Megacystis (enlarged urinary bladder) can occur as a primary congenital malformation, a manifestation of a syndrome (i.e. megaureter-megacystis syndrome, vesicoureteral reflux, megacystis-microcolon-intestinal hypoperistalsis), or it can be secondary to urethral obstruction from posterior urethral valves, urethral stricture, urethral agenesis, or persistent cloaca with associated urethral agenesis. Associated vesicoureteral reflux results from abnormally situated and deformed distal ureteral orifices that are too high and somewhat loosely attached to the angles of a poorly developed trigone. Embryologically the ureteral bud arises more caudally than normal from the mesonephric duct resulting in a short trigone precursor. Absorption of the duct is completed early and as a consequence allows the ureteral duct a longer period to migrate cranially and laterally. The trigone is poorly muscularised and the ureteral orifices are positioned more laterally than normal, and are patulous with deficient submucosal tunnels.


A massively dilated and persistent fluid-filled structure arising from the fetal pelvis in the presence of oligohydramnios after 6 weeks is strong evidence for megacystis secondary to complete bladder obstruction. The bladder often assumes a fusiform or pear-shaped appearance with the neck of the pear representing the bladder neck and dilated upper portion of the urethra. Obstruction is not always complete or constant and associated features can be variable. Oligohydramnios is thought to occur in approximately 50% of fetuses with urethral level obstruction and 40% have ureterectasis or hydronephrosis. Detection of renal cortical cysts in the presence of urinary tract obstruction indicates renal dysplasia. Following aspiration, shunt placement, or spontaneous decompression of the urinary bladder, thickening of the bladder wall is seen secondary to hypertrophy of the detrusor muscle. Spontaneous decompression may cause extravasation of urine and result in urinary ascites or a paranephric urinoma. Decompression through the urachus may result in a dilated urachus or urachal cyst. With megaureter-megacystis syndrome or vesicoureteral reflux the degree of hydronephrosis is usually less severe and nonprogressive. Occasionally it is severe but in some cases it can resolve spontaneously. Since the mechanism of megacystis is not urethral obstruction but rather incomplete bladder emptying due to either poor musculature or reflux, the amniotic fluid volume may be normal or only slightly decreased. In megacystis-microcolon-hypoperistalsis syndrome the cardinal features include an enlarged bladder, dilated small bowel and polyhydramnios. Hydroureter and hydronephrosis are almost always present. This syndrome is frequently lethal so a precise diagnosis is extremely important.

Differential Diagnosis

Urethral obstruction due to posterior urethral valves is more common in male fetuses and is characterised by bilateral hydronephrosis, hydroureter and a distended bladder. Urethral obstruction in females is more commonly due to urethral atresia. Megacystis-microcolon-intestinal hypoperistalsis syndrome is a frequently lethal condition that is more common in females; familial cases have been reported. It should always be considered in a female fetus with bladder distention in the presence of polyhydramnios. Urethral obstruction in females (4:1) is usually due to urethral atresia or detrusor hypertrophy and is associated with severe oligohydramnios. Simple vesicoureteral reflux may be unilateral or bilateral and hydronephrosis is usually mild to moderate in severity and nonprogressive. The amniotic fluid volume can be normal or slightly decreased and spontaneous resolution of the hydronephrosis can occur. The bladder is not obstructed but empties incompletely. There is an increased incidence of reflux in the siblings of children with reflux.

Sonographic Features

Enlarged fetal urinary bladder

Hydronephrosis and tortuous dilated ureters in 40%, often asymmetrical

Cystic dysplasia of kidneys in severe cases

Evidence of rupture may be present (thickened bladder wall, urinary ascites, paranephric urinoma, dilated urachus)

Oligohydramnios after 16 weeks, 50% with urethral obstruction

Lung hypoplasia,

Potterís syndrome in severe cases

Consider megacystis-microcolon-intestinal hypoperistalsis syndrome in a female fetus with a dilated stomach and normal to increased amniotic fluid

Associated Syndromes

  • Brachio-oto-renal
  • Megacystis-microcolon-intestinal hypoperistalsis
  • Megaduodenum (intestinal pseudo-obstruction)
  • Posterior urethral valves
  • Prune belly
  • Vesico-ureteral reflux


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