal view of the normal kidney. d The same fetus in a sagittal longitudinal view of the contralateral side and empty
renal bed. Note the large adrenal gland. e GW 22 + 4, sagittal longitudinal view of the normal kidney. f Same fetus on the contralateral side. The kidney’s space is occupied by the intestines
and adrenal gland, which makes the diagnosis challenging.
Fig. 9 Frontal view of a fetus with pelvic kidney at GW 21 + 4 B-mode (a) and with color Doppler (b). The pelvic kidney with normal renal structure can be seen near the pelvic vessels.
Pelvic Kidney
Pelvic kidney can be unilateral or bilateral (which is rare). Again, during the ultrasound
examination you typically visualize the normally filled urinary bladder, then look
for the kidneys and find that one or both of them are not in the usual location. In
this case, you must look for the kidney in the small pelvis of the fetus. Typically,
it is located towards the center in front of the aortic bifurcation. The vascular
supply mainly comes from the iliac artery. The structure may be completely unremarkable.
However, various signs of urinary retention may occur, usually due to an atypical
joining of the ureter and the urinary bladder [2]
[18]. Malformations of pelvic kidneys can also occur ([Fig. 9], [10], [11]).
Fig. 10 Frontal view of a fetus with pelvic kidney at GW 33 + 5. The normal kidney in an
orthotopic location can be seen in (a) and the pelvic kidney with normal structure cranial to the urinary bladder can be
seen in (b).
Fig. 11 A fetus at GW 28 + 5 with a normal orthotopic kidney (a) and a multicystic dysplastic pelvic kidney (b).
The prognosis for pelvic kidney is in principle good.
If unilateral renal agenesis is suspected, always think of pelvic kidney. Follow-up
of these findings at around GW 28 is recommended.
Horseshoe Kidney
Renal fusion, or horseshoe kidney, is a special form of renal abnormality in which
the two lower poles of both kidneys fuse at the center. This fusion is located ventral
to the aorta; the position of the kidney often appears slightly deeper than expected.
A typical feature is vascular supply of the medial junction from the aorta, which
can be visualized in a transverse view. Generally the horseshoe kidney functions normally,
which means that both urinary bladder filling and the quantity of amniotic fluid may
appear unremarkable. Because horseshoe kidney is usually associated with other malformations,
especially chromosomal abnormalities (trisomy 13/18), genetic diagnostic testing is
always recommended [2] ([Fig. 12]).
Fig. 12 GW 28 + 5. Illustration of a horseshoe kidney with fusion of both kidneys ventral
to the aorta in a fetus with a diaphragm hernia on the left side.
Cystic Kidney Disease
Cystic kidney disease leads to structural changes in one or both kidneys. These changes
can be multicystic or polycystic; in both cases, the entire organ is always affected.
It is important to distinguish this condition from rare cases of isolated renal cysts
with normal renal structure [2]
[3].
Multicystic Dysplastic Kidney (MCDK)
Multicystic Dysplastic Kidney (MCDK)
With an incidence of 1:1400 live births, this is a common renal malformation. It is
usually diagnosed in the second trimester. Ultrasound shows multiple cysts that do
not communicate with each other, and no normal renal structure is detectable. The
affected kidney is enlarged and dysfunctional.
Bilateral occurrence (20 % of cases) is characterized by anhydramnios due to renal
failure. Unilateral MCDK with contralateral renal agenesis (10 % of cases) is also
characterized by anhydramnios; in both cases, the prognosis is poor.
Unilateral MCDK with a normal contralateral kidney has a favorable prognosis. In the
course of the disease, it is important to watch out for the possible development of
a urinary transport disorder in the healthy contralateral kidney. If necessary, induced
early birth or intrauterine shunting should be considered, although such cases are
fairly rare. As a rule, the kidney affected by MCDK becomes significantly smaller
after birth, so surgical removal is only performed in rare cases.
Genetic diagnostics are recommended, preferably using a high-resolution method (genotyping
array, trio exome sequencing), as genetic changes are found in up to 15 % of cases
[19]
[20]
[21]
[22]
[24] ([Fig. 13], [14]).
Fig. 13 Various fetuses with a multicystic dysplastic kidney: a Transverse view at GW 25 + 5. b Same fetus in a longitudinal view. The size of the change can be seen. c Transverse view of a fetus with a multicystic dysplastic kidney (on the side distant
from the transducer, with a normal kidney close to the transducer, GW 22 + 6. d The same fetus in a frontal view. e Transverse view of a fetus with multicystic dysplastic kidney at GW 22 + 6. The affected
kidney crosses the midline. The healthy kidney near the transducer is visible. f Sagittal longitudinal view of the affected kidney of the same fetus.
