Hypoplastic left heart syndrome (HLHS) is a very serious congenital heart disease in which the chambers, valves and related blood vessels on the left side of the heart are so malformed that they cannot efficiently pump blood to the rest of the body. The heart defects that may be associated with HLHS include:

Closure of the fetal communication between the two receiving chambers of the heart (premature closure of the foramen ovale) may result in underdevelopment of the left atrium and left ventricle as well as producing severe, extra workload on the lungs.  This particular form of HLHS may produce symptoms even in the fetus and often carries the worse prognosis because of the damage to the blood vessels in the lungs. Fortunately, this form of HLHS is very rare - aortic atresia and mitral atresia account for the vast majority of HLHS cases.

The diagnosis of HLHS has made great strides with the advent of fetal ultrasound. This common test is able to allow a diagnosis of HLHS at 16 to 18 weeks of gestation. This allows pediatric cardiologists time to review the defects, various options, and outcomes with the parents, and offers them valuable time for counseling and education. Additionally, this will allow the surgical team time for planning and transportation of the mother and baby to a surgical center with experiencing in HLHS. Usually delivery at that site is suggested rather than having the child born at a community hospital and then transporting an ill newborn to the medical center.

If the condition is not diagnosed before birth, it will likely be diagnosed shortly afterward. During fetal growth, oxygen-rich blood is supplied through the ductus arteriosus, a special blood vessel that connects the pulmonary artery to the aorta, allowing blood to bypass the lungs and left side of the heart. Because of this detour, the fetus is protected to a certain extent and usually shows no ill-effects from the disorder.  

If the infant has not been diagnosed before birth with a fetal ultrasound, the baby will likely appear to be normal at birth. Throughout this period, the severity of symptoms depends on the balance of blood flow between the lungs and the ductus arteriosus. If there is a good balance, the condition may not manifest itself immediately. If the balance is less than optimal, the child may appear slightly blue, or cyanotic, because of a lack of oxygen-rich blood. However, when the ductus arteriosus closes, which usually occurs in the first few days of life, the child will go into shock and cardiovascular failure. Surgery is required in most cases of HLHS for survival.  

HLHS is relatively common among congenital heart defects, affecting between 4 percent and 9 percent of all children with congenital heart disease. An estimated 1,000 babies are born every year in the United States with HLHS. Before the advent of modern treatments, HLHS was responsible for 25 percent of all infant deaths related to congenital heart disease.