This is a routine ultrasound scan performed at 20 weeks of gestation. There is a cystic structure in the posterior mediastinum (note its relation to the descending aorta).

The above antenatal scans show a cystic mass in the posterior mediastinum, above the diaphragm. No flow is seen within the mass. No other abnormalities were noted. Among the many likely diagnoses were hiatal or diaphragmatic hernia, neuroblastoma, neurenteric cyst, esophageal duplications...

The baby was born at 38 weeks of gestation as a result of normal spontaneous vaginal delivery. She had no feeding or respiratory difficulties. There were no pertinent positive findings on physical examination.

A CT of the chest was performed, which revealed a posterior mediastinal mass that was subcarinal and retrocarinal in location. The mass was low in density and encircled the esophagus and aorta. Follow-up MRI demonstrated a cystic structure consistent with a posterior mediastinal cystic hygroma and was atypical for both a neurenteric cyst and a duplication cyst. A neuroblastoma was less likely. The surgical service was consulted and they felt that the patient could be followed as an outpatient with serial imaging studies.