Hypoplastic Left Heart Syndrome (HLHS)

What is hypoplastic left heart syndrome?

Hypoplastic left heart syndrome (HLHS) represents a spectrum of heart diseases in which the left-heart structures are underdeveloped. These structures include the mitral valve, the left ventricle, the aortic valve and the aorta. HLHS is a congenital heart defect that forms progressively during pregnancy. The cause of HLHS is unclear, and there are no known risk factors for this disease. It is not preventable.

An understanding of HLHS begins with blood flow to a baby during pregnancy. Normally, blood flows from the placenta into the heart via the umbilical vein, carrying oxygen from the mother to the baby. Some of this blood enters the right ventricle through a valve called the tricuspid valve, and some of it passes across a hole in the atrium into the left ventricle. This oxygen-rich blood is pumped directly to the head and neck. The blood, which passes through the tricuspid valve, enters the right ventricle and is pumped into the pulmonary artery.

Vinny, who had a life-saving surgery for hypoplastic left heart syndrome (HLHS), pictured here in scrubs

Normally, a small fraction of this blood actually enters the lungs, which are filled with amniotic fluid. The remainder of this blood passes through a special artery called the ductus arteriosus, which connects the pulmonary artery to the aorta.

When a child has hypoplastic left heart syndrome, this flow pattern is altered — sometimes dramatically — before birth. In the most extreme cases, the connection between the left ventricle and the aorta is closed off completely, a condition known as aortic atresia, which can compromise oxygen delivery to the head and neck vessels.

Another problem is that the blood returning to the heart from the lungs, entering the left side of the heart, must go somewhere. In patients with milder forms of HLHS, this blood can enter the left side of the heart and be pumped out normally. But in more extreme cases, most or all of this blood crosses the hole in the top part of the heart (which normally allows blood to pass from the right to the left side) from left to right. 

In a small number of children, this flow is restricted by the size of the hole in the atrial septum. When this happens, the pressure in the lungs becomes excessively high, and the lungs themselves can become diseased. In some cases, a hole can be made in the atrial septum during fetal development to decrease the amount of lung disease a baby is born with.

Once the PDA begins to close (a natural occurrence), the baby will become extremely sick, due to lack of blood supply to the body. At this point, treatment is necessary for the baby to survive.

How we care for HLHS at Boston Children’s Hospital

At Boston Children’s Hospital, we have a team of specialists ready to evaluate your baby at the Advanced Fetal Care Center. This team includes cardiologists who specialize in fetal assessments and interventions, and a team of nurses and other specialists who will make sure your baby is in the best possible condition when born and will help you plan for every step along the way.

Boston Children’s surgeons treat a high volume of some of the most complex pediatric heart conditions in the world, with survival rates more than 98 percent. Our approach to treating HLHS is different for every child, based upon their individual needs. We are one of the few hospitals in the country to offer complex biventricular repair, an approach pioneered by Boston Children’s.

Our areas of innovation for HLHS

The Boston Children’s Heart Center offers a unique combination of technical expertise, a state-of-the-art intensive care unit and a continuous drive towards innovation that allows us to help when others cannot. We are here to serve as a resource to you.

We have experience in repairing the tricuspid valve in infants with HLHS who suffer from tricuspid regurgitation and can address obstruction of the reconstructed aortic arch using a catheterization.

In other cases, when the heart muscle may be failing, we have recently been approved for an FDA trial, testing mesoblastic stem cells as an experimental therapy to help augment growth and function of the left ventricle. In addition, we have begun a trial of mitochondrial transplantation as a means to rescue failing ventricles. These therapies are experimental and your child may not meet the criteria necessary to receive them; however, if you would like to discuss these options, contact our Heart Access Center at 617-919-6185.