Tetralogy of Fallot results from a defect in the conotruncal division of the embryonic outflow tract during cardiac development, between the 4th and 8th weeks of gestation.
This defect causes an anterior and superior displacement of the infundibular septum, leading to the four (hence "tetra") hallmark abnormalities:
This combination of defects alters pulmonary and systemic blood flow: pulmonary perfusion is reduced, and a portion of deoxygenated venous blood is diverted directly into the systemic circulation via the VSD, resulting in a right-to-left shunt that leads to cyanosis and hypoxemia.
Tetralogy of Fallot may occur as an isolated malformation or be associated with other congenital anomalies, the most common being hypoplastic main pulmonary artery, persistent left superior vena cava, and, in some cases, coronary artery anomalies. It is also frequently associated with genetic syndromes, particularly the 22q11.2 deletion (DiGeorge syndrome).
Clinically, it is one of the most typical causes of late-onset neonatal cyanosis and represents a major indication for early surgical repair within the first months of life.
Tetralogy of Fallot results from abnormal development of the infundibular (conal) septum during embryogenesis, particularly between the fifth and eighth weeks of gestation. Under normal conditions, the infundibular region aligns and fuses correctly with the ventricular septum and endocardial cushions, allowing for separation of pulmonary and systemic circulations.
The precise causes of infundibular misalignment remain unclear, but in a subset of patients, genetic abnormalities involving conotruncal development have been identified, including mutations in the TBX1, NKX2.5, and JAG1 genes. Tetralogy is also commonly associated with the 22q11.2 microdeletion, found in approximately 15% of cases.
In tetralogy, there is an anterior and superior displacement of the infundibular septum, resulting in a complex malformation consisting of four interrelated components:
From a pathophysiological perspective, the right ventricular outflow tract obstruction is the primary determinant. When the stenosis is severe, right ventricular pressure equals or exceeds that of the left ventricle, promoting a right-to-left shunt through the VSD and causing systemic hypoxemia. Conversely, in cases of mild obstruction, pulmonary blood flow increases and the shunt may be bidirectional or left-to-right, resulting in minimal cyanosis but risk of pulmonary overcirculation.
Progressive right ventricular hypertrophy represents an adaptive response to chronic pressure overload but may eventually impair contractile function and diastolic compliance, worsening the hemodynamic state. In uncorrected cases, chronic cyanosis may lead to polycythemia, hypercoagulability, and systemic vascular complications.
In cases with mild infundibular obstruction, pulmonary blood flow may be relatively preserved and the shunt across the VSD may remain bidirectional or predominantly left-to-right. In these patients, arterial oxygen saturation remains within or near normal limits, and clinical cyanosis may be absent. This condition is referred to as “pink Fallot” and may delay diagnosis until later in childhood or even adulthood, when signs of pulmonary overcirculation or ventricular dysfunction become apparent.
The clinical presentation of tetralogy of Fallot is dominated by cyanosis, the severity of which depends on the degree of right ventricular outflow tract obstruction. In neonates with mild stenosis, symptoms may initially be absent or subtle, and the diagnosis is often made upon detection of a systolic murmur during physical examination. In some cases, such as "pink Fallot", cyanosis may be entirely absent. In contrast, in cases with severe obstruction, cyanosis is evident early and may be noticeable within the first hours of life.
Central cyanosis is typically more pronounced during crying, feeding, or physical effort, when increased systemic vascular resistance exacerbates the right-to-left shunt. In infants, the condition may worsen over time due to progressive infundibular hypertrophy and increasing obstruction. Many patients present with growth retardation in both weight and height, secondary to chronic hypoxemia and increased energy expenditure.
A hallmark feature is the occurrence of hypoxic spells (or “Tet spells”), which are paroxysmal episodes of acute desaturation characterized by labored breathing, deep cyanosis, pallor, irritability, and possible syncope. These spells typically occur between 2 and 6 months of age, often upon awakening or during crying, and result from a sudden decrease in pulmonary blood flow due to infundibular spasm. If not treated promptly, they may lead to cerebral hypoxia, seizures, or cardiopulmonary arrest.
In older children, cyanosis may be mitigated by postural compensatory mechanisms. A classic clinical sign is the tendency to squat after exertion, a position that increases systemic vascular resistance and reduces the right-to-left shunt, temporarily improving arterial oxygen saturation. Over time, chronic hypoxemia may result in polycythemia, digital clubbing, and symptoms of chronic fatigue.
In certain cases, particularly in the presence of a coronary anomaly or hypoplastic pulmonary artery, the clinical picture may be more complex, with episodes of myocardial ischemia, arrhythmias, or progressive hypoxia even at an early age. A variant known as “pink Fallot” is characterized by minimal obstruction and absent or mild cyanosis, but with significant pulmonary overcirculation and risk of heart failure in infancy.
On physical examination, beyond cyanosis, the most consistent finding is an ejection systolic murmur along the left sternal border, due to right outflow tract stenosis. The murmur's intensity does not always correlate with the severity of the defect: in cases of very severe obstruction, pulmonary flow may be so reduced that the murmur becomes paradoxically soft. The second heart sound is often single or widely split, with a diminished or absent pulmonary component.
The diagnosis of tetralogy of Fallot can be made prenatally, in the neonatal period, or during infancy, depending on the degree of right ventricular outflow obstruction and the presence of cyanosis. In neonates with mild stenosis, clinical signs may be subtle, while in those with severe obstruction, suspicion arises early due to persistent central cyanosis.
