Hypoplastic left heart syndrome (HLHS) is a very serious disease present at birth (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. This condition is also frequently referred to as left heart hypoplasia by physicians.

HLHS can manifest in a variety of malformations, including an overly muscular wall between the right and left side of the heart (septum) that pushes into the left heart; abnormal aortic and mitral valves on the left side of the heart; and a abnormally constricted aorta. The most common abnormality is aortic valve atresia, with or without mitral valve atresia.

Infants born with HLHS also have additional heart defects, such as hole in the septum (e.g., a ventricular septal defect). There may also be other abnormalities not associated with the heart. For example, in one review, about 30 percent of infants born with HLHS also suffered from major abnormalities in the central nervous system, and about 30 percent had genetic disorders.

The diagnosis and treatment of HLHS has made great advances in recent decades. In the 1970s, HLHS was invariably fatal. Today, however, physicians can diagnose the disease with a standard fetal ultrasound at 16 to 18 weeks into pregnancy. This extra time allows for physicians to counsel and prepare the parents as well as review the various options available to them and the possible modes of therapy.

After the baby is born, whether or not he or she has already been diagnosed, the question of when to perform surgery depends on the severity of the defects. In general, surgery will be performed a quickly as possible. Today's open-heart surgeries offer new hope for a longer life, though researchers do not yet know if surgery allows HLHS patients to mature past early adulthood.

The open-heart surgeries currently available are initially heart transplant surgery or a Norwood procedure, followed by a series of surgeries, each done at a different age, ranging from infancy through the toddler years. Both heart transplantation and the Norwood procedure carry risks and should only be done by a fully qualified pediatric heart surgeon.