Truncus Arteriosus

Introduction

Truncus arteriosus is a rare and complex congenital heart defect characterized by a single arterial trunk arising from the heart that supplies the systemic, pulmonary, and coronary circulations. This condition occurs in approximately 1 in 10,000 live births and accounts for about 1-4% of all congenital heart defects. Understanding the embryology, anatomy, clinical presentation, diagnosis, and management of truncus arteriosus is crucial for pediatricians to provide optimal care for affected patients.

Embryology and Pathophysiology

Truncus arteriosus results from incomplete septation of the embryonic truncus arteriosus into the aorta and pulmonary artery during cardiac development. Normally, this process occurs between weeks 5 and 8 of gestation. The failure of the aorticopulmonary septum to develop properly leads to a single arterial trunk that overrides both ventricles.

The embryological basis of truncus arteriosus involves abnormalities in neural crest cell migration and differentiation. These cells are crucial for the formation of the aorticopulmonary septum and the development of the cardiac outflow tract. Genetic factors, environmental influences, and their interactions may contribute to the disruption of this process.

Associated with truncus arteriosus is a ventricular septal defect (VSD), which is typically large and located in the outlet septum. This combination results in mixing of oxygenated and deoxygenated blood in the common arterial trunk, leading to varying degrees of cyanosis and overcirculation of the pulmonary vasculature.

Anatomical Classification

The most widely used classification system for truncus arteriosus is the Collett and Edwards classification, which categorizes the defect into four types based on the origin of the pulmonary arteries:

Type I (50-70%): A short main pulmonary artery arises from the truncus and then bifurcates into right and left pulmonary arteries.
Type II (20-30%): The right and left pulmonary arteries arise separately but close together from the posterior aspect of the truncus.
Type III (10%): The right and left pulmonary arteries arise separately from the lateral aspects of the truncus.
Type IV (5-10%): The pulmonary arteries arise from the descending aorta (now considered a form of pulmonary atresia with VSD rather than true truncus arteriosus).

Another classification system proposed by Van Praagh and Van Praagh focuses on the presence or absence of a VSD and the relationship of the truncal valve to the ventricles:

Type A1: VSD present, truncal valve primarily committed to the right ventricle
Type A2: VSD present, truncal valve equally committed to both ventricles
Type A3: VSD present, truncal valve primarily committed to the left ventricle
Type A4: VSD absent (extremely rare)

Associated Cardiac Anomalies

Several cardiac anomalies may be associated with truncus arteriosus, including:

Truncal valve abnormalities (present in 50-60% of cases):
- Stenosis
- Insufficiency
- Abnormal number of cusps (bicuspid, quadricuspid, or more)
Right aortic arch (30-35% of cases)
Interrupted aortic arch (10-15% of cases)
Coronary artery anomalies (30% of cases)
Atrial septal defect
Patent ductus arteriosus

Extracardiac Manifestations

Truncus arteriosus may be associated with various extracardiac manifestations, particularly in the context of genetic syndromes. The most common association is with 22q11.2 deletion syndrome (DiGeorge syndrome), which occurs in approximately 30-35% of patients with truncus arteriosus. Features of 22q11.2 deletion syndrome may include:

Characteristic facial features
Thymic hypoplasia or aplasia
Parathyroid gland abnormalities
Cleft palate or velopharyngeal insufficiency
Immune deficiency
Developmental delay
Psychiatric disorders

Other genetic syndromes occasionally associated with truncus arteriosus include CHARGE syndrome, VACTERL association, and Noonan syndrome.

Clinical Presentation

The clinical presentation of truncus arteriosus varies depending on the anatomy of the defect, the degree of pulmonary blood flow, and the presence of associated anomalies. Most infants with truncus arteriosus present with symptoms in the neonatal period or early infancy.

Neonatal Presentation

In the immediate neonatal period, infants may appear relatively well due to the persistence of elevated pulmonary vascular resistance. As pulmonary vascular resistance decreases over the first few days of life, symptoms typically develop and may include:

Tachypnea and increased work of breathing
Poor feeding and failure to thrive
Mild to moderate cyanosis
Sweating, particularly during feeding
Irritability

Physical Examination Findings

Key physical examination findings in infants with truncus arteriosus may include:

Tachypnea and increased work of breathing
Mild to moderate cyanosis (may be difficult to detect in dark-skinned infants)
Hyperdynamic precordium
Single, loud second heart sound (S2)
Systolic ejection murmur at the left upper sternal border
Diastolic murmur of truncal valve regurgitation (if present)
Hepatomegaly
Bounding peripheral pulses

Late Presentation

In rare cases, patients with truncus arteriosus may present later in infancy or childhood. These patients typically have more balanced pulmonary and systemic blood flow, often due to pulmonary stenosis. Late presentation may include:

Exercise intolerance
Recurrent respiratory infections
Failure to thrive
Progressive cyanosis

Diagnostic Evaluation

A comprehensive diagnostic evaluation is essential for confirming the diagnosis of truncus arteriosus, delineating the anatomy, and identifying associated anomalies. The diagnostic workup typically includes:

