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Hypoplastic Left Heart Syndrome

What is Hypoplastic Left Heart Syndrome (HLHS)?

Hypoplastic left heart syndrome (HLHS) is a rare and complex congenital (present at birth) heart defect that represents an underdevelopment of the entire left side of the heart. HLHS is found in less than 1 percent of all live births, but it is among the most severe of all heart defects found in children. While this condition is very serious, the specialized physicians affiliated with Children’s Memorial Hermann Hospital provide comprehensive care to children with HLHS.

In a child with a normal-functioning heart, the right side of the heart receives oxygen-poor (blue) blood from the body and then pumps the blood through a large blood vessel called the pulmonary artery to the lungs to pick up oxygen. Then, the left side of the heart receives the oxygen-rich (red) blood from the lungs and -pumps it to the rest of the body through another blood vessel called the aorta.

In a baby with HLHS, most of the structures on the left side of the heart, including the left ventricle and certain valves, are too small and underdeveloped such that the heart cannot effectively pump blood to fulfill the body’s needs.

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What are the causes?

The causes of heart defects such as HLHS among most babies are unknown. Some babies have heart defects because of changes in their genes or chromosomes.  A combination of environmental exposures and genetic factors are likely responsible, but little is known about the specific cause.

How is it diagnosed?

HLHS may be diagnosed during pregnancy with a fetal echocardiogram, which is a specialized ultrasound of the fetal heart. The affiliated physicians in the Fetal Cardiology Program at The Fetal Center will confirm a diagnosis and prepare a delivery plan for both mom and baby. A multidisciplinary team of specialists will also develop the baby's immediate care plan following delivery.

If HLHS is not diagnosed in utero, and suspicion of a heart defect occurs after the baby is born, a pediatrician will refer the patient to a pediatric cardiologist or neonatologist to determine the diagnosis.

Some babies that are not diagnosed with HLHS before they leave the hospital may not show warning signs for several days.  Once signs are present, the infant may have an acute life-threatening event, such as a pounding heart rate, difficulty breathing, a weak pulse or a gray or blue cast to their skin.

How is it treated?

Typically, your child can be delivered without inducement and without a cesarean section (C-section), unless your obstetrician deems it necessary for other reasons.

A delivery plan will be developed carefully by your multidisciplinary team of affiliated physicians at The Fetal Center and the Children’s Heart Institute and will be discussed with you and your obstetrician. Your baby should be delivered at a tertiary care hospital that is well prepared to care for babies with congenital heart defects.  There should be a neonatal intensive care unit with the capability to provide specialized care and pediatric cardiology and pediatric cardiovascular surgery services available at that institution.

Immediately following delivery, doctors will evaluate your baby carefully and, in most cases, transfer the newborn to a neonatal intensive care unit, where he or she will be stabilized and assisted with any breathing and blood pressure concerns. Medicine will be given to help stabilize your baby and support him or her until surgery can be performed.

The Fontan Palliation Sequence

STAGE ONE | Norwood Procedure: The Norwood procedure is the first stage in a series of three open heart procedures to palliate HLHS. Typically, a Norwood procedure is done within the first two weeks of life and represents a high-risk, extensive operation. The principle behind the Norwood procedure is that the only good pumping chamber (the right ventricle) must pump blood to the body. There is no natural, permanent connection between the right ventricle and the body. Thus, the Norwood procedure involves dividing the blood vessel that takes blood from the right ventricle to the lungs and rerouting it to take blood to the body.

By dividing the blood vessel to the lungs, the blood supply to the lungs is taken away. Part of the Norwood procedure involves restoring some blood flow to the lungs. There are two methods generally used:

  • Modified Blalock-Taussig (MBT) shunt – a small tube from the main blood vessel to the body (aorta or branch of the aorta) that connects to the blood vessel to the lungs (pulmonary arteries)
  • Sano shunt - tube goes from the pumping chamber (right ventricle) to the blood vessel to the lungs (pulmonary arteries)

Both methods are accepted. The decision to use one versus the other is individualized based on the infant’s anatomy and the surgeon’s preference.

STAGES TWO & THREE: the blood which returns from the body is directly routed to the lungs without passing through the heart. (There is no ventricle pumping blood through the lungs as there is in a normal heart.)


Bidirectional Glenn Procedure and Fontan Procedure:

Among doctors and nurses, a Fontan refers to the procedure where the two big veins that bring blood back from the body (one from the upper body and one from the lower body) are connected directly to the lungs.  This is done at two separate operations. The upper vein is usually connected at about 6 months of age), and the lower vein is usually connected between 2 ½ and 5 years of age. The first operation is called a bidirectional Glenn procedure, named after Dr. Glenn, who first performed the procedure. The second operation is referred to as a Fontan procedure, named for Dr. Fontan. 

