Dr. Alessandro Giardini
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A diagnosis of hypoplastic left heart syndrome is one of the most confronting things a parent can be told. Whether it comes during a routine antenatal scan at twenty weeks, or in the hours after birth when a midwife notices something wrong with a newborn's colour or breathing, the words carry an immediate and devastating weight. The left side of the heart has not formed properly. Surgery will be needed. Several surgeries. Beginning within days of birth.
What follows that moment is a long journey, and families navigate it better when they understand what is actually happening, what the surgical plan is trying to achieve, and what to expect at each stage. This page is written for parents: not to minimise the seriousness of the diagnosis, but to give a clear and accurate account of what HLHS is, how it is managed, and what life after palliation can look like for children who complete the surgical pathway.
Hypoplastic left heart syndrome is a complex congenital heart condition in which the entire left side of the heart fails to develop normally during pregnancy. The left ventricle, which in a healthy heart is the most powerful pumping chamber and is responsible for pushing oxygen-rich blood out to the body, is severely underdeveloped and cannot perform this function. The structures closely associated with it are almost always affected as well: the mitral valve, the aortic valve, and the ascending aorta are all typically hypoplastic (small), narrowed, or atretic (fully blocked) to varying degrees.
In a baby born with HLHS, the body's systemic circulation depends entirely on the right ventricle pumping blood to the body. That ventricle, designed to pump blood only to the lungs, is required instead to supply the whole body. It can do this in the neonatal period only because of two foetal connections that are normally present at birth and have not yet closed: the ductus arteriosus, a vessel that links the pulmonary artery to the aorta, and the foramen ovale, a communication between the two upper chambers. As long as these remain open, the right ventricle can sustain some systemic output. When they close, as they normally do in the first hours and days of life, a baby with HLHS cannot survive without intervention.
HLHS occurs in approximately 1 in 4,000 to 5,000 live births in the UK. It accounts for around 25% of all cardiac deaths in the first week of life and represents the most complex end of the congenital heart disease spectrum. It is not caused by anything the parents did during pregnancy.
The cause in most cases is not fully understood, and families should hear this clearly: HLHS is not caused by anything the mother ate, did, or failed to do during pregnancy. It arises from a disruption in the normal development of the left heart structures during the first eight weeks of foetal life, when the heart is forming.
Research has identified genetic associations in a proportion of cases, including variants in genes such as NOTCH1, GJA1, and MYH6, and chromosomal abnormalities are found in a minority of children with HLHS. The condition occurs more commonly in male babies and in babies with a family history of congenital heart disease, suggesting a partially heritable component. But for most families, a specific cause is never identified, and this is not a failure of investigation. It reflects the genuinely complex and incompletely understood embryology of the developing heart.
The great majority of HLHS cases in the UK are now identified before birth. Antenatal cardiac screening at the 18 to 20-week anomaly scan includes a four-chamber view of the foetal heart, and the asymmetry of HLHS, with a small or absent left ventricle, is usually visible at this stage. Any abnormality on the standard scan is followed by a detailed foetal echocardiogram performed by a specialist in foetal cardiology.
A prenatal diagnosis is genuinely important. It allows the birth to be planned at or near a cardiac surgical centre, ensures the medical team is ready at delivery, and means prostaglandin therapy to keep the ductus arteriosus open can be started immediately, before the baby deteriorates. Survival is meaningfully better in babies diagnosed before birth compared to those presenting as a postnatal emergency.
Some cases are still diagnosed after birth, when a baby develops grey or blue skin colour, difficulty breathing, weak pulses, or signs of cardiovascular collapse as the ductus begins to close. Any of these features in a newborn requires immediate assessment, and HLHS should be considered until excluded.
After birth, the diagnosis is confirmed by echocardiogram. This provides detailed information about the anatomy: the degree of left ventricular hypoplasia, the morphology of the mitral and aortic valves, the size of the aortic arch, and the presence of any additional cardiac lesions that affect the surgical plan.
A baby with HLHS requires immediate and specific neonatal management. Prostaglandin is started as soon as the diagnosis is made, or suspected, to keep the ductus arteriosus open and maintain systemic blood flow. This buys time for stabilisation and surgical planning.
Oxygen is administered carefully: contrary to instinct, giving high-flow oxygen to a baby with HLHS can be harmful. High oxygen levels cause the pulmonary blood vessels to dilate, drawing too much blood flow to the lungs and away from the body, worsening systemic bodyperfusion. The balance of pulmonary and systemic blood flow is managed precisely by the neonatal intensive care or high dependency unit teams in the days before surgery.