Fig. 14 Various fetuses with MCDK and complications: a Unilateral MCDK and contralateral urinary retention at GW 28 + 5 (b). c Bilateral MCDK at GW 21. Anhydramnios can be seen. d Unilateral MCDK and contralateral renal agenesis and anhydramnios GW 22 + 2. e MCDK as horseshoe kidney with additional brain malformations GW 28 + 2.
Polycystic Renal Dysplasia
Polycystic Renal Dysplasia
A distinction is made between autosomal recessive polycystic kidney disease (ARPKD)
and autosomal dominant polycystic kidney disease (ADPKD) [2]
[3]. Both appear similar on ultrasound.
Autosomal Recessive Polycystic Kidney Disease (ARPKD)
Autosomal Recessive Polycystic Kidney Disease (ARPKD)
The autosomal recessive form (ARPKD) is considered to be a ciliopathy; ultimately,
it always affects several organs (kidney, liver, bile ducts and pancreas). A mutation
of the PKHD1 gene may be a fundamental cause, which is why genetic diagnostic testing
that includes the parents is always recommended. If this mutation is detected in the
parents, there is a 25 % risk of it recurring in their offspring (autosomal recessive).
The incidence is reported to be 1:40 000 live births.
On ultrasound, the kidneys appear massively enlarged and hyperechoic; they are unable
to form a typical renal structure with a distinct renal cortex and renal medulla.
It is not possible to distinguish individual cysts. This disease always affects both
kidneys. To date, there have been no reports of cysts being detected in the other
organ systems during the prenatal stage.
Because the disease is always bilateral, it is assumed in all cases that the affected
child will be clinically ill. If anhydramnios is detected prenatally, prenatal renal
failure can be assumed. This often results in typical myocardial hypertrophy of the
heart. The prognosis is poor.
If a normal amount of amniotic fluid is still detectable prenatally, after birth the
disease characteristically takes a chronic course involving pulmonary hypertension,
liver fibrosis, and biliary dysgenesis, with a need for dialysis and transplantation.
In such cases, the progression of the disease over time cannot be predicted during
the prenatal stage. Consequently, providing prognostic advice to the parents is very
challenging [2]
[3]
[19]
[22]
[25]
[26]
[27]
[28] ([Fig. 15], [16]).
Fig. 15 Polycystic kidney disease at GW 23 + 2. a Transverse view, the enlarged echogenic kidneys can be seen on both sides. b Frontal view, there is virtually no normal renal structure visible. Renal function
is still present, the amount of amniotic fluid is normal.
Fig. 16 PCKD GW 22 + 3 in transverse view. The bilateral hyperechoic kidneys are easily visible.
Because this form manifests clinically during childhood, it is also called the infantile
form.
Autosomal Dominant Polycystic Kidney Disease (ADPKD)
Autosomal Dominant Polycystic Kidney Disease (ADPKD)
Autosomal dominant polycystic kidney disease (ADPKD) is significantly more common,
with an incidence of 1:1000. Again, this is a ciliopathy which may affect the liver,
bile ducts, and pancreas in addition to both kidneys. The majority of cases are due
to a single mutation in the PKD1 gene (85 %), or more rarely in the PKD2 gene (15 %).
The postnatal course is characterized by delayed onset of renal failure; accordingly,
this is called the adult form. Because of the dominant inheritance, it is recommended
that the parents undergo ultrasound examination and genetic testing.
Ultrasound diagnosis is usually made in the second trimester based on hyperechoic
kidney enlargement without normal renal structure. It may be possible to visualize
the renal pelvis and pelvicalyceal system. Usually there is a normal amount of amniotic
fluid present. Prenatal renal failure is fairly rare [1]
[2]
[3]
[19]
[25]
[26]
[27] ([Fig. 17]).
Fig. 17 Illustration of fetuses with PCKD, all with a normal quantity of amniotic fluid:
a Longitudinal view, GW 22. b Transverse view, GW 23 + 2. c Longitudinal view, GW 28 + 2. d Frontal view, GW 28.
Polycystic Renal Dysplasia with Other Malformations
Polycystic Renal Dysplasia with Other Malformations
Bilateral polycystic renal dysplasia can also occur as a ciliopathy (multi-organ disease)
in the context of other malformations (see examples in [Table 1]).
