Fetal megacystis

Fetal megacystis occurs in the antenatal period in around 1-in-1,500 pregnancies. It is much more commonly seen in male than female fetuses.

Diagnosis

• In the first trimester, fetal megacystis is defined as a longitudinal bladder diameter greater than seven millimetres.
• In the second and third trimesters, it is defined as an enlarged bladder failing to empty during an extended ultrasound examination lasting at least 40 minutes.
• The main cause of fetal megacystis is bladder outlet obstruction, also known as lower urinary tract obstruction.

Prognosis

• If the bladder diameter is seven millimetres to 15 millimetres at the first trimester scan, resolution occurs in around 90% of cases.
• If the bladder diameter is greater than 15 millimetres, it is likely to be associated with a poor prognosis: usually hydronephrosis is progressive and results in dysplastic, non-functioning kidneys.
• Additionally, where the bladder obstruction is significant, there may be oligohydramnios and resultant pulmonary hypoplasia (poor lung development), which is associated with perinatal death.

You can view the ultrasound video on this page to see an example of bladder obstruction, courtesy of the Fetal Medicine Foundation.

Megacystis diagnosed during a first trimester ultrasound can be associated with aneuploidy, usually trisomy 18 (Edward syndrome), trisomy 13 (Patau syndrome) or rarer genetic syndromes.

The risk of an underlying genetic diagnosis varies depending on the extent of obstruction and bladder dilatation. There is:

• a 25% risk where the measurement is seven millimetres to 15 millimetres; and
• a 10% risk where the measurement is greater than 15 millimetres.

Megacystis is not thought to be an inherited condition itself. If found in association with other anomalies, it is more likely to be linked to a chromosomal syndrome or a genetic condition. These may have different inheritance risks to the megacystis in isolation.

The cause of megacystis determines the management. Serial ultrasound scans are recommended, as megacystis can spontaneously resolve or progress over the antenatal period.

Postnatal treatment options depend on the type of obstruction. Children with megacystis are more susceptible to kidney infections and kidney failure. Some may eventually require renal replacement therapy, including transplantation. Individuals affected with antenatal megacystis may also experience long-term bladder symptoms and/or bladder dysfunction, poor growth and musculoskeletal problems.

Long-term follow-up is overseen by a paediatric urologist and nephrologist.

For clinicians

• NHS England: National Genomic Test Directory
• Fetal Medicine Foundation: Urethral obstruction

References:

• Lesieur E, Barrois M, Bourdon M and others. ‘Megacystis in the first trimester of pregnancy: Prognostic factors and perinatal outcomes‘. PLOS One 2021: volume 16, issue 9. DOI: 1371/journal.pone.0255890

For patients

• International Society of Ultrasound in Obstetrics and Gynaecology: Fetal megacystis