In cases where the atrial septum is very restrictive, causing inadequate mixing of oxygenated and deoxygenated blood, an emergency cardiac catheter procedure may be needed to open the communication before surgery can be planned.
HLHS cannot be corrected. The heart cannot be made to work like a normal heart. What the surgical pathway achieves is palliation: a series of reconstructive operations that progressively reorganise the circulation so that the right ventricle becomes the sole systemic pump, and systemic venous blood travels to the lungs without needing to pass through a ventricle at all.
Three operations are performed in staged sequence, typically over the first two to four years of life. Each stage carries its own risks and its own period of recovery. Families are supported through each by the cardiac surgical, cardiology, nursing, and allied health teams at the surgical centre.
The first operation, known as the Norwood procedure or stage one palliation, is performed in the first week of life. It is the most technically demanding of the three stages and carries the highest procedural risk. The surgical goals are to create an unobstructed outflow from the right ventricle to the systemic bodycirculation by reconstructing the aorta using the pulmonary artery as the new aortic root, and to establish a controlled source of blood flow to the lungs, either through a modified Blalock-Taussig shunt from a systemic artery to the pulmonary arteries, or through a right ventricle to pulmonary artery conduit known as the Sano modification. The details of first-stage surgical options are covered in the dedicated blog post on stage one palliation.
The second operation, the Glenn procedure, is performed at around four to six months of age when the pulmonary vascular resistance has fallen sufficiently to allow passive blood flow to the lungs. The superior vena cava (the vein taking the poorly oxygenated blood from the upper part of the body to the heart) is connected directly to the pulmonary arteries, and the previous shunt is taken down. From this point, blood returning from the upper body flows directly to the lungs without passing through the heart. The Glenn procedure is explained in detail here.
The third operation, the Fontan procedure, is performed at around two to four years of age. The inferior vena cava (the vein taking the poorly oxygenated blood from the lower part of the body to the heart) is connected to the pulmonary arteries, completing the separation of the circulations. Blood from the entire body now flows passively to the lungs, and the single right ventricle pumps only oxygenated blood to the body. This is the endpoint of the surgical pathway. The Fontan operation and its implications are discussed here.
Between the first and second stages lies the interstage period, a particularly vulnerable time when the infant is at home but the circulation remains precarious. Careful monitoring and parental education are essential. The interstage period is covered in detail here.
For a small number of children, the staged surgical pathway is not feasible: the anatomy is too complex, the right ventricular function deteriorates severely, or other organ systems are significantly affected. In these cases, cardiac transplantation is discussed as an alternative or as a bridge when palliation fails. Transplantation carries its own substantial risks and the lifelong burden of immunosuppression, and the waiting list for donor organs in infancy is often very long. For children who receive a transplant and do well, long-term outcomes can be excellent.
The numbers have improved significantly over the past three decades, but they remain sobering when compared to most other congenital heart conditions.
In the most experienced surgical centres today, early survival after the Norwood procedure is around 85 to 90%. Interstage mortality, between stage one and the Glenn, adds further attrition. Cumulative survival to the completion of the Fontan is approximately 60 to 70% in current surgical series. UK population-based data from England and Wales show survival of around 64% at one year, 59% at five years, and 55% at ten years for children born in the period 1998 to 2012, reflecting improving but still significant mortality.
The most recent long-term data, published in the Journal of the American College of Cardiology in 2025 from a cohort of over 2,000 patients followed for up to 35 years, found transplant-free survival of 31% at 35 years. Within that, a meaningful subgroup of survivors reported good to excellent health and quality of life, representing what investigators describe as the high-performing Fontan phenotype.
These figures are not intended to diminish hope. They are shared because parents making decisions about management, and families living with a child through the surgical stages, deserve to understand the realistic landscape. The children who do well after HLHS palliation often do genuinely well: attending school, developing normally, participating in life in ways that are meaningful and rich.
Children with HLHS face a higher risk of neurodevelopmental difficulties than the general population. This is a direct consequence of the altered foetal cerebral circulation before birth, the complexity of neonatal surgery performed on a very young brain, and the periods of reduced cardiac output that occur across the staged pathway. Cognitive difficulties, problems with attention and executive function, motor delays, and learning differences are all more prevalent in this group than in children without heart disease.
This does not mean every child with HLHS will have significant learning difficulties. Many do not. But it does mean that developmental surveillance should begin early and continue throughout childhood. Formal neurodevelopmental assessment, school liaison, and early access to educational support make a substantial difference to long-term outcomes for children who do experience difficulties.
Children who complete the three-stage pathway live with a Fontan circulation: a fundamentally different arrangement from the normal circulation, in which systemic venous blood flows passively to the lungs and a single right ventricle sustains the systemic output. This circulation is not normal, and it carries long-term consequences that families need to understand.
