Hypoplastic Left Heart Syndrome (HLHS)

Hypoplastic Left Heart Syndrome (HLHS) is a severe congenital heart defect characterized by underdevelopment of the left ventricle, mitral valve, aortic valve, and ascending aorta. This condition prevents the heart from pumping blood into the systemic circulation, making the patency of the ductus arteriosus essential for neonatal survival. Without immediate intervention, mortality within the first days of life is extremely high. HLHS accounts for approximately 2-3% of congenital heart defects and affects an estimated 2-4 newborns per 10,000 live births. It is more common in males and may be associated with genetic syndromes such as Turner syndrome or 22q11 deletion syndrome.

Anatomy and Pathophysiology

In newborns with HLHS, the left ventricle is too small to support systemic circulation. Oxygenated blood from the lungs reaches the left atrium but, due to the obstruction of the left ventricle, it crosses a patent foramen ovale to reach the right atrium. From there, the right ventricle becomes the main pumping chamber, sending blood both to the lungs and to the systemic circulation via a patent ductus arteriosus. Spontaneous closure of the ductus arteriosus leads to cardiogenic shock, metabolic acidosis, and rapid death.

Signs and Symptoms

Newborns with HLHS may appear normal at birth, but their condition rapidly deteriorates as the ductus arteriosus closes. Cyanosis is the first visible sign, followed by tachypnea, lethargy, and respiratory distress. Hypothermia and peripheral hypoperfusion occur rapidly, with weak or absent pulses in the lower extremities. Without treatment, these neonates quickly develop cardiogenic shock and metabolic acidosis, leading to fatal outcomes within hours or days.

Diagnosis

HLHS can be diagnosed prenatally through fetal echocardiography, typically between the 18th and 22nd weeks of gestation. After birth, neonatal echocardiography confirms left-sided heart hypoplasia and dependence on the ductus arteriosus. Electrocardiogram (ECG) may show right ventricular hypertrophy. Chest X-ray reveals a normal or slightly enlarged heart with decreased pulmonary vascular markings.

Treatment

Initial treatment includes administration of prostaglandins (PGE1) to maintain ductal patency and ensure systemic blood flow. Oxygen therapy should be used cautiously to avoid lowering pulmonary vascular resistance, which could impair systemic circulation. Surgical treatment is performed in three stages:

Norwood Procedure → Performed within the first few days of life, this procedure creates a new aortic arch by connecting the pulmonary artery to the aorta.
Glenn Shunt → Performed at 4-6 months of age, it connects the superior vena cava to the pulmonary artery, reducing right ventricular workload.
Fontan Procedure → Completed at approximately 2-4 years of age, it directs systemic venous blood flow directly to the lungs, bypassing the right ventricle.

When surgery is not feasible, heart transplantation is an alternative option, though it is limited by the availability of neonatal donors.

Prognosis

Without treatment, mortality is 100% within the first month of life. With staged surgical repair, survival rates have improved significantly, but lifelong follow-up is necessary to monitor cardiac function. Long-term complications include right ventricular dysfunction, arrhythmias, venous thrombosis, and chronic heart failure.

References

Rychik J. et al. Hypoplastic Left Heart Syndrome: From Origin to Outcome. J Am Coll Cardiol. 2012;59(1):S1-S8.
Tweddell J.S. et al. Intermediate-Term Follow-Up After the Fontan Procedure in Patients With Hypoplastic Left Heart Syndrome. Ann Thorac Surg. 2014;97(6):2139-2145.
Mahle W.T. et al. Outcome of Patients With Hypoplastic Left Heart Syndrome: A National Perspective. Cardiol Young. 2004;14(3):288-293.
Brigham K.L. et al. Long-Term Challenges in HLHS Management. J Am Coll Cardiol. 2020;75(4):560-578.
Bove E.L. et al. Evolution of Surgical Techniques for HLHS. Pediatr Cardiol. 2019;40(6):1105-1120.