Ultrasound Diagnosis of Fetal Megacystis

Real World Experience Fetal Megacystis

Definition
First trimester (11–14 weeks)
• Longitudinal bladder diameter (LBD) ≥7 mm
• Measured in midsagittal plane
• Persistent filling, no emptying during scan

Sub-classification:
• 7–15 mm → mild megacystis
• >15 mm → severe megacystis
This cut-off is critical because prognosis shifts dramatically at 15 mm.

Second / third trimester
No strict universal size cut-off.

Megacystis is suspected when:
• Bladder persistently enlarged
• Fails to empty over 30–45 minutes
• Often associated with upper tract dilation

Pathophysiology
1. Mechanical outlet obstruction
o Posterior urethral valves (PUV)
o Urethral atresia
o Obstructing urethral membrane

2. Functional obstruction
o Prune belly syndrome
o Neurogenic bladder
o Visceral myopathy

3. Chromosomal / syndromic association
o Aneuploidy
o Microdeletions
o RASopathies

4. Transient / physiologic early imbalance
o Mild cases in first trimester
o Often resolve spontaneously

Etiology by Trimester
First Trimester Megacystis

If LBD 7–15 mm
• ~70–90% resolve spontaneously
• Chromosomal abnormality risk ~10%
• Structural obstruction less common

If LBD >15 mm
• High likelihood of true obstruction
• Increased risk of renal dysplasia
• Higher risk of pulmonary hypoplasia
• Poorer prognosis

Second Trimester Megacystis
• PUV (if male fetus)
• Urethral atresia
• Cloacal anomaly (especially female)
• Severe neurogenic dysfunction

Associations
A. Chromosomal Abnormalities
• Trisomy 18
• Trisomy 13
• Trisomy 21

Risk is highest in:
• Mild first trimester megacystis
• Associated increased NT
• Other structural anomalies

Chromosomal risk decreases when:
• Isolated
• Male fetus
• Large LBD >15 mm (more likely obstructive than chromosomal)

Structural Associations
• Bilateral hydronephrosis
• Hydroureter
• Thick bladder wall
• Oligohydramnios
• “Keyhole sign” (classic PUV)
• Renal cortical echogenicity

Syndromic / Genetic Associations
• Prune belly syndrome
• Megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS)
• Visceral myopathy genes
• Rare smooth muscle actin mutations
• Some cases with RASopathies

If megacystis + increased NT → genetic workup becomes important.

Role of NT
• Megacystis + increased NT → Think chromosomal / syndromic
• Megacystis isolated → Think obstructive

Prognosis by Bladder Size (First Trimester)

  LBD   Spontaneous Resolution   Obstructive Uropathy Risk   Chromosomal Risk   Overall Prognosis
  7–10 mm   ~80–90%   Low   ~5–10%   Usually good
  10–15 mm   ~50–70%   Moderate   ~10%   Variable
  >15 mm   Rare resolution   High   Lower than mild cases   Often poor

Second Trimester Features That Worsen Prognosis
• Early oligohydramnios (<20 weeks)
• Bilateral renal dysplasia
• Echogenic cortex
• Loss of corticomedullary differentiation
• Progressive bladder enlargement
• Fetal ascites (bladder rupture)

Pulmonary hypoplasia risk rises sharply with early severe oligohydramnios.

Scenario A
11+5 weeks, LBD 9 mm, NT normal, male fetus
→ High chance of spontaneous resolution
→ Offer genetic testing but prognosis good

Scenario B
12 weeks, LBD 18 mm, NT 4 mm
→ High obstruction risk
→ High renal damage risk
→ Genetic testing strongly indicated

Scenario C
18 weeks, large bladder, oligohydramnios, echogenic kidneys
→ High risk renal dysplasia
→ Guarded to poor prognosis

• Not all megacystis equals PUV.
• Size at diagnosis matters more than absolute gestational age.
• Early oligohydramnios is the strongest poor prognostic marker.
• Resolution before 16 weeks changes everything.
• Female severe megacystis is usually more serious.