Fontan-associated liver disease develops progressively in most patients with a Fontan circulation, as elevated venous pressures transmit to the hepatic veins over years and decades. Regular liver surveillance is part of long-term HLHS management. Protein-losing enteropathy (loss of proteins in the gut), plastic bronchitis (loss of proteins and cast formation in the lungs), arrhythmias (heart rhythm problems), and declining ventricular pumping function are all recognised late complications of the Fontan circulation. The long-term outlook post addresses these in detail.
Most children with a Fontan circulation are advised to avoid competitive high-intensity sport, though moderate regular physical activity is actively encouraged and beneficial. Regular exercise testing, echocardiographic review, and cardiac catheterisation at defined intervals form the backbone of long-term surveillance. Transition to an adult congenital heart disease service in the mid-teens is a planned and important step, not an abrupt handover.
Children who embark on the surgical palliation for HLHS require multiple appointments to visit their treatment teams and they require at times prolonged periods of hospital admission. Additionally children with HLHS require medical treatment with medications troughout their lives. One of these, aspirin, is used to thin blood and minimise the risk of clots forming and is something that these children need to take all their life.
A referral will be made to a foetal cardiology service at a specialist congenital heart centre. You will have a detailed foetal echocardiogram to fully characterise the anatomy, and appointments with the paediatric cardiology and cardiac surgery teams to discuss what the diagnosis means, what the surgical pathway involves, and what the options are. The birth will be planned at or near the surgical centre so the baby can be stabilised and operated on without delay. Dr Giardini provides pre-natal counselling for families receiving this diagnosis and can advise on what to expect from that first specialist appointment.
In a small number of cases where the left heart structures are borderline, foetal intervention or a biventricular repair strategy may be considered. In established HLHS with a severely hypoplastic left ventricle, the left heart does not grow sufficiently to sustain normal circulation, and the single ventricle pathway is the standard approach. This is a question the foetal cardiology team can answer specifically based on the detailed echocardiographic anatomy.
No. HLHS is not caused by anything the mother did or did not do during pregnancy. It arises from a disruption in the normal formation of the left heart structures, most likely involving a combination of genetic susceptibility and developmental factors that are not yet fully understood. There is nothing in the standard advice given to pregnant women, including diet, exercise, work, or lifestyle, that causes HLHS.
The Norwood procedure is the first of the three palliative operations. It reconstructs the aorta and establishes a controlled source of blood flow to the lungs. It is performed in the first week of life on a very small baby with a heart the size of a walnut, and it involves reconstructing the major blood vessels in a way that fundamentally changes the architecture of the circulation. It is among the most technically demanding operations in cardiac surgery. Mortality in experienced centres is now around 10 to 15%, compared to over 50% two decades ago. The dedicated page on stage one palliation covers this in full.
Many children with HLHS who complete the surgical pathway attend mainstream school, make friends, and participate in everyday life in ways that are genuinely normal. Some need educational support because of neurodevelopmental differences. Most will have physical activity limitations and will need ongoing cardiac follow-up throughout life. The Functional outcomes vary considerably between individuals. The children who do well often do very well. Quality of life data from adolescent survivors of HLHS show self-reported health scores comparable to healthy peers in some cohorts.
The Fontan circulation is a remarkable achievement of surgical imagination, but it places the body under a sustained physiological burden over decades. Liver disease, arrhythmias, protein-losing enteropathy, and declining exercise capacity are all recognised long-term complications. Regular specialist follow-up is not optional: it is the mechanism by which complications are detected early and managed before they become irreversible. The long-term outlook page addresses this in detail.
Some children and young adults with HLHS do require cardiac transplantation, either because the Fontan circulation fails or because the single right ventricle deteriorates over time. This is a recognised long-term trajectory for a proportion of patients, not for all. The likelihood depends on many factors including the underlying anatomy, the quality of palliation, ventricular function, and the development of Fontan-related complications. Regular surveillance is specifically designed to identify those at risk before transplantation becomes urgent rather than planned.
There is a genetic component in a proportion of cases, and having one child with HLHS does increase the risk for subsequent pregnancies above the background population rate, though the absolute risk remains low. Genetic testing and cardiac genetic counselling can help families understand their specific situation. Foetal echocardiography is offered in subsequent pregnancies for families with a history of congenital heart disease.
Most children with a Fontan circulation are advised to avoid competitive high-intensity sport, particularly endurance events and contact sports. Moderate activity, swimming, cycling, walking, and recreational activities are not only permitted but actively beneficial for cardiovascular and psychological health. Exercise testing helps define the individual child's safe activity range, and Dr Giardini tailors this advice to each child based on their current cardiac function and exercise capacity.