1. Chest Radiography

Chest X-ray findings in truncus arteriosus may include:

Cardiomegaly
Increased pulmonary vascular markings
Narrow mediastinum due to thymic hypoplasia (in cases associated with 22q11.2 deletion)
Right aortic arch (if present)

2. Electrocardiography (ECG)

ECG findings are nonspecific but may show:

Right ventricular hypertrophy
Biventricular hypertrophy
Right axis deviation
Left ventricular hypertrophy (in cases with significant truncal valve regurgitation)

3. Echocardiography

Echocardiography is the primary imaging modality for diagnosing and characterizing truncus arteriosus. It provides detailed information about:

The single arterial trunk arising from the heart
Origin and course of the pulmonary arteries
Size and location of the VSD
Truncal valve anatomy and function
Ventricular size and function
Associated cardiac anomalies
Coronary artery anatomy (to the extent possible)

Both transthoracic and transesophageal echocardiography may be used, with the latter providing superior imaging in some cases, particularly for truncal valve assessment.

4. Cardiac Catheterization

While not routinely necessary for diagnosis, cardiac catheterization may be performed in select cases to:

Assess pulmonary vascular resistance
Evaluate coronary artery anatomy
Perform interventional procedures (e.g., balloon dilation of branch pulmonary artery stenosis)

5. Advanced Imaging

Advanced imaging modalities may be used to complement echocardiography, particularly in complex cases or when planning surgical intervention:

Cardiac CT: Provides detailed anatomical information, especially useful for evaluating the pulmonary arteries and coronary artery anatomy.
Cardiac MRI: Offers excellent soft tissue contrast and can provide functional assessment of the ventricles and valves. It may be particularly useful in older patients or those with poor echocardiographic windows.

6. Genetic Testing

Given the association with 22q11.2 deletion syndrome and other genetic conditions, genetic testing should be considered in all patients with truncus arteriosus. This typically includes:

Fluorescence in situ hybridization (FISH) or microarray analysis for 22q11.2 deletion
Broader genetic testing panels if indicated based on clinical features or family history

Management

The management of truncus arteriosus requires a multidisciplinary approach involving pediatric cardiologists, cardiac surgeons, intensivists, and other specialists. The primary goal of treatment is to separate the systemic and pulmonary circulations, establish right ventricle to pulmonary artery continuity, and repair associated defects.

Medical Management

Initial medical management is aimed at stabilizing the patient and optimizing their condition for surgical repair. This may include:

Prostaglandin E1 (PGE1): May be used in cases with associated interrupted aortic arch to maintain ductal patency and systemic perfusion.
Diuretics: To manage pulmonary overcirculation and congestive heart failure.
Inotropic support: In cases of ventricular dysfunction or low cardiac output.
Respiratory support: Including supplemental oxygen or mechanical ventilation as needed.
Nutritional support: To optimize growth and development prior to surgery.

Surgical Management

Surgical repair is the definitive treatment for truncus arteriosus and is typically performed in early infancy, ideally between 2-6 weeks of age. The primary goals of surgical repair include:

Separation of the pulmonary arteries from the truncus
Closure of the VSD, committing the truncus (now functioning as the aorta) to the left ventricle
Establishment of right ventricle to pulmonary artery continuity using a valved conduit
Repair of associated anomalies (e.g., truncal valve repair or replacement, if necessary)

The most common surgical approach involves:

Cardiopulmonary bypass with moderate hypothermia
Detachment of the pulmonary arteries from the truncus
VSD closure using a patch (typically Gore-Tex or bovine pericardium)
Implantation of a valved conduit (e.g., homograft or bovine jugular vein graft) from the right ventricle to the pulmonary arteries
Reconstruction of the aorta (former truncus) if necessary

In cases with associated interrupted aortic arch, arch reconstruction is performed concurrently.

Postoperative Care

Postoperative management in the intensive care unit focuses on:

Hemodynamic stabilization
Ventilator management and weaning
Pain control and sedation
Fluid and electrolyte management
Monitoring for and treating postoperative complications

Common postoperative complications may include:

Low cardiac output syndrome
Arrhythmias
Pulmonary hypertensive crises
Bleeding
Chylothorax
Wound infections

Long-term Follow-up

Patients with repaired truncus arteriosus require lifelong cardiology follow-up. Key aspects of long-term management include:

Regular clinical assessment and echocardiography to monitor:
- Ventricular function
- RV-PA conduit function (stenosis and regurgitation)
- Neo-aortic (truncal) valve function
- Pulmonary artery anatomy and growth
Assessment for arrhythmias (ECG, Holter monitoring)
Exercise testing to evaluate functional capacity
Cardiac MRI or CT as needed for detailed anatomical and functional assessment
Management of associated conditions (e.g., 22q11.2 deletion syndrome)
Endocarditis prophylaxis as per current guidelines
Counseling regarding physical activity, pregnancy, and genetic risks

Reinterventions

Many patients with repaired truncus arteriosus will require reinterventions over their lifetime. Common indications for reintervention include:

RV-PA conduit stenosis or regurgitation
Branch pulmonary artery stenosis
Neo-aortic valve regurgitation or stenosis
Neo-aortic root dilation
Residual or recurrent VSD

Reinterventions may be performed surgically or, in select cases, via catheter-based techniques. The choice of intervention depends on the specific anatomical and physiological issues, patient age and size, and institutional expertise.

