Mondo Description Lethal hydranencephaly-diaphragmatic hernia syndrome is a rare, genetic, lethal, multiple congenital anomalies syndrome characterized by hydranencephaly and diaphragmatic hernia, as well as macrocephaly, a widely open anterior fontanel, scaphoid abdomen and hypotonia. Additionally, congenital heart defects, polyhydramnios and pulmonary hypertension have also been associated.