In a bidirectional Glenn procedure (Glenn), the large vein which brings blood without oxygen back from the upper body is connected directly to the lungs. This blood is oxygenated by the lungs, and then is returned to the heart. The blood which comes back from the lower body goes directly to the heart itself, and then gets re-pumped to the body without going to the lungs. Since unoxygenated blood from the lower body gets mixed in the heart (with the oxygenated blood from the upper body), and then is pumped to the body, the blood that goes to the body is not fully oxygenated. The usual oxygen saturation for a child with a Glenn is around 80%.  Children tolerate this level quite well, even though it is lower than a normal child’s oxygen saturation.

In a Fontan procedure, the large vein which carries unoxygenated blood to the heart form the lower body is connected directly to the artery that carries blood to the lungs.  At the end of this operation, all of the unoxygentated blood that comes from the body goes to the lungs, and all of the blood that comes from the lungs goes to the body.  The outcome is a normal blood flow pattern; however, the child’s heart is not structurally normal.

There are a number of reasons why the procedure is staged; that is, the upper vein is connected to the lungs at one operation, and the lower vein is connected at a different operation. Over time, this has shown to be a safer and better strategy for children, rather than doing the upper and lower veins during the same operation. Recovering from a Fontan can be very hard, and the heart has to be in the best shape possible in order for the Fontan to be successful.

A Glenn is not as hard to recover from as the complete Fontan, and a child whose heart is not in perfect shape will have a safer surgery with a Glenn than with a Fontan. A Glenn does not require additional work by the heart. Thus, when the Fontan surgery is performed, the heart is entering the operation in better condition.

Although the Fontan procedure is the best that can be offered for single ventricle patients at this time, it is far from perfect. With a Fontan, there is no ventricle to pump blood through the lungs. Only the “pressure” within the veins drives blood through the lungs. If there is any significant resistance to flow through the lungs, a Fontan will not work.


What are the long-term effects?

There is good reason to have hope for babies born with HLHS.  The national statistics for survival after a Norwood procedure are very favorable.  The hospital stay after a Norwood procedure can be up to 6 weeks or even more.  A Norwood procedure is the highest risk surgery in the field pediatric cardiovascular surgery – the surgery is performed on a newborn, the resulting blood flow pattern after surgery is not normal, and the surgery itself is very difficult.  The chances of a child having a catastrophic event from the second and third operations are much less likely than the first procedure.

Although commonly accepted as the standard of care for HLHS, the Fontan palliation sequence does not provide assurances that children will live a normal lifespan.  With current medical technology, there is hope that most children will live into adulthood with few long-term complications. The reality for a child with HLHS is their heart will never be normal, but they may be able lead a relatively normal life.  On-going research is being conducted to look at the neurodevelopmental and cognitive impact of HLHS in order to improve current treatments and medical therapies.

What follow-up care is needed?

Children with HLHS will need lifelong follow-up with a pediatric cardiologist.  During the first few years of life, these visits will be almost monthly.  After completion of the Fontan palliation, monitoring may be reduced to once or twice a year. 

Prior to the second and third stage surgeries, a child will have to undergo a heart catheterization to evaluate the structures of the heart and measure pressure gradients within the heart and lungs.  This catheterization will help determine the suitability of the next stage surgery.  Unfortunately, some children are unable to undergo the second or third stage palliation.  For some patients, despite positive results from surgery, the heart muscle becomes weak and consideration for heart transplant may become necessary. 

Patient Stories

Mason Brown Thumb

Mason: Hypoplastic Left Heart Syndrome

When our daughter was about 2 1/2 years old, my husband, Kevin, and I began discussing having another child. We wanted our daughter Macie to have a sibling, and we wanted them close in age. It was not long after that I found out I was pregnant for the second time.

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Why choose the Children’s Heart Institute?

As part of the Children’s Heart Institute at Children’s Memorial Hermann Hospital, the affiliated cardiologists and affiliated cardiovascular surgeons collaborate as a multidisciplinary team to treat babies prior to birth, infants, children and young adults with congenital and acquired heart disorders.  The team of affiliated physicians, nurses and coordinators work together with you to determine the best treatment plan for your child.  Your child’s referring physician is also kept fully informed of your child’s treatment plan. 

At Children’s Memorial Hermann Hospital, the affiliated cardiologists and affiliated cardiovascular surgeons understand the unique challenges, circumstances and intricacies of caring for young patients with heart conditions. In addition to the team’s medical expertise, Children’s Memorial Hermann Hospital provides families with patient-centered care, keeping families fully involved as part of the child’s treatment team as well as offering resources to meet the needs of your entire family.

Contact Us

Pediatric Cardiology Clinic
The University of Texas Health Science Center Professional Building
6410 Fannin, Suite 370
Houston, TX 77030
Phone: (713) 486-6755 (Appointment Line)

Pediatric and Congenital Heart Surgery Clinic
The University of Texas Health Science Center Professional Building
6410 Fannin, Suite 370
Houston, TX 77030
Phone: (713) 500-5746

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Note: This page is meant to be a helpful, informative introduction on the subject of HLHS in children. The information may not be applicable to all cases, especially if there are additional defects. It is not meant to replace the opinion of a personal physician.

03/2016 – This page was updated and approved by an affiliated pediatric physician at the Children’s Heart Institute.