Dilated Renal Pelvis
What is this and what is its significance?
Urinary tract disorders are common. They occur in a large percentage of the population,
are generally mild, and usually have no significant clinical implications. Major
defects are relatively rare.
On each side of the body there is a kidney and a renal pelvis. The renal pelvis
is the "funnel" that receives the secretions (urine) of the kidney tissue and transfers
them to the ureter (urine duct) that leads to the urinary bladder - from here, the
urine is excreted into the amniotic fluid.
Dilation of the renal pelvis is defined as an accumulation of excess fluid in these
"funnels". This can be seen on
ultrasound examination. Repeated examinations should be performed to confirm
that it is a persistent finding. It can arise from a blockage in different parts
of the urinary tract, although the cause may often not be known.
Several studies have shown that it is possible to grade the degree of dilation as
either mild or moderate depending on the week of pregnancy when it is diagnosed.
3.5 - 5 mm between the 15th and the 20th week.
5.5 - 7 mm between the 21st and the 24th week.
5 - 6 mm between the 15th and the 20th week.
7 - 8 mm between the 21st and the 24th week.
Severe dilation: values in
excess of the above indices.
The implications of mild dilation of the renal pelvis
In most cases this is not a kidney defect, and the dilation remains mild through
to the end of the pregnancy or even recedes.
In order to confirm that this is the case, it is important to monitor the pregnancy
by performing an ultrasound scan every two to three weeks in order to ascertain
that the degree of dilation is not increasing.
If there is a significant increase in the degree of dilation, this indicates that
there is a high probability of an obstructive defect in the urinary tract.
In these cases the condition will probably not worsen. If the dilation continues
through to term (birth), it is advisable to refer the mother to a nephrologist for
The condition is of no significance in 97% of the cases with a dilation of less
than 10 mm after birth and there is no need for surgical intervention. A number
of studies have found a higher frequency of chromosome abnormalities in fetuses
with mild dilation of the renal pelvis. In cases where this is an isolated finding,
chromosome abnormalities have been reported in about 1% - 3% of cases, whereas when
it is associated with other defects, the risk of chromosome abnormality is increased
to 31%. The most common chromosome abnormality in these cases is Down syndrome. Some studies, however, have found that
when the dilation is an isolated finding, the risk of a chromosome abnormality is
less than 1%, and others have calculated that the risk is only 1.5 times greater
than that for other women of the same age.
What should be done when this condition is diagnosed?
The width of the renal pelvis and any coexisting defects should be monitored by
performing repeated ultrasound examinations. The quantity of amniotic fluid should
also be monitored since this is another index of the normal functioning of the kidneys.
should be recommended in order to discuss the significance of the condition and
also to decide whether
amniocentesis should be performed. There are differing opinions. Amniocentesis
is undoubtedly the only test that can categorically rule out chromosome disorders.
However, it is neither medically indicated nor recommended that every pregnant woman
should undergo amniocentesis - only those who are at high risk of having a specific
problem that can be examined by this test (see Why is there no medical recommendation
for amniocentesis in all cases?). Most opinions hold that in the majority
of the cases in which mild renal pelvis dilatation is the sole finding, and taking
into account the other parameters mentioned above, which do not raise the risk for
a chromosome disorder, the weighted risk is not in the high-risk range for which
amniocentesis is indicated.
The "gold-standard" threshold for recommendation of amniocentesis is a risk of Down
syndrome greater than 1:386 based on the results of the biochemical marker screening tests,
and when the weighted risk is equal to or higher than this amniocentesis is generally
recommended. However, the presence of an mild renal pelvis dilatation may statistically
increase the risk of Down syndrome. Therefore in these cases some physicians suggest
that this should be integrated with the results of the biochemical screening tests
and the new threshold for recommending amniocentesis is a risk for Down syndrome
of greater than 1:1000 in the biochemical marker screening tests. If other abnormal
findings are also present, these guidelines are insufficiently established and the
woman should be referred for genetic counseling.
The implications of moderate-to-severe dilation of the renal pelvis
Moderate-to-severe dilation of the renal pelvis may occur as an isolated finding.
In some cases it may be possible to see an obstruction on ultrasound examination.
In cases of moderate-to-severe dilation, monitoring the width of the pelvis by ultrasound
examination every two to three weeks is very important so that signs of further
dilation and the appearance of
oligohydramnios (too little amniotic fluid) can be detected. The presence
of oligohydramnios is a bad prognostic sign for the fetus because it indicates inadequate
kidney function. Also, the paucity of amniotic fluid may lead to damage to the fetus's
lungs and disrupt their normal development. In cases that have been reported in
the literature where the width of the kidney pelvis was between 10 and 15 mm after
birth, most were healthy infants and about 40% needed some surgical intervention.
Various studies have reported an increase in the frequency of chromosome abnormalities
in cases of moderate-to-severe dilation of the renal pelvis. When this is an isolated
finding, the risk is estimated to be 4%, but when it is associated with other defects,
the risk increases to 35%. The most common chromosome abnormalities are trisomy 18 and trisomy
What should be done when this condition
It is important to monitor the width of the renal pelvis and to look for additional
defects in the urinary tract and in other systems by performing an ultrasound scan
every two to three weeks.
The parents should be referred for professional counseling in a multidisciplinary
clinic in a hospital where they will be able to consult with a nephrologist, a genetic
counselor, and an expert in ultrasound examinations. Genetic counseling should be
recommended in order to discuss the significance and implications of the condition,
whether it might be part of a syndrome, and also to decide whether further investigation
and amniocentesis are required.
In addition, recommendations for further investigation/monitoring will be discussed,
and based on the findings and their severity, a decision may be made whether or
not to continue the pregnancy. In cases where there is severe oligohydramnios, the
advisability of terminating the pregnancy will be discussed.