Lifelong specialist follow-up at least every 6 months is standard practice for children who have completed the Fontan procedure. This typically includes echocardiography, ECG, blood tests including liver function and protein monitoring, and periodic exercise testing and cardiac MRI at defined intervals. The frequency and composition of review increases if any complications are identified.
Research is active in several areas. Foetal cardiac intervention, aimed at improving left heart growth before birth in borderline cases, is performed at a small number of specialist centres. Stem cell therapies to strengthen the right ventricle are in clinical trials in the United States. Improvements in hybrid palliation techniques and better interstage monitoring programmes continue to reduce the attrition between surgical stages. The surgical pathway itself has remained largely stable in structure, but outcomes at each stage continue to improve incrementally as surgical technique, anaesthesia, and perioperative management advance.
A baby with HLHS may appear entirely normal at birth if the ductus arteriosus is still open. As it begins to close, typically in the first hours to days of life, the signs of cardiovascular compromise emerge: a grey or blue colour to the skin, lips, and tongue; rapid or laboured breathing; weak or absent pulses in the arms and legs; a soft, limp appearance; and poor or absent feeding. In severe cases the baby can deteriorate to collapse very rapidly. Any newborn showing these features requires immediate emergency assessment. A baby diagnosed before birth will not necessarily show these signs, because prostaglandin is started at delivery to keep the ductus open before deterioration occurs.
On a standard anomaly scan, the four-chamber view of the foetal heart shows a striking asymmetry: the left ventricle is visibly much smaller than the right, often appearing as a tiny sliver of tissue rather than the robust pumping chamber it should be. The aorta, which should be clearly visible emerging from the left ventricle, may appear very small or difficult to identify. The right ventricle and pulmonary artery tend to appear relatively enlarged in comparison. Not every sonographer performing routine anomaly scans will recognise HLHS immediately, but the four-chamber asymmetry is the key finding that should prompt referral for a specialist foetal echocardiogram. At that detailed assessment, the morphology of the mitral valve, aortic valve, and aortic arch can be fully characterised.
No. Without intervention, HLHS is uniformly fatal within days to weeks of birth. As the ductus arteriosus closes after birth, the baby loses the only route by which the right ventricle can pump blood to the body. Without a functioning systemic circulation, cardiovascular collapse follows rapidly. Prostaglandin can keep the ductus open temporarily, but this is a bridge to surgery, not a treatment in its own right. The only alternatives to surgical palliation are cardiac transplantation or compassionate care, in which families choose to allow natural death rather than proceed with the surgical pathway. That is a choice some families make, and it is a valid and deeply personal decision that the clinical team supports without judgement.
Yes, though less commonly now than in previous decades. The routine anomaly scan at 18 to 20 weeks includes a four-chamber view specifically to detect conditions like HLHS, and detection rates in the UK have improved substantially with better equipment and training. However, HLHS can still be missed, particularly in cases where the left ventricle is borderline rather than severely hypoplastic, where image quality is limited by foetal position or maternal body habitus, or where the sonographer does not have extensive experience in foetal cardiac abnormalities. Approximately 20 to 30% of HLHS cases in the UK are still diagnosed after birth rather than before. This is one reason why recognising the postnatal symptoms, detailed above, remains clinically important even in an era of widespread antenatal screening.
No. The left heart structures do not grow or develop after HLHS is established, and no surgical or medical treatment restores the left ventricle to a functioning state. What the surgical pathway achieves is palliation: a reorganisation of the circulation that allows the child to survive and function using the right ventricle as the sole systemic pump. Palliation is not cure, and families need to understand that their child will live with an abnormal circulation, requiring lifelong specialist follow-up and medications, throughout their life. In a very small number of cases where the left heart is borderline, foetal intervention or a biventricular repair may be possible, effectively giving the child a two-ventricle circulation. This is not applicable to established HLHS with a severely hypoplastic left ventricle.
Yes. Children who complete the three-stage surgical pathway and are managed carefully can and do reach adulthood. The first generation of survivors treated with the Norwood procedure in the 1980s are now in their thirties and forties. Long-term data published in 2025 show transplant-free survival of 31% at 35 years, which means that while HLHS carries substantial long-term mortality, a meaningful proportion of patients are alive and living independent lives decades after surgery. Many report good health. Adulthood brings its own challenges for HLHS survivors, including the need to transition to adult congenital heart disease care, the long-term consequences of the Fontan circulation, and decisions about work, relationships, and family. These are navigable challenges, and growing numbers of HLHS survivors are navigating them.