Hypoplastic Right Heart Syndrome (HRHS) is a rare and severe congenital heart disease characterized by an incomplete development of the structures that form the right heart: the right atrium, tricuspid valve, right ventricle, pulmonary valve, and pulmonary outflow tract. This anomaly leads to a significant impairment of blood flow from the systemic venous circulation to the lungs, forcing blood to follow abnormal pathways through interatrial, interventricular, or ductal shunts to ensure oxygenation. The clinical presentation is often critical within the first few days of life, with severe cyanosis, hypoxia, and rapid development of heart failure, making HRHS one of the most challenging conditions in pediatric cardiology and cardiac surgery.
The definite cause of hypoplastic right heart syndrome lies in a defect of embryological development of the structures destined to form the right heart. Specifically, during the 4th to 7th week of gestation, disruptions in the processes of growth, differentiation, and remodeling of the cardiac bulbus and atrioventricular septum lead to a variable reduction in the size of the right atrium and ventricle and/or hypoplasia of the tricuspid and pulmonary valves. Frequently, the presence of tricuspid or pulmonary valve stenosis or atresia during the embryonic period causes underdevelopment of the right-sided chambers due to lack of forward flow, in line with the “flow-dependent development” principle also seen in hypoplastic left heart syndrome.
Risk factors are mostly related to genetic and syndromic conditions, although most cases are sporadic. Identified risk factors include:
The key pathogenic mechanism consists of a progressive reduction of antegrade flow through the tricuspid and/or pulmonary valve already during intrauterine life. This leads to insufficient growth of the right-sided chambers and outflow tracts, resulting in a condition where, at birth, the right ventricle is unable to sustain the workload required to direct blood to the lungs. The severity of the syndrome depends on the extent of hypoplasia and the association with other anomalies such as interatrial septal defects, pulmonary stenosis or atresia, tricuspid hypoplasia or atresia, and the presence of a patent ductus arteriosus.
From a pathophysiological standpoint, hypoplastic right heart leads to systemic venous blood encountering a critical obstruction en route to the pulmonary circulation. This necessitates blood diversion through interatrial shunts (patent foramen ovale, atrial septal defect), interventricular shunts (ventricular septal defect), or a patent ductus arteriosus. The efficacy of oxygenation depends on the size and functionality of these shunts. In the most severe cases, spontaneous closure of the ductus arteriosus or restriction of interatrial passages can lead to rapid clinical deterioration with severe cyanosis, metabolic acidosis, and risk of sudden death.
Hypoplastic right heart syndrome must therefore be regarded as a highly complex condition, characterized by precarious hemodynamic balance, where survival depends on the presence and patency of intracardiac communications and timely therapeutic intervention.
Clinical manifestations of hypoplastic right heart syndrome (HRHS) are generally early and severe, reflecting the profound impairment of blood flow to the lungs and the critical dependence on intracardiac and ductal shunts for survival. Onset typically occurs within the first hours or days of life, with symptoms worsening rapidly in the absence of prompt intervention.
Common symptoms in neonates include:
In the most critical forms, spontaneous closure of the ductus arteriosus or restriction of interatrial communications can result in a dramatic worsening with rapid development of hypoxia, metabolic acidosis, circulatory collapse, and high risk of sudden death. The severity of symptoms is directly proportional to the degree of right-sided hypoplasia and the ability to maintain an alternative route for pulmonary blood flow.
On physical examination, findings may include intense cyanosis, weak and tachycardic pulse, shallow and labored breathing, muffled heart sounds, and, in cases with significant interventricular shunt, a holosystolic parasternal murmur. Hepatomegaly and signs of peripheral venous congestion appear quickly when right atrial pressure is elevated.
In rare cases where HRHS presents in a less severe form, symptoms may arise later with chronic cyanosis, subacute onset of heart failure, easy fatigability, and poor growth. In all cases, spontaneous progression is invariably unfavorable without specific treatment.
The diagnosis of hypoplastic right heart syndrome (HRHS) is based on a careful integration of clinical evaluation, first-level instrumental tests, and advanced cardiac imaging. Clinical suspicion arises in a neonate presenting with severe and persistent cyanosis, poorly responsive to oxygen therapy, signs of respiratory distress, and a rapidly worsening hemodynamic picture. In some cases, diagnosis can be made prenatally through fetal echocardiography.
The electrocardiogram (ECG) in neonates with HRHS may show sinus rhythm with right axis deviation and signs of right atrial hypertrophy; however, due to reduced right ventricular mass, typical right ventricular hypertrophy may be absent.
The chest X-ray often reveals moderate cardiomegaly, a globular cardiac silhouette, and alterations in the pulmonary vascular pattern, which can be reduced in cases with limited pulmonary blood flow.
