Hypoplastic left heart syndrome (HLHS) is a severe congenital heart defect in which the entire left side of the heart is too underdeveloped to pump blood to the body. Without treatment, it is fatal within the first weeks of life. It is one of the most complex birth defects a baby can be born with, but a series of surgeries performed over the first few years of life can reroute blood flow so the heart’s working right side does the job of both sides.
What Happens Inside the Heart
In a healthy heart, the left side receives oxygen-rich blood from the lungs and pumps it out to the rest of the body through the aorta, the body’s largest artery. In HLHS, four structures on that left side fail to develop properly during pregnancy. The left ventricle, the chamber responsible for pumping blood to the body, is drastically undersized. The mitral valve (which lets blood into the left ventricle) and the aortic valve (which lets blood out) are either missing entirely or far too small to function. The ascending aorta itself is also underdeveloped.
The result is that the left side of the heart is essentially non-functional. The right ventricle, which normally only pumps blood to the lungs, must take over circulation for the entire body. In the first hours after birth, a small natural opening between the two sides of the heart and a temporary blood vessel called the ductus arteriosus allow some blood to reach the body. But these connections close on their own within days, which is why HLHS becomes life-threatening so quickly.
Signs in a Newborn
Babies with HLHS typically appear ill within hours to days of birth as those temporary circulatory pathways begin to close. The most visible signs include a bluish or grayish tint to the skin, lips, and nail beds, caused by oxygen-poor blood reaching the body. Breathing becomes rapid and labored. Feeding is difficult, and the baby may seem unusually lethargic or unresponsive. A weak pulse and cool extremities are also common. These symptoms can escalate rapidly into shock if the condition is not treated.
How HLHS Is Diagnosed
Most cases of HLHS are now caught before birth. A routine prenatal ultrasound can raise suspicion when the heart’s four chambers look uneven, and a specialized fetal echocardiogram confirms the diagnosis. This is most commonly performed during the second trimester, though there are documented cases of HLHS being identified as early as 12 to 14 weeks of pregnancy. Early diagnosis is a significant advantage because it allows the delivery to be planned at a hospital with a pediatric cardiac surgery team standing by.
When HLHS is not caught prenatally, it is usually diagnosed in the first few days of life based on the newborn’s symptoms and confirmed with an echocardiogram after birth.
What Causes It
The exact cause of HLHS is not fully understood in most cases, but genetics play a clear role. Some cases are linked to chromosomal conditions like Turner syndrome (where a female is missing part or all of one X chromosome) or Trisomy 18. Mutations in several genes involved in heart development have also been identified in HLHS patients. The defect can run in families: having one child with a congenital heart defect increases the chance of a subsequent child having one. In many cases, though, no specific genetic cause is found, and HLHS likely results from a combination of genetic and environmental factors during early fetal development.
The Three-Stage Surgical Path
There is no way to repair the underdeveloped left side of the heart. Instead, surgeons perform a series of three operations over the first few years of life that gradually reconfigure the circulatory system so the right ventricle alone can sustain blood flow to both the lungs and the body. Each surgery builds on the last.
Stage 1: The Norwood Procedure
This is the most critical and highest-risk operation, typically performed within the first week of life. The goal is to restructure the heart so the right ventricle can pump blood to the body through a rebuilt aorta. Surgeons also create a new pathway to supply blood to the lungs, either through a small tube from an artery or directly from the right ventricle. After this surgery, oxygenated and deoxygenated blood still mix inside the heart, so the baby’s oxygen levels remain lower than normal, but the circulation is stable enough to support growth.
Stage 2: The Glenn Procedure
Performed between 3 and 6 months of age, this surgery reduces the workload on the right ventricle. Surgeons reroute blood returning from the upper body so it flows directly to the lungs by gravity and passive pressure, bypassing the heart entirely for that portion of circulation. This significantly lightens the pumping burden on the single working ventricle, which is critical for its long-term health. The procedure cannot be done before about 2 months of age because the blood pressure in a young infant’s lungs is still too high to allow blood to flow passively through them.
Stage 3: The Fontan Procedure
The final surgery, usually performed between ages 2 and 4, completes the separation of oxygen-rich and oxygen-poor blood. Surgeons connect the blood returning from the lower body directly to the lungs as well, so that all returning blood now reaches the lungs passively without the heart needing to pump it there. After the Fontan, the right ventricle only pumps oxygenated blood to the body. Oxygen levels improve substantially, and the child’s skin color typically normalizes.
Survival and Long-Term Outlook
Survival after these surgeries has improved dramatically since the procedures were first developed in the 1980s. Most children who complete all three stages survive into adulthood. However, the long-term picture is sobering. A large study published in the Journal of the American College of Cardiology found that transplant-free survival was 31% at 35 years, meaning fewer than one-third of patients who underwent staged reconstruction as newborns were alive without needing a heart transplant by their mid-thirties. The highest-risk period is between the first and second surgeries, when the single ventricle is under the greatest strain.
Children and adults living with the Fontan circulation generally face reduced exercise tolerance compared to their peers. Many lead active lives, attend school, and work, but they require lifelong cardiology follow-up because the single-ventricle system, while life-sustaining, is not a permanent fix. It is a palliative solution that the body gradually outgrows or that leads to complications over decades.
Long-Term Complications After Fontan
The Fontan circulation creates chronically elevated pressure in the veins, because blood must flow to the lungs without a pump driving it. Over years and decades, this sustained venous congestion takes a toll on multiple organs.
The liver bears the brunt. A condition called Fontan-associated liver disease is now recognized as highly prevalent among survivors. The chronic congestion damages liver tissue, progressing silently from scarring to bridging fibrosis and eventually cirrhosis. Liver nodules are detected in more than half of Fontan patients on imaging, and complications of portal hypertension, including fluid accumulation in the abdomen and enlarged veins, appear in over half of patients with advanced disease. In some cases, this liver damage progresses to liver cancer, and an increasing number of patients are now undergoing combined heart-liver transplantation.
Kidney function, lung efficiency, and chronic low-grade inflammation are also concerns. Oxygen saturation in Fontan patients tends to remain mildly but persistently below normal due to the way blood flows through the lungs without active pumping. These issues develop gradually and often without obvious symptoms for years, which is why routine screening of the liver, kidneys, and lungs is a standard part of follow-up care for Fontan patients.
The Hybrid Approach
Some centers offer a hybrid procedure as an alternative to the traditional Norwood for the first stage, particularly for higher-risk newborns. Rather than open-heart surgery in the first days of life, the hybrid combines a smaller surgical procedure with catheter-based techniques to maintain blood flow. It was initially developed in the 1990s and has been refined since.
For high-risk patients, the hybrid approach does appear to lower mortality in the immediate newborn period. However, a systematic review found that hybrid patients had higher mortality between the first and second stages and higher mortality at one year compared to those who had the traditional Norwood. They also required more unplanned interventions and had longer hospital stays after the first procedure. By three and five years, survival rates between the two approaches were similar. The overall takeaway is that the hybrid may buy time for fragile newborns but comes with trade-offs in the months that follow.