Allantoic cysts and posterior urethral valves: a case report.

Allantoic cysts are a somewhat rare entity. They are identifiable on antenatal ultrasound examination but are not easily distinguishable from pseudocysts, which are often associated with a poor prognosis. Their etiology remains obscure and obstructive uropathies have been proposed to be one of the underlying mechanisms. We report on a case in which both allantoic cysts and a patent urachus were detected antenatally and turned out to be associated with posterior urethral valves (PUV). The cysts were first seen in the early second trimester, with a full fetal bladder and patent urachus. They disappeared by the 29(th) week. During the subsequent sonographic examinations, the bladder was emptying regularly. The kidneys remained normal throughout the pregnancy. It is hypothesized that the increased pressure within the urinary tract kept the urachus patent, led to the formation of allantoic cysts and, later, to their perforation; this allowed the fetus to empty his bladder and kept him from the usual complications of PUV. This case emphasizes the importance of detection and characterization of umbilical cord cysts on antenatal ultasound examination and suggests that obstructive uropathies should be included in the differential diagnosis of umbilical cord cyst communicating with the fetal bladder.