What is hypoplastic left heart syndrome (HLHS)?

Hypoplastic left heart syndrome (HLHS) is a very complex heart condition seen in newborn babies. It is a single-ventricle heart defect, meaning one side of the heart is not developed correctly.

Babies born with HLHS have underdeveloped structures in the left side of the heart, including:

  • Left ventricle: The left ventricle or the lower left chamber of the heart is part of the heart that pumps out oxygen-rich blood to the aorta.
  • Aorta: The aorta is the largest blood vessel that carries blood from the heart to the rest of the body.
  • Mitral and aortic valves: Heart valves are flaps of tissue that open and close to control blood flow through the heart. The aortic valve allows blood to pass from the left ventricle to the aorta. The mitral valve controls blood flow from the top heart chamber (atrium) to the lower heart chamber (ventricle).

Anatomy of the heart | Cleveland Clinic

Normal anatomy in the left side of the heart.

How common is hypoplastic left heart syndrome?

Around one in 4,000 babies are born with hypoplastic left heart syndrome each year. It accounts for about 2% to 3% of all congenital heart diseases (CHD).

How does hypoplastic left heart syndrome affect a baby’s body?

In hypoplastic left heart syndrome, the heart’s left side is too small to pump enough blood to the rest of the body. The right ventricle or right lower chamber of the heart then takes over to pump blood to both lungs and whole body through a blood vessel called the ductus arteriosus. The ductus arteriosus is the vessel that all fetuses have and typically gets smaller until completely closing after babies are born. Without the ductus arteriosus, the condition is usually fatal if babies do not receive treatment.

What causes hypoplastic left heart syndrome (HLHS)?

Most of the time, there is no known cause for hypoplastic left heart syndrome. Sometimes, the cause of genetic. Babies with mutations (changes) in specific genes may have a higher risk of HLHS. These genes are known as GJA1 or NKX2-5.

What are the symptoms of hypoplastic left heart syndrome (HLHS)?

Newborns with HLHS don’t always show symptoms immediately. Instead, signs may develop within a few hours or days of birth.

Babies with hypoplastic left heart syndrome may experience:

  • Cyanosis, or a grayish (in dark-skinned people) or bluish (in light-skinned people) discoloration of skin, lips and nails.
  • Difficulty breathing.
  • Difficulty feeding.
  • Lack of energy (lethargy).
  • Rapid heartbeat.
  • Sweaty, clammy or cool skin.
  • Weak pulse.

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