Surgical Reinterventions

Common surgical reinterventions include:

RV-PA conduit replacement: This is the most frequent reintervention, typically required 10-15 years after initial repair due to patient outgrowth, stenosis, or regurgitation of the conduit.
Pulmonary artery reconstruction: To address branch pulmonary artery stenosis or distortion.
Neo-aortic valve repair or replacement: For significant regurgitation or stenosis of the native truncal valve.
Neo-aortic root replacement: In cases of progressive aortic root dilation.

Catheter-Based Interventions

Catheter-based interventions may be used as a bridge to surgery or as definitive treatment in select cases:

Balloon angioplasty and stenting: For RV-PA conduit stenosis or branch pulmonary artery stenosis.
Transcatheter pulmonary valve replacement: May be an option in patients with suitable anatomy and appropriate conduit size.
Device closure: For residual VSDs or other septal defects.

Outcomes and Prognosis

The prognosis for patients with truncus arteriosus has improved significantly over the past few decades due to advancements in surgical techniques, perioperative care, and long-term management. However, it remains a complex condition with potential for long-term complications and need for reinterventions.

Short-term Outcomes

Early surgical mortality has decreased substantially and is now generally reported to be less than 10% in experienced centers. Factors associated with increased early mortality include:

Younger age and lower weight at time of repair
Presence of significant truncal valve regurgitation
Associated interrupted aortic arch
Coronary artery anomalies

Long-term Outcomes

Long-term survival for patients with repaired truncus arteriosus continues to improve. Recent studies report 20-year survival rates of 70-80%. However, patients face several long-term issues:

Conduit longevity: The need for conduit replacement remains a significant issue, with many patients requiring multiple reoperations over their lifetime.
Truncal (neo-aortic) valve function: Progressive neo-aortic valve regurgitation can develop over time, potentially requiring valve repair or replacement.
Aortic root dilation: This can occur in a subset of patients and may necessitate aortic root surgery.
Arrhythmias: Both atrial and ventricular arrhythmias can occur, particularly as patients reach adulthood.
Exercise capacity: Many patients have reduced exercise capacity compared to healthy peers, even after successful repair.
Neurodevelopmental outcomes: Patients may be at risk for neurodevelopmental issues, particularly those with associated genetic syndromes.

Quality of Life

Despite the complexities of their condition, many patients with repaired truncus arteriosus achieve good quality of life. However, they may face challenges related to:

Repeated hospitalizations and interventions
Restrictions on physical activity
Educational or occupational limitations
Psychosocial issues related to living with a chronic condition

Comprehensive care should address these aspects through multidisciplinary follow-up, including psychological support and vocational counseling as appropriate.

Special Considerations

Pregnancy

Women with repaired truncus arteriosus who are considering pregnancy require careful preconception counseling and multidisciplinary management. Key considerations include:

Assessment of cardiovascular status, including ventricular function, valve function, and pulmonary pressures
Evaluation of RV-PA conduit function
Genetic counseling regarding the risk of congenital heart disease in offspring
Close monitoring during pregnancy, labor, and delivery
Adjustment of medications as needed

Transition to Adult Care

As patients with repaired truncus arteriosus reach adulthood, a structured transition to adult congenital heart disease (ACHD) care is crucial. This process should:

Begin in early adolescence
Involve both pediatric and adult congenital heart disease specialists
Include education about the condition, importance of lifelong follow-up, and potential long-term complications
Address psychosocial aspects of transition, including education, employment, and independence

Endocarditis Prophylaxis

Current guidelines recommend endocarditis prophylaxis for patients with repaired truncus arteriosus who have:

Prosthetic cardiac valve or prosthetic material used for cardiac valve repair
Previous history of infective endocarditis
Unrepaired cyanotic congenital heart disease or repaired congenital heart disease with residual defects at or adjacent to the site of a prosthetic patch or prosthetic device

Future Directions

Ongoing research and technological advancements continue to shape the management of truncus arteriosus:

Tissue-engineered conduits: Development of growth-potential conduits that could reduce the need for reoperations.
Improved transcatheter interventions: Expanding options for pulmonary valve replacement and other catheter-based procedures.
Fetal interventions: While not currently applicable to truncus arteriosus, advances in fetal cardiac interventions may eventually play a role in management.
Genetic and molecular research: Better understanding of the genetic basis of truncus arteriosus may lead to improved risk stratification and potentially targeted therapies.
Long-term outcomes research: Continued study of adults with repaired truncus arteriosus will inform management strategies and improve counseling for patients and families.

In conclusion, truncus arteriosus remains a complex congenital heart defect that requires lifelong, specialized care. While significant challenges persist, advancements in surgical techniques, medical management, and interventional procedures have dramatically improved outcomes for these patients. Continued research and innovation promise to further enhance the care of individuals with this rare but significant cardiac anomaly.