Table 1
Syndromes (ciliopathies) that present bilateral hyperechoic cystic renal dysplasia
(selection).
|
Malformation/syndrome
|
Malformations
|
Gene mutation
|
|
Meckel-Gruber
|
Hyperechoic polycystic renal degeneration, CNS malformations (e. g., encephalocele),
polydactylia
|
MKS1, MKS2, or MKS3 gene
|
|
Bardet-Biedl/McKusick-Kaufman
|
Hyperechoic polycystic kidneys, polydactylia, genital malformation
|
BBS1–14 genes
|
|
Joubert
|
Hyperechoic polycystic kidneys, brain malformations (molar tooth sign)
|
37 genes
|
|
ARPKD
|
Large hyperechoic medullary sponge kidney
|
PKHD1 gene
|
|
ADPKD
|
Hyperechoic kidneys, discreetly enlarged
|
PKD1, PKD2
|
[2]
[3]
[22], ([Fig. 18], [19]).
Fig. 18 PCKD in the context of other malformations: a GW 13, trisomy 18 with AVSD and increased intracranial translucency and growth restriction.
b GW 15 + 5, trisomy 13 with alobar holoprosencephaly and cleft lip, jaw, and palate
cleft. c Frontal view both kidneys appearing hyperechoic in this fetus.
Fig. 19 PCKD in a fetus with Zellweger syndrome in transverse view (a), frontal view (b), and sagittal view (c). The bilateral hyperechoic dysplastic kidneys can be seen. The quantity of amniotic
fluid is normal.
In cases of bilateral hyperechoic kidney, high-resolution genetic diagnostic testing
of the amniotic fluid is always recommended.
Renal Cysts
Isolated renal cysts may occur. In such cases the renal tissue is completely normal,
which enables this condition to be distinguished from dysplastic kidney disease. The
isolated cysts (there may be several of them) are not connected to the urinary system.
This condition can be difficult to distinguish from pronounced urinary obstruction.
The prognosis is good [2] ([Fig. 20]).
Fig. 20 Illustration of a fetus with renal cysts: a Transverse view, GW 23 + 2. b The same fetus in longitudinal view. c Transverse view. On the contralateral side, the renal pelvis shows minimal urinary
retention. d The cyst extends far into the ventral area. e Fetus at GW 28 + 2, transverse view. f GW 28 + 2, longitudinal view.
Duplex Kidney
Duplex kidney can be either unilateral or bilateral. If both parts of the duplex kidney
are structurally normal and there is no drainage impairment, the diagnosis is challenging.
You may occasionally notice a kidney that appears enlarged, with significant enlargement
confirmed through biometry. Both renal pelvises are then significantly more marked
in appearance, especially in the later weeks of pregnancy. In these cases, the ureteropelvic
junction can be seen twice. It is also possible to detect two renal arteries, although
there are a large number of normal variants. Furthermore, the suprarenal arteries
are also located in the area adjacent to the adrenal glands, so that the usefulness
of this for diagnostic purposes is hard to assess.
Because a duplex kidney has two ureters, the development of a ureterocele is often
observed; this is similar in appearance to a cystic spatial growth in the fetal urinary
bladder, and is virtually conclusive for the diagnosis of duplex kidney.
If a portion of the duplex kidney is affected by urinary retention, often due to incorrect
joining of the ureter and the urinary bladder, the diagnosis is much easier. This
often affects the cranial pole of the kidney, which then shows varying degrees of
urinary retention. As a rule, especially in the later weeks of pregnancy, it is possible
to visualize the kinked and distended ureter.
Both parts of a duplex kidney on the same side may also be affected.
The prognosis is fairly favorable. As a rule, the contralateral kidney and/or second
part of the duplex kidney is unaffected, so there is no need for treatment until after
birth [2]
[29] ([Fig. 21], [22], [23], [24]).
Fig. 21 Fetuses with duplex kidney: a GW 24. There is massive urinary retention in the cranial pole; the path of the associated
ureter can be visualized. b GW 22 + 2, unremarkable duplex kidney, no signs of urinary retention. c GW 22 + 3, urinary retention in the cranial pole. d The same fetus from c at GW 28. No progression of findings.
Fig. 22 Fetus with duplex kidney over time: a GW 22, urinary retention in the cranial pole. b The same fetus in transverse view with visible ureter. c GW 23, clearly visible ureter. d Detection of ureterocele in the urinary bladder. e) GW 27 + 6, unchanged urinary retention in the cranial pole. f Detection of ureterocele at GW 27 + 6. g GW 33 + 3, detection of a massively dilated ureter.
Fig. 23 Fetus with duplex kidney, with various ultrasound techniques: a B-scan, GW 23, illustration of the ureter affected by urinary retention. b Static 3 D image (minimum mode), GW 27, spatial image of the path of the ureter.
c Inversion of the image from the B-scan with digital contrast, so to speak (inversion
mode).
