Fetal megacystis refers to the presence of an unusually large urinary bladder in a fetus.
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Epidemiology
The estimated incidence of antenatal imaging is at ~1:1500 pregnancies.
Associations
Associated anomalies are common 6 and include:
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on a first trimester scan (10-14 weeks)
if the longitudinal bladder diameter is 7-15 mm, the risk of chromosomal defects is estimated at ~25% 4
if the bladder diameter is >15 mm, the risk of chromosomal defects is estimated at ~10% 4
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megacystis microcolon intestinal hypoperistalsis (MMIH) syndrome (Berdon syndrome)
Pathology
It can result from a number of causes but the main underlying mechanism is either distal stenosis or reflux.
Radiographic assessment
Antenatal ultrasound
It is generally defined as a bladder diameter of:
>7 mm in the 1st trimester 3
>30 mm in the 2nd trimester ref
>60 mm in the 3rd trimester ref
Ancillary sonographic findings:
may show evidence of oligohydramnios
may show associated renal anomalies
Treatment and prognosis
The overall prognosis can be variable from progressive obstruction to spontaneous resolution. A follow-up ultrasound is necessary to correctly interpret the significance of megacystis detected in the first trimester.
If the fetus is chromosomally normal and there is megacystis on the 1st trimester scan:
there is a spontaneous resolution of the megacystis in about 90% of cases when the 1st trimester longitudinal bladder diameter is between 7-15 mm 4
if the bladder diameter is >15 mm, there is a very high likelihood of progressive obstructive uropathy 4
Management will depend on the underlying pathology.