Hypoplastic left heart syndrome, often abbreviated HLHS, is a severe congenital heart defect in which structures on the left side of the heart are underdeveloped. The left ventricle, mitral valve, aortic valve, and ascending aorta may all be affected to varying degrees. Because the left side of the heart is responsible for pumping oxygenated blood to the body, this underdevelopment can lead to critical circulatory compromise soon after birth. ^[1][2][3]

Many cases are identified before birth during prenatal ultrasound or fetal echocardiography. When the condition is not recognized prenatally, a newborn may appear relatively stable at first and then deteriorate as the ductus arteriosus begins to close. Symptoms can include poor feeding, rapid breathing, gray or blue skin color, weak pulses, cool extremities, lethargy, and signs of shock. HLHS is therefore considered a medical emergency in the neonatal period. ^[1][2][4]

HLHS is not caused by anything a parent did or did not do. It develops during fetal heart formation, and while genetics may contribute in some cases, the exact cause is often not identified. Because the circulation depends on fetal shunts immediately after birth, prostaglandin treatment is often started to keep the ductus arteriosus open while the infant is stabilized and evaluated for intervention. ^[1][2][5]

Diagnosis is made with echocardiography, which defines the anatomy and the pattern of blood flow. Additional testing may assess oxygen levels, organ function, and any associated cardiac or noncardiac abnormalities. Management usually begins in a tertiary center with pediatric cardiology and cardiac surgery support. ^[1][2][3]

Treatment most often involves staged surgical palliation rather than a single curative procedure. These staged operations are designed to reroute blood flow so that the right ventricle can support systemic circulation. The exact sequence and timing depend on the infant’s anatomy and clinical status. In some cases, heart transplantation may be considered. Families should understand that treatment is complex and long-term follow-up is essential. ^[1][2][5][6]

Even after successful surgery, children with HLHS require ongoing monitoring for growth, feeding difficulties, rhythm problems, neurodevelopmental concerns, and changes in heart function or oxygenation. Some children do well for years with structured care, while others require additional interventions. Longitudinal pediatric cardiology follow-up is a core part of survival and quality-of-life support. ^[2][5][6]

Emergency evaluation is necessary in any newborn with poor feeding, cyanosis, rapid breathing, profound sleepiness, or signs of shock. For families already carrying a prenatal diagnosis, delivery planning at a center prepared for neonatal cardiac care can be important. ^[1][2][4]

FAQ

What is hypoplastic left heart syndrome?

It is a severe congenital heart defect in which the left-sided pumping structures of the heart are underdeveloped. ^[1][3]

Can HLHS be diagnosed before birth?

Yes. Many cases are identified with prenatal ultrasound and fetal echocardiography. ^[1][2]

Why do babies worsen after birth?

Symptoms often become more apparent when the ductus arteriosus starts to close and blood flow to the body becomes inadequate. ^[1][2]

Is one surgery enough?

Treatment usually involves staged procedures and lifelong cardiology follow-up rather than a single one-time cure. ^[2][5][6]

Which newborn symptoms are emergencies?

Blue or gray color, poor feeding, fast breathing, weak pulses, marked sleepiness, and signs of shock require urgent care. ^[1][2]