A pleural effusion is a collection of excess fluid that builds up in the space that surrounds the lungs (pleural space). The pleural effusion can be detected during the pregnancy by ultrasound. On ultrasound, the lungs appear to float within the excess fluid in the pleural space. The causes of pleural effusions can be varied, and is usually split up into primary and secondary causes.
The most common cause of primary pleural effusions is chylothorax. Chylothorax is often due to malformation of the thoracic duct within the chest. The pleural effusions are usually not symmetric, and often displace the heart to one side of the chest. The excess fluid in the chest may cause underdevelopment of the lung tissue and lethal pulmonary hypoplasia, as well as compression of cardiovascular structures and eventual fetal hydrops (heart failure) and death. Pleural effusions can also be caused by an underlying fetal condition, such as a chromosomal abnormality, infection, tumor, cardiovascular abnormality, and/or other syndromes. Despite a thorough evaluation of the fetus for these secondary causes of the pleural effusion, the diagnosis may not be made until after the baby is born.
Fetal pleural effusions from chylothorax have a variable outcome, from complete spontaneous resolution to fetal hydrops and death. Improved perinatal survival has been documented in fetuses with persistent pleural effusions with hydrops or impending hydrops treated by thoracoamniotic shunting. A thoracoamniotic shunt is a tube that is placed into the chest of the fetus so that one end is in the pleural space and the other end is protruding out of the chest and into the amniotic cavity. This tube allows for drainage of the pleural effusion into the amniotic fluid. By decompressing the pleural effusion, the lungs can develop more normally and the heart can function better.