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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.

Mild dilation:
3.5 - 5 mm between the 15th and the 20th week.
5.5 - 7 mm between the 21st and the 24th week.

Moderate dilation:
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 investigation.

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.

Genetic counseling 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 13.

What should be done when this condition is diagnosed?

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.

 
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