Hypoplastic left heart syndrome (HLHS) is a serious birth defect in which the left side of the heart is severely underdeveloped and cannot pump blood to the body. It accounts for a small but significant portion of congenital heart defects, and without treatment in the first days of life, it is fatal. With a series of three open-heart surgeries, many children now survive into adulthood, though the condition requires lifelong cardiac care.
What Happens Inside the Heart
In a healthy heart, the left side does the heavy lifting. The left ventricle, the heart’s largest and most muscular chamber, pumps oxygen-rich blood out through the aorta to every organ in the body. In HLHS, the left ventricle and several surrounding structures never fully develop during pregnancy. The mitral valve (the door into the left ventricle), the left ventricle itself, the aortic valve (the door out), and the aorta are all dramatically undersized or, in some cases, nearly absent.
Because the left side can’t do its job, the right side of the heart has to pump blood to both the lungs and the rest of the body. This is only possible in the first hours and days after birth because of two natural openings that all babies are born with: a small hole between the upper chambers of the heart (the foramen ovale) and a short vessel connecting the pulmonary artery to the aorta (the ductus arteriosus). These temporary passages allow oxygen-rich blood to bypass the useless left side and reach the body through the right side instead.
Why the First Days Are Critical
For the first few days of life, a baby with HLHS may seem fine. The two natural openings keep blood flowing to the body. But those openings are designed to close shortly after birth, and when they do, the baby loses its only route for delivering oxygenated blood. The result is a rapid decline: the baby’s skin may turn gray or blue, breathing becomes labored, and the body begins to go into shock as organs are starved of oxygen.
To prevent this, doctors give a continuous infusion of a medication that keeps the ductus arteriosus open. This buys time until surgery can be performed, typically within the first two weeks of life.
Prenatal Diagnosis
HLHS can be detected as early as 11 to 14 weeks of pregnancy on a fetal echocardiogram, though it is more commonly identified during the standard anatomy ultrasound at 18 to 22 weeks. Prenatal diagnosis is a significant advantage because it allows the birth to be planned at a hospital with a pediatric cardiac surgery team already in place, avoiding the dangerous period where the ductus arteriosus might close before anyone realizes something is wrong.
The Three-Stage Surgical Repair
There is no way to “fix” the left side of the heart or make it grow to normal size. Instead, surgeons perform a series of three operations over the first few years of life that gradually reconfigure the heart’s plumbing so the single working ventricle (the right one) can handle all the pumping on its own.
Stage 1: The Norwood Procedure
This is the most complex and highest-risk operation. It takes place in the first two weeks after birth. The surgeon connects the small, underdeveloped aorta to the larger pulmonary artery and uses a patch to widen the aorta so it can carry blood to the body. A small tube called a shunt is placed to maintain blood flow to the lungs. The wall between the two upper chambers is also opened to allow blood to mix freely. After this surgery, the right ventricle is doing double duty, pumping blood to both the lungs and the body.
Stage 2: The Glenn Procedure
Performed when the child is 3 to 6 months old, this surgery begins to lighten the right ventricle’s workload. The surgeon disconnects the large vein that carries oxygen-poor blood from the upper body (the superior vena cava) and connects it directly to the pulmonary artery. Blood from the head and arms now flows passively into the lungs without the heart having to pump it there. The heart still pumps blood from the lower body to the lungs, but it’s handling less volume than before.
Stage 3: The Fontan Procedure
The final surgery typically happens between ages 2 and 4. The surgeon connects the large vein carrying blood from the lower body (the inferior vena cava) directly to the pulmonary artery using a tube. After this, all oxygen-poor blood flows passively to the lungs without passing through the heart at all. The single working ventricle can now focus entirely on pumping oxygen-rich blood to the body. This also reduces the mixing of oxygen-rich and oxygen-poor blood, improving the child’s oxygen levels.
Survival After Surgery
The Norwood procedure remains the riskiest stage. Overall survival through the first year of life, which includes the Norwood and Glenn procedures, is approximately 64%. Once a child completes all three stages and reaches the Fontan, the outlook improves considerably. One-year survival after the Fontan procedure is about 92%. Survival rates have been steadily improving over the past two decades as surgical techniques and post-operative care have advanced, and many Fontan patients now live into their 30s and beyond.
Living With Fontan Circulation
Completing all three surgeries is not the end of the story. The Fontan circulation, while life-sustaining, is fundamentally different from a normal two-ventricle heart. Blood reaches the lungs through passive flow rather than being actively pumped, which means the system operates at higher venous pressures than normal. Most Fontan patients have reduced exercise tolerance compared to their peers and need regular follow-up with a cardiologist experienced in congenital heart disease.
One of the most significant long-term concerns is liver damage. Because blood from the lower body no longer passes through a pumping chamber before reaching the lungs, it backs up slightly in the veins, creating chronic congestion in the liver. Over years and decades, this leads to a condition called Fontan-associated liver disease. The liver gradually develops scarring (fibrosis), and in more advanced cases, this can progress to cirrhosis or portal hypertension, which involves enlarged veins and fluid buildup in the abdomen. Abnormal liver nodules appear in 20% to 30% of patients after the Fontan, and studies have reported rates of 3% to 15% for the development of liver cancer up to 22 years after surgery.
Because of these risks, Fontan patients undergo regular liver imaging and blood work as part of their long-term care. In severe cases where both the heart and liver deteriorate, combined heart-liver transplantation has become an increasingly recognized option. Other potential long-term complications include abnormal heart rhythms, protein loss through the intestines, and gradual decline in the function of the single ventricle itself.
What Causes HLHS
In most cases, the cause is unknown. HLHS develops during the first trimester when the heart is forming, and the left-side structures simply fail to grow. Some cases are associated with genetic conditions, and the defect does appear to run in families at a slightly higher rate than would be expected by chance. Environmental risk factors have been studied but none have been definitively linked to HLHS. It is not caused by anything a parent did or didn’t do during pregnancy.