Tetralogy of Fallot (TOF) is a congenital cardiovascular anomaly involving a combination of four defects: pulmonary stenosis, ventricular septal defect (VSD), overriding aorta (the aorta straddles the VSD), and right ventricular hypertrophy. It is the most common cyanotic congenital heart disease, accounting for nearly 10% of adult congenital heart diseases.
Pathological Anatomy
The primary defect in TOF is a large ventricular septal defect, typically located in the perimembranous region, resulting in equal pressure between the left and right ventricles. Pulmonary stenosis can be valvular, supravalvular, or subvalvular, with subvalvular (infundibular) stenosis being the most common. The degree of aortic overriding over the right ventricle can range from 15% to 95%. Right ventricular hypertrophy is a secondary change caused by hemodynamic alterations. TOF is often associated with other anomalies, such as atrial septal defect, which, if present, is referred to as pentalogy of Fallot.
Pathophysiology
The large VSD causes equal pressure in the left and right ventricles, effectively functioning as a single ventricle pumping blood to both the systemic and pulmonary circulations. The increased right ventricular pressure, combined with pulmonary stenosis, reduces pulmonary artery pressure and flow. A significant portion of right ventricular blood is shunted through the overriding aorta into the systemic circulation, leading to decreased arterial oxygen saturation and cyanosis. Chronic hypoxia may result in secondary polycythemia.
Clinical Manifestations
Symptoms
The main symptoms are progressive cyanosis and dyspnea from early childhood, fatigue, and squatting posture during exertion to relieve symptoms. Severe hypoxia may cause syncope. Prolonged right ventricular pressure overload and hypoxia can lead to heart failure.
Signs
Patients often exhibit marked cyanosis, clubbed fingers and toes, and reduced or absent pulmonary valve second heart sound. A systolic ejection murmur is typically heard along the left sternal border. Common complications include cerebrovascular accidents (e.g., stroke), infective endocarditis, and pulmonary infections.
Auxiliary Examinations
Complete Blood Count
Elevated red blood cell count, hemoglobin, and hematocrit due to chronic hypoxia can be seen.
Electrocardiography (ECG)
ECG shows right axis deviation and right ventricular hypertrophy.
Chest X-ray
It reveals evidence of right ventricular hypertrophy, concave pulmonary artery segment, and a boot-shaped heart silhouette. Decreased pulmonary vascular markings are also seen.
Echocardiography
It demonstrates right ventricular hypertrophy, VSD, and an overriding aorta. It also provides details on right ventricular outflow tract obstruction and pulmonary valve anatomy.
Magnetic Resonance Imaging (MRI)
MRI offers clear visualization of anatomical abnormalities.
Cardiac Catheterization
It determines the nature and severity of the defects and identifies associated anomalies, aiding in surgical planning.
Diagnosis and Differential Diagnosis
The diagnosis is suggested by clinical presentation, chest X-ray, and ECG findings and is typically confirmed by echocardiography. Differential diagnoses include transposition of the great arteries with pulmonary stenosis, double outlet right ventricle, and Eisenmenger syndrome.
Treatment
Patients with TOF should undergo surgical repair. In recent years, a combination of catheter-based interventions and surgical procedures has significantly improved treatment outcomes.
Prognosis
Without surgical intervention, the prognosis is poor. Most untreated children die before the age of 20 due to heart failure, cerebrovascular accidents, infective endocarditis, or other complications.