The cornerstone examination is transthoracic echocardiography (TTE), which allows direct visualization of hypoplasia or atresia of right-sided structures (atria, valves, ventricle, and pulmonary outflow tract), the presence and size of interatrial or interventricular shunts, the patency of the ductus arteriosus, and the anatomy of the pulmonary arteries.
Doppler echocardiography enables assessment of pressure gradients, direction and volume of blood flow, and identification of associated lesions (such as aortic coarctation, pulmonary venous return anomalies, or other complex congenital heart diseases).
In cases with complex anatomy or in preparation for surgery, advanced imaging techniques such as cardiac magnetic resonance imaging (CMR) or cardiac computed tomography (CT) are indicated. These modalities provide three-dimensional reconstruction of structures, detailed evaluation of the pulmonary and systemic circulation, and precise surgical planning.
Cardiac catheterization may be necessary in selected cases both for accurate measurement of intracardiac pressures and pulmonary vascular resistance and for performing palliative interventional procedures (e.g., ductal stenting or balloon atrial septostomy).
The differential diagnosis includes hypoplastic left heart syndrome, tricuspid atresia, critical pulmonary stenosis or atresia, and other cyanotic congenital heart diseases. Accurate distinction between these conditions is essential to establish an appropriate and personalized therapeutic approach.
The treatment of hypoplastic right heart syndrome (HRHS) is among the most complex in pediatric cardiology, requiring multidisciplinary management in highly specialized centers. The main objective is to ensure adequate pulmonary and systemic blood flow through a combination of medical, interventional, and surgical strategies tailored to the specific anatomy and clinical status of the patient. Long-term survival depends on early diagnosis, timely care, and the ability to maintain stable hemodynamic balance during growth.
In the neonatal phase, the utmost priority is maintaining ductal patency through infusion of prostaglandin E1, allowing blood flow to the lungs in the presence of pulmonary outflow tract atresia or critical hypoplasia. Ventilatory support, intensive monitoring, and correction of metabolic disturbances (acidosis, hypoglycemia, electrolyte imbalances) are often required to stabilize the patient. In cases of significant interatrial flow restriction, balloon atrial septostomy or dilation of the foramen ovale may be indicated to improve blood mixing.
The surgical strategy generally involves multiple stages, similarly to hypoplastic left heart syndrome. The first stage consists of an early palliative procedure such as a systemic-to-pulmonary shunt (e.g., modified Blalock-Taussig shunt), ductal stenting, or, in selected cases, direct connection of a pulmonary artery to the ascending aorta (central shunt). These interventions aim to provide adequate pulmonary blood flow during the first months of life.
Subsequently, a two-stage univentricular palliation is planned: the Glenn procedure (anastomosis between the superior vena cava and the right pulmonary artery) around 4–6 months of age, followed by the Fontan circulation (connection of the inferior vena cava to the pulmonary circulation), typically between 2 and 4 years of age. These surgeries allow systemic venous blood to reach the lungs bypassing the hypoplastic right heart, entrusting the left ventricle with the main pumping function.
The prognosis of HRHS is closely related to residual anatomy, early diagnosis, and success of staged surgical repairs. Five- and ten-year survival rates after Fontan completion have progressively improved due to surgical and intensive care advances; however, quality of life can be limited by medium- to long-term complications including ventricular dysfunction, arrhythmias, pulmonary venous hypertension, protein-losing enteropathy, thromboembolic complications, and risk of organ failure.
A multidisciplinary follow-up is essential to monitor cardiac function, systemic oxygen saturation, growth, and neurodevelopment, allowing early intervention in case of complications or need for additional interventional procedures.
Hypoplastic right heart syndrome (HRHS) carries a high risk of complications, both in the natural untreated course and during long-term follow-up after palliative or definitive surgical repairs. Prognosis strongly depends on the initial anatomy, success of surgical interventions, timing of correction, and presence of associated anomalies.
In the untreated natural course, the most severe complications include progressive systemic hypoxia, rapid onset of heart failure, and risk of hemodynamic collapse upon spontaneous closure of the ductus arteriosus or restriction of interatrial communications. During the first days of life, severe metabolic acidosis, multiorgan damage, severe respiratory infections, and sudden death can occur if adequate therapy is not instituted.
After palliative surgical correction or Fontan circulation, the risk of complications remains significant even in the long term. The main complications include:
Management of complications requires constant surveillance and multidisciplinary follow-up. Timely intervention on arrhythmias, organ dysfunction, nutritional issues, or thromboembolic events is fundamental to improve survival and quality of life for these patients.