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Bladder emptying into an umbilical cord cyst: prenatal sonographic sign of allantoic cyst with patent urachus

Accurate diagnosis of allantoic cyst of the umbilical cord is of major importance as it is commonly associated with omphalocele, urinary tract obstruction and urachal anomalies1. A 30-year-old primigravida was referred to our department for investigation after abnormal findings on firsttrimester ultrasound. Sonographic evaluation at 13 weeks’ gestation revealed a fetus with a large umbilical cyst (33 mm) surrounded by the umbilical vessels and in close contact with the anterior abdominal wall. The bladder was visualized and its appearance varied abruptly from filled with a diameter of 7 mm to empty with an irregular shape. Given the large size of the cyst and its proximity to the fetus it was difficult to exclude an abdominal wall defect. Serial ultrasound examinations showed persistence of the umbilical cyst and a progressive decline in amniotic fluid volume. A connection between the bladder and the cyst was clearly visualized at 18 weeks’ gestation as the bladder emptied into the umbilical cyst (Figure 1). Fetal magnetic resonance imaging showed the presence of a large umbilical cord cyst (100 mm) not attached to the abdominal wall (Figure 2). At 23 weeks’ gestation, the umbilical cyst showed a significant reduction in size (24 mm), the amniotic fluid volume was normal and the fetal bladder was clearly visualized within the pelvis but was irregular in shape. An allantoic cyst with patent urachus was suspected and this diagnosis was confirmed postnatally. On ultrasound, an allantoic cyst is typically seen at the center of the cord, between the umbilical vessels. A clue for first-trimester diagnosis of patent urachus, suggested by Sepulveda et al.1,2, is the detection of megacystis associated with an umbilical cord cyst. In our case, ultrasound clearly depicted retrograde micturition through the patent urachus to the umbilical cord at 18 weeks (Videoclip S1). After this gestational age the diameter of the umbilical cyst decreased and the amniotic fluid was normal. The cyst did not rupture, despite its size, and the bladder appeared normally filled. This suggests that the fluid from the cyst was drained, through its connection to the bladder, with fetal micturition. This is in agreement with findings previously published by Persutte et al.3 who suggested that disappearing fetal umbilical cord masses may be associated with urachal anomalies. Partial bladder exstrophy has been described in association with patent urachus. One prenatal characteristic feature is a non-visible fetal bladder4. Nevertheless, a single report indicated an umbilical cord cyst as the sole abnormal prenatal finding5. Magnetic resonance imaging can detect abdominal or bladder wall defects, working as a complementary useful diagnostic tool in the assessment of cystic masses of the umbilical cord6–8. In our case, this method was useful for demonstrating an umbilicalcyst separated from the abdominal wall and excluding bladder exstrophy.