Obstetric ultrasound performed between the 18th and 22nd week of gestation may detect suggestive findings such as a ventricular septal defect, reduced caliber of the pulmonary artery, and malposition of the aorta. The presence of extracardiac anomalies or suspected genetic syndromes significantly increases screening accuracy.
In the newborn, the combination of cyanosis unresponsive to oxygen therapy and an ejection systolic murmur along the left sternal border should prompt suspicion of a cyanotic congenital heart disease.
Transthoracic echocardiography is the first-line test in the postnatal period to confirm the diagnosis and define the defect's anatomy, allowing visualization of the four classic components of tetralogy: ventricular septal defect, right outflow tract obstruction, overriding aorta, and right ventricular hypertrophy. It also enables detection of associated anomalies such as pulmonary branch hypoplasia or coronary variants, which are essential for surgical planning.
In older children or adults, cardiac magnetic resonance imaging provides detailed information on anatomy, ventricular function, and the extent of pulmonary regurgitation in previously operated patients. This modality is frequently used in long-term follow-up.
Electrocardiography typically reveals signs of right ventricular hypertrophy and rightward axis deviation.
Chest radiography may show a "boot-shaped" cardiac silhouette and decreased pulmonary vascular markings, but it is not sufficient for definitive diagnosis.
Cardiac catheterization plays a limited role in initial diagnosis but may be indicated in cases of equivocal echocardiographic findings, complex anatomy, or for assessing pressure gradients and pulmonary vascular resistance in surgical candidates. It is also useful for selective coronary artery evaluation prior to corrective surgery.
The diagnosis relies on a correct integration of clinical data, echocardiographic imaging, and advanced studies when needed. Timely recognition is essential to schedule appropriate treatment and prevent complications of chronic hypoxemia.
The management of tetralogy of Fallot requires corrective cardiac surgery, which is the standard of care in the vast majority of cases. The goal of surgery is to restore a univentricular circulation with adequate pulmonary blood flow by closing the ventricular septal defect and relieving the right outflow tract obstruction. Timing, technique, and preoperative planning depend on the clinical presentation and anatomical complexity of the defect.
In symptomatic neonates with severe cyanosis and recurrent hypoxic spells, a temporary palliative procedure may be necessary before definitive repair. The most commonly used approach is the modified Blalock-Taussig shunt, which connects the subclavian artery to the ipsilateral pulmonary artery, increasing pulmonary perfusion and improving oxygen saturation.
Complete repair is ideally performed between 3 and 6 months of age, although many centers prefer a primary early approach during the neonatal period in stable patients. The procedure includes:
In patients with an anomalous coronary artery crossing the infundibulum, an alternative surgical technique is employed to avoid ischemic damage, such as the creation of an extracardiac conduit between the right ventricle and pulmonary artery.
Prognosis after complete repair is very favorable. In specialized centers, long-term survival exceeds 90%, with good quality of life. However, long-term cardiological follow-up is essential, as surgery is not free from consequences. The main sequelae include chronic pulmonary regurgitation, progressive right ventricular dilation, ventricular dysfunction, and ventricular arrhythmias, particularly in patients operated on later or undergoing multiple interventions.
Long-term monitoring is based on regular clinical evaluations, morpho-functional imaging (echocardiography, MRI), and exercise testing. In cases of severe pulmonary regurgitation and functional deterioration, surgical or percutaneous reintervention for pulmonary valve replacement is indicated, and today may also be performed using transcatheter techniques.
In patients treated early and followed regularly, life expectancy approaches that of the general population. However, age at repair, presence of residual defects, and right ventricular function are the main long-term prognostic factors.
Despite the high success rate of surgical repair for tetralogy of Fallot, there are short- and long-term complications that require careful follow-up. The nature and frequency of these complications depend on multiple factors, including age at repair, surgical technique used, initial anatomy, and presence of associated anomalies.
The most frequent long-term complication is pulmonary regurgitation, especially in patients who received a transannular patch. Initially well tolerated, this condition may lead over time to right ventricular dilation and dysfunction, reduced exercise tolerance, arrhythmias, and, in advanced cases, heart failure.
Another common complication is the development of ventricular arrhythmias, which can arise years after surgery. Their pathogenesis is multifactorial: surgical scars, ventricular dilation, residual hypertrophy, and conduction system abnormalities are all contributing factors. In severe cases, sustained ventricular tachycardia may occur, with a risk of sudden cardiac death. Periodic monitoring and, when indicated, implantable cardioverter-defibrillators are essential preventive measures.
In patients operated on late or with complex anatomy, there may be residual right outflow tract obstruction or a persistent ventricular septal defect, requiring further intervention. Progressive dilation of the ascending aorta has also been observed in some cohorts, likely due to hemodynamic and structural factors.
In adulthood, some patients may develop left ventricular dysfunction secondary to chronic interventricular interaction, particularly in cases with severe right ventricular dilation. Biventricular impairment worsens the prognosis and requires comprehensive management.
Infective endocarditis remains a lifelong risk, particularly in patients with prosthetic materials or residual shunts. Appropriate prophylaxis is recommended in high-risk settings.
Follow-up should be structured and multidisciplinary, with regular reassessment of cardiac function, valvular status, rhythm, and functional capacity. Early detection of complications allows targeted intervention and significantly improves survival and quality of life in patients with repaired tetralogy of Fallot.