Any part of the renal
tract can become dilated and common sites are the
collecting system within the kidney (hydronephrosis),
the ureter (megaloureter), or the bladder (megacystis).
Blockage of the renal/urinary
tract can be physical, where there is a narrowing
(stenosis) or complete block (atresia) to the flow of urine, or functional,
where the tubes are open (patent) but are not working properly to propel
the urine along. A failure of function of the pelvi-ureteric
junction (PUJ obstruction),
reflux of urine back up the ureter from the bladder, a blockage at the uretero-vesicle
junction or blockage along the urethra.
Complete obstruction
in early fetal life causes renal hypoplasia and
dysplasia. Intermittent obstruction may allow normal
renal development. Obstruction in later pregnancy
will result in hydronephrosis and severity of renal
impairment depends on the degree and duration of
obstruction.
Hydronephrosis
Mild hydronephrosis,
sometimes called dilated renal pelvis (DRP) or
pyelectesis, is a common finding on antenatal ultrasound
scans. There is some evidence that this finding
on a mid-pregnancy scan increases the chances of
the fetus being affected by Down's syndrome, and
some authorities have recommended karyotyping for
this reason. Most fetal medicine specialist in
this country would not recommend this approach
unless there is some other risk factor for Down's
syndrome, such as advanced maternal age, other
structural anomaly, or abnormal serum screening
results.
Hydronephrosis is the
most common fetal renal anomaly. Mild degrees of
transient hydronephrosis are present in many fetuses
during pregnancy, possibly due to the high levels
of maternal hormones that cause the smooth muscle
of the urinary tract to relax.
Dilation of the urinary tract signifies obstruction. The size of the renal
pelves is dependent on gestational age but calyceal dilation is more significant
and usually indicates significant hydronephrosis. It is important to delay
postnatal assessment until 48 hours as mild to moderate hydronephrosis may
not be present in the first day of life due to dehydration.
