Fetal megacystis is a condition characterized by an abnormal enlargement of the bladder. In the first trimester, megacystis is defined as a longitudinal (lengthwise) bladder dimension greater than or equal to 7 millimeters. In the second and third trimesters, megacystis is defined as an enlarged bladder that fails to empty during an ultrasound examination lasting for a minimum of 40 minutes. The vast majority of cases are diagnosed between 10 and 14 weeks gestation.
The incidence rate of fetal megacystis is estimated to range from 1 in 1,330 and 1 in 1,670 pregnancies. Megacystis is more commonly seen in male than female fetuses with a ratio of 8:1.
Early diagnosis of megacystis is possible due to the highly visible nature of the bladder during ultrasound examinations. At 10 weeks’ gestation, the bladder is visible in 50% of fetuses, and by 13 weeks, the bladder is visible in 100% of fetuses. Early detection through routine perinatal care is critical, as it leads to early identification of underlying causes and/or additional malformations and/or anomalies. Early detection allows for genetic counseling and a multidisciplinary approach to management perinatally and postnatally.
The prognosis and recurrence risk for fetal megacystis varies widely and is highly dependent on the underlying cause of the condition, any associated malformations, and the presence of additional anomalies. In cases of first-trimester megacystis where there are no additional anomalies, and the bladder diameter is between 7 millimeters and 15 millimeters, spontaneous resolution is likely to occur, resulting in no long-term adverse outcomes. In cases in which the bladder diameter is more than 15 millimeters, additional diagnoses, adverse outcomes, and a poor prognosis are likely.
The most common underlying cause of megacystis is bladder outlet obstruction, also referred to as lower urinary tract obstruction, and when the obstruction is significant, additional anomalies like oligohydramnios, echogenic kidneys, and pulmonary hypoplasia are common. Oligohydramnios is estimated to occur in 45% of megacystis cases and is associated with an increased risk of perinatal death. Associated urological malformations occur in 32% of megacystis cases and are associated with an increased risk of perinatal death or long-term adverse outcomes postnatally. Aneuploidy, particularly Trisomy 13, Trisomy 18, and Trisomy 21 is estimated to occur in 15% of megacystis cases. In cases where malformations and/or aneuploidy or other anomalies are present, the prognosis rate is determined by the specific malformation or anomaly.
On its own, megacystis is not considered a genetic or inherited condition. If megacystis occurs in conjunction with additional anomalies or malformations, the risk of a chromosomal abnormality or genetic condition is higher and determines the inheritance risk.
Management of megacystis during and after pregnancy is determined by the underlying cause and associated diagnoses. Serial ultrasounds are recommended for the duration of the pregnancy, as megacystis can spontaneously resolve or progress. Long-term management for children born with megacystis is vital, as the risks of kidney infections, kidney failure, bladder dysfunction, poor growth, and musculoskeletal issues are higher. [84]