Fig. 1: Cross section of the umbilical cord. The umbilical arteries (a) and vein (v) are separated by the cyst (c).

Both the umbilical cord and the umbilical vein were dilated distal to the cyst. The umbilical cord insertion into the fetal abdomen appeared normal. The fetal bladder was enlarged and distorted. In an oblique longitudinal scanning plane, a direct communication between the bladder and the umbilical cord cyst was documented. The umbilical artery and vein could clearly be seen entering the fetus adjacent to the fistula tract (fig. 2).

Fig. 2: Connection between the fetal bladder (b) and umbilical cord cyst (c). The umbilical artery (a) and vein (v) are adjacent to the urachus.

Pulsed wave Doppler of the umbilical arteries proximal and distal to the cyst demonstrated normal indices for gestational age. Also, Doppler analysis of the cyst failed to document flow. The fetus was male, and the fetal bladder was seen to fill and empty. No other fetal anomalies were seen, and the amniotic fluid volume was normal. It was felt that this represented a congenital patent urachus. The patient was counseled regarding these findings and offered genetic studies, which she declined.

Serial ultrasound exams documented normal fetal growth. The size of the umbilical cord cyst remained unchanged throughout the remainder of the pregnancy. Serial umbilical artery Doppler indices proximal and distal to the cyst showed no vascular occlusion and remained normal for gestational age. The fetal biophysical profile and non-stress test also remained normal. A healthy male infant weighing 3690g with Apgars of 9 and 9 at 1 and 5 minutes, respectively, was delivered vaginally without complications.

Initial neonatal examination revealed a 3x2 cm cyst at the inferior margin of the umbilicus which was observed to drain urine. Voiding also was documented from the penile urethra. Renal ultrasound and a voiding cystourethrogram were both normal. The baby initially had hyperbilirubinemia secondary to ABO incompatibility which resolved with phototherapy. On the eighth day of life the urachus was surgically ligated. The surgical pathology revealed transitional epi­the­lium consistent with urachus. The baby was discharged in good condition, and follow-up exams revealed no complications.

Discussion

Congenital patent urachus is a rare anomaly with an estimated incidence of 0.25:10,000 deliveries¹. Males are affected twice as commonly as females.

Embryology

Embryologically, the urachus is a derivative of the allantois. The allantois appears on day 16 post-conception as an outpouching of the caudal wall of the yolk sac. It functions as an embryonic bladder, in early blood formation and in formation of the definitive bladder. Normally, the extraembryonic part of the allantois degenerates during the second month of gestation. Occasionally, trace remnants of the allantois remain in the proximal umbilicus and may be seen between the umbilical arteries on pathologic examination of fetuses at this gestational age. The intraembryonic portion forms a connection from the umbilicus to the apex of the bladder. As the bladder enlarges, the allantois involutes to form the urachus. The urachus has little function after the second month of gestation. After birth it becomes a fibrous cord which remains in the adult as the median umbilical ligament. Failure of the urachal lumen to close can result in a variety of anomalies including complete luminal patency (patent urachus), distal urachal patency (urachal sinus), proximal patency (urachal diverticulum), and urachal cysts.

Differential diagnosis

The differential diagnosis of patent urachus includes anterior abdominal wall defects, bladder exstrophy, vascular lesions of umbilical cord (hemangioma, varix, true knot) or allantoic or omphalomesenteric cysts. Documentation of a normal umbilical cord insertion into the fetal abdomen along with integrity of the anterior abdominal wall help exclude omphalocele and gastroschisis. Lack of Doppler signals in the cyst excludes vascular lesions of the cord. The prenatal diagnosis of bladder exstrophy has been described³, and this lesion can be ruled out by the anechoic nature of the cyst and documentation of a fetal bladder. The presence of a direct communication between the bladder and the umbilical cord cyst will confirm the diagnosis of patent urachus. The umbilical cord may be dilated distal to the cystic mass and this may be related to absorption of fetal urine by Wharton"s jelly⁴.

Associated anomalies

Isolated patency of the urachus was not associated with any other congenital defects in those cases diagnosed prenatally. Rich et al, however, reported associated anomalies in 46% of children presenting with urachal anomalies⁶ including omphalocele, omphalomesenteric remnant, meningomyelocele, unilateral kidney, hydronephrosis and vaginal atresia. Careful fetal evaluation to rule out associated anomalies is indicated when patent urachus is diagnosed in utero.

Management

In the previously described cases where patent urachus was diagnosed prenatally, the umbilical cord cyst was seen to decrease in size as the gestation advanced. This did not occur in the present case with the cyst remaining constant in size until delivery. The possibility of vascular occlusion secondary to enlargement of the cyst has been suggested⁵, but there was no Doppler or ultrasonic evidence of this in the present case. This may be related to patency of the urinary outflow tract and the umbilical cord absorption of fetal urine.

References

1. Persutte WH, Lenke RR, Kropp K, Ghareeb C. Antenatal diagnosis of fetal patent urachus. J Ultrasound Med 7:399-403, 1988.

2. Persutte WH, Lenke RR. Disappearing fetal umbilical cord masses: Are these findings suggestive of urachal anomalies? J Ultrasound Med 9:547-551, 1990.

3. Jaffe R, Schoenfeld A, Ovadia J. Sonographic findings in the prenatal diagnosis of bladder exstrophy. Am J Obstet Gynecol 162:675-8, 1990.

4. Tsuchida Y, Ishida M. Osmolar relationship between enlarged umbilical cord and patent urachus. J Pediatr Surg 4:465- 67, 1969.

5. Romero R, Gianluigi P, Jeanty P et al: Allantoid cyst. In: Romero R, ed. Prenatal diagnosis of congenital anomalies. Norwalk, Connecticut: Appleton and Lange pp392, 1988.

6. Rich RH, Hardy BE, Filler RM. Surgery for anomalies of the urachus. J Pediatr Surg 18:370-372, 1983.