Fig. 24 Unremarkable duplex kidney at GW 22 (a) and bilateral duplex kidney with bilateral urinary retention at GW 28 (b).
Urinary Retention
With urinary retention, there is a drainage obstruction which leads to a backlog of
urine in the urinary system. The cause may be found in the area of the urethra (LUTO
– always bilateral!) or in the area where the ureter joins the urinary bladder (may
be unilateral or bilateral), or may be due to valves in the area of the Ureter (either
unilateral or bilateral), or due to an incorrect outflow of the ureter from the renal
pelvis (either unilateral or bilateral). In the literature, urinary retention is synonymous
with pyelectasia and hydronephrosis. Its severity is determined based on the anterior-posterior
diameter of the renal pelvis (largest diameter), the description of the renal tissue
(typical or echogenic), and possibly the ureter if visible (diameter!) ([Fig. 25], [26]) [2]
[3]
[30]. The normal values according to gestational age are shown in [Table 2]
[31].
Fig. 25 Illustration of fetuses with urinary retention: a Mild bilateral pylelectasia at GW 28. b Mild bilateral pylelectasia at GW 23 + 2. c Massive enlargement of the renal pelvic-calyceal system at GW 33. d Massive unilateral urinary retention at GW 36 with markedly dilated ureter. e Pronounced unilateral pyelectasia at GW 27. f Significant enlargement of the renal pelvic-calyceal system at GW 27.
Fig. 26 a Mild bilateral urinary retention with correct kidney dimensions. b Pronounced unilateral urinary retention at GW 27. c+d Massive unilateral grade IV urinary retention with dilated ureter (c), with normal contralateral kidney (d). e Bilateral urinary retention with correct kidney dimensions.
Table 2
Normal values for distention of the urinary tract.
|
Ultrasound findings
|
GW 16–27
|
>/ = GW 28
|
Postnatal > 48 hours
|
|
Renal pelvic-calyceal system – anterior-posterior diameter
|
< 4 mm
|
< 7 mm
|
< 10 mm
|
|
Central enlargement of calyces
|
No
|
No
|
No
|
|
Peripheral enlargement of calyces
|
No
|
No
|
No
|
|
Parenchymal thickness
|
Normal
|
Normal
|
Normal
|
|
Parenchymal structure
|
Normal
|
Normal
|
Normal
|
|
Ureter
|
Normal/not visible
|
Normal/not visible
|
Normal/not visible
|
|
Urinary bladder
|
Filled
|
Filled
|
Filled
|
|
Oligohydramnios
|
No
|
No
|
|
Male fetuses are more likely to have a urinary obstruction than female fetuses.
Depending on the cause, more or fewer parts of the urinary system may be affected
by urinary retention. Unilateral findings are always located in the area between the
kidney and urinary bladder. As the fetus grows larger and urine production increases,
in the case of a permanent drainage obstruction, there is always an increase in urinary
retention during the course of the pregnancy. If the finding remains stable or decreases,
this is more suggestive of a temporary problem, such as an external pressure, or restriction
of the renal function on the affected side.
As part of the broader diagnosis, the possibility of genetic diagnostic testing of
the fetus should be discussed. Serial follow-up examinations (approx. 4–6 weeks apart)
should be performed to record these dynamics. The condition of both kidneys, the filling
state of the urinary bladder, and the quantity of amniotic fluid should be documented.
Furthermore, any visible signs of urinary retention in the ureter should be looked
for.
The earlier the drainage impairment becomes clinically relevant, the earlier dysplastic
restructuring with impairment of renal function may occur in the area of the affected
kidney. These dysplastic changes appear on ultrasound as a hyperechoic texture in
the affected kidney, which is reduced in size and features cortical cysts.
Multicystic renal degeneration may appear similar on ultrasound, but the kidney is
always enlarged during the prenatal stage.
In very rare cases, the pressure caused by the backlog is so great that urine leaks
into the surrounding tissue; this is called urinoma. Urinomas appear on ultrasound
as hypoechoic space-occupying lesions of varying size in the area of one of the kidneys
(or both in the case of a bilateral finding). These space-occupying lesions can compress
the kidney and displace it from its normal position. Urinoma should be considered
an indication of severe functional impairment of the affected kidney. Due to previous
damage to the kidney, shunt insertion is not useful in these cases [2].
Fetuses benefit from full-term delivery, regardless of the severity of the finding;
induction of premature birth is not recommended. In the case of very pronounced bilateral
findings, prenatal shunt insertion may be useful. The basic prognosis depends on the
severity of the urinary retention, but is usually very good. Very pronounced findings
usually require prompt postnatal pediatric surgery [2]
[3]
[30]
[31].
