Hypoplastic Right Heart Syndrome (HRHS)

Hypoplastic Right Heart Syndrome (HRHS) is a rare congenital heart defect characterized by an underdevelopment of right-sided cardiac structures, including the right ventricle, tricuspid valve, and pulmonary artery. This anomaly leads to a reduced pulmonary blood flow, resulting in neonatal hypoxemia. Without immediate treatment, survival is significantly compromised.
HRHS is less common than hypoplastic left heart syndrome, accounting for approximately 1% of congenital heart defects. It can occur as an isolated anomaly or in association with other malformations, such as anomalous pulmonary venous return, atrial septal defects, and abnormal pulmonary vascular connections.

Anatomy and Pathophysiology

HRHS is characterized by a hypoplastic right ventricle, which is unable to ensure adequate pulmonary perfusion. The main anatomical abnormalities include:

Tricuspid valve atresia or stenosis: prevents normal filling of the right ventricle.
Pulmonary artery hypoplasia: restricts blood flow to the lungs.
Dependence on the ductus arteriosus for pulmonary circulation: patency of the ductus arteriosus (PDA) is essential for neonatal survival.
Atrial shunting: allows blood to pass between the atria to compensate for the reduced pulmonary blood supply.

These abnormalities lead to progressive hypoxemia, which, if left untreated, can result in metabolic acidosis and multiorgan failure.

Signs and Symptoms

Newborns with HRHS develop symptoms early, which worsen after spontaneous closure of the ductus arteriosus. The clinical presentation includes:

Severe cyanosis due to reduced pulmonary blood flow.
Respiratory distress and tachypnea.
Peripheral hypoperfusion, with weak pulses and cold extremities.
Metabolic acidosis, indicative of multiorgan failure.

Diagnosis

HRHS can be identified prenatally using fetal echocardiography, which reveals a hypoplastic right ventricle and valvular anomalies. After birth, the diagnosis is confirmed with:

Echocardiography: defines the defect's anatomy and confirms dependence on the ductus arteriosus.
Electrocardiogram (ECG): may show left ventricular hypertrophy.
Chest X-ray: may reveal a small heart with decreased pulmonary vascularity.

Treatment

Initial treatment is pharmacologic and includes the administration of prostaglandins (PGE1) to maintain ductal patency and ensure minimal pulmonary blood flow. In severe cases, mechanical ventilation may be required to support oxygenation. Surgical management is staged to optimize blood flow redistribution:

Systemic-to-pulmonary shunt (in the first days of life): connects the aorta to the pulmonary artery to ensure pulmonary perfusion.
Glenn procedure (at 4-6 months of age): connects the superior vena cava to the pulmonary artery to reduce ventricular workload.
Fontan procedure (at 2-4 years of age): completes the separation of systemic and pulmonary venous circulation.

In complex cases, where no surgical alternatives are feasible, heart transplantation may be considered.

Prognosis

Without treatment, survival is extremely poor, with high mortality in the first days of life. Advances in surgical techniques have significantly improved the prognosis, allowing for acceptable long-term survival. However, many patients require lifelong follow-up and potential corrective interventions in adulthood. The main long-term complications include:

Arrhythmias, related to the altered cardiac flow.
Ventricular dysfunction, due to increased left ventricular workload.
Risk of thrombosis, especially in patients with Fontan circulation.

References

Tweddell J.S. et al. Hypoplastic Right Heart Syndrome: Surgical Management and Outcomes. J Thorac Cardiovasc Surg. 2020;160(5):1305-1317.
Rao P.S. et al. Right Heart Hypoplasia: Diagnosis and Treatment. Am Heart J. 2019;117(1):89-102.
Gatzoulis M.A. et al. Long-Term Outcomes in Hypoplastic Right Heart Syndrome. Circulation. 2018;137(12):2099-2115.
Brigham K.L. et al. Fontan Physiology in Hypoplastic Right Heart Syndrome. J Am Coll Cardiol. 2021;145(3):320-332.
Bove E.L. et al. Fontan Procedure: Evolution and Future Perspectives. Pediatr Cardiol. 2020;42(5):600-612.