Late Onset LUTO
Urethral urinary retention may also occur for the first time late in pregnancy. In
contrast to LUTO that is diagnosed early in the pregnancy, the prognosis is significantly
better here.
Occasionally, if LUTO persists over a prolonged period, the fetus develops hypertrophy
of the bladder wall. This can be diagnosed based on a clearly increased distance between
the umbilical arteries and the lumen of the urinary bladder. This can also be measured
to assess the dynamics [2]
[3] ([Fig. 27]).
Fig. 27
a Bilateral urinary retention in the presence of a known left diaphragm hernia, frontal
view. b Detection of dilated ureters in the fetus from a at GW 21. c Same fetus at GW 29 with signs of urinary retention in the kidney and ureter. d 3 D representation (minimum mode) of the ureter affected by urinary retention in
the same fetus. e Initial diagnosis of LUTO at GW 31 with hypertrophy of the bladder wall, enlargement
of the urethra, and bilateral urinary retention affecting the renal pelvis (f).
Renal Hypoplasia
Renal hypoplasia can be unilateral or bilateral. It may be caused by an early and
severe urinary flow disorder with subsequent dysplastic remodeling of the kidney.
However, even with completely normal renal structure, the kidney may fail to increase
in size. In the course of the disease, oligohydramnios often develops as an expression
of impaired renal function. This condition is diagnosed through renal biometry. The
cause is unclear and the prognosis is difficult to assess ([Fig. 28]).
Fig. 28 Fetus with mosaic trisomy 20: a Empty renal bed in unilateral renal agenesis. b Hypoplastic pelvic kidney adjacent to the filled urinary bladder. c Illustration of the biometry values over time in a fetus with bilateral renal hypoplasia
and postnatal death. d Illustration of the amniotic fluid quantities over time in the fetus from c).
Decreased Amniotic Fluid
The quantity of amniotic fluid during the 2nd and 3 rd trimesters is mostly a function
of the fetus, especially the fetal kidneys. This is a dynamic situation, and accordingly
there is a high degree of variability; however, there are methods for quantification
using the single deepest pocket method and the amniotic fluid index.
A decrease in the quantity of amniotic fluid in conjunction with typical renal findings
on ultrasound may be an expression of fetal renal failure. As differential diagnoses,
a rupture of membranes (normal fetal kidneys, normal urinary bladder filling) and
intrauterine growth restriction with pathological Doppler (abnormal biometry, abnormal
Doppler, normal kidneys, normal urinary bladder filling) should be excluded.
Changes in the kidneys and urinary system in the second half of pregnancy are noticeable
due to changes in the quantity of amniotic fluid (oligo-anhydramnios), structural
changes, or due to fluid-filled areas of varying size. Both kidneys may be affected.
Diagnostic algorithm for malformations of the kidney and urinary system:
First trimester:
|
Urinary bladder filled
|
There must be at least one kidney present
|
|
Urinary bladder empty
|
Look for kidneys
|
|
Megacystis
|
Visualize kidneys (echogenicity, urinary retention)
|
|
Urinary bladder biometry
|
|
Concomitant malformations?
|
|
Follow-up (7 mm after a week, transiently larger diameter)
|
|
Diagnostic puncture recommended
|
Second/third trimester:
|
Renal pelvic-calyceal system – Distention:
|
Unilateral/bilateral?
|
|
Bladder filling?
|
|
Quantity of amniotic fluid?
|
|
Visualize ureter(s)
|
|
Concomitant malformations?
|
|
Consultation on genetic diagnostic testing
|
|
Progress monitoring
|
|
MCDK
|
Unilateral/bilateral?
|
|
Bladder filling
|
|
Quantity of amniotic fluid?
|
|
Concomitant malformations?
|
|
Consultation on genetic diagnostic testing?
|
|
Progress monitoring/management
|
|
PCKD
|
Bladder filling?
|
|
Quantity of amniotic fluid?
|
|
Concomitant malformations
|
|
Consultation on genetic diagnostic testing
|
|
Progress monitoring/management
|
|
Kidney cannot be visualized
|
Unilateral/bilateral?
|
|
Pelvic kidney?
|
|
Quantity of amniotic fluid?
|
|
Structure?
|
|
Cardiac muscle
|
|
With pelvic kidney and in female fetuses: Look out for urogenital tract malformations
|
|
Duplex kidney
|
Kidney biometry in 3 planes
|
|
Number of renal pelvises?
|
|
Unilateral/bilateral?
|
|
Ureterocele in the urinary bladder? – Look for it again
|
|
Quantity of amniotic fluid?
|
|
Follow-up examinations
|
