Tetralogy of Fallot – Causes, Symptoms and Treatment

What is Tetralogy of Fallot?

Tetralogy of Fallot (TOF) is a complex congenital heart defect present at birth. It was first described in 1888 by French physician Étienne-Louis Arthur Fallot. The term tetralogy refers to the simultaneous presence of four characteristic cardiac malformations. The condition is classified as a cyanotic heart defect, because it leads to reduced oxygen levels in the blood, causing the characteristic bluish discoloration of the skin known as cyanosis. Tetralogy of Fallot is the most common complex cyanotic heart defect in childhood, affecting approximately 3 to 5 in every 10,000 newborns worldwide.

The Four Cardiac Defects

Tetralogy of Fallot is defined by four simultaneously occurring structural heart anomalies:

• Pulmonary stenosis: A narrowing of the right ventricular outflow tract or pulmonary valve, which restricts blood flow to the lungs.
• Ventricular septal defect (VSD): A hole in the wall (septum) separating the left and right ventricles, allowing oxygen-poor and oxygen-rich blood to mix.
• Overriding aorta: The aorta (main artery) is positioned directly over the ventricular septal defect, receiving blood from both ventricles instead of only from the left ventricle.
• Right ventricular hypertrophy: The muscular wall of the right ventricle thickens due to the increased resistance it must overcome to pump blood.

Causes and Risk Factors

In most cases, the exact cause of Tetralogy of Fallot is unknown. The defect develops during the embryonic period of heart development, typically between weeks 3 and 8 of pregnancy. Known risk factors include:

• Genetic conditions: chromosomal abnormalities such as Down syndrome (trisomy 21) or DiGeorge syndrome (22q11.2 deletion) are frequently associated.
• Maternal illnesses during pregnancy, such as diabetes mellitus or rubella infection.
• Maternal alcohol or drug use during early pregnancy.
• Certain medications taken during pregnancy.
• Family history: a slightly increased risk exists if a parent was born with Tetralogy of Fallot.

Symptoms

The severity of symptoms in Tetralogy of Fallot depends largely on the degree of pulmonary stenosis. Common signs and symptoms include:

• Cyanosis: Bluish discoloration of the lips, fingernails, and skin due to low blood oxygen levels.
• Hypoxic spells (Tet spells): Sudden episodes of severe cyanosis and breathlessness, often triggered by physical activity or crying, most common in infancy.
• Clubbing of fingers and toes: Widening and rounding of the fingertips due to chronic oxygen deficiency.
• Reduced exercise tolerance and rapid fatigue.
• Heart murmur detectable during medical examination.
• Squatting behavior: Older children may spontaneously squat during episodes of cyanosis, as this position increases blood flow to the lungs by raising systemic vascular resistance.

Diagnosis

Tetralogy of Fallot is diagnosed using several examination methods:

• Prenatal echocardiography: The defect can often be identified before birth via fetal ultrasound of the heart.
• Echocardiography (cardiac ultrasound): The most important diagnostic method after birth. It clearly visualizes all four cardiac abnormalities.
• Electrocardiogram (ECG): Shows characteristic changes resulting from right ventricular hypertrophy.
• Chest X-ray: Often reveals the characteristic boot-shaped heart silhouette (coeur en sabot) due to the enlarged right ventricle.
• Pulse oximetry: Measures oxygen saturation in the blood, which is typically reduced in patients with Tetralogy of Fallot.
• Cardiac catheterization or MRI: Used in complex cases to plan surgical intervention in detail.

Treatment

Surgical Repair

Tetralogy of Fallot requires surgical treatment in virtually all cases. The goal of surgery is complete correction of all four cardiac defects. Total corrective surgery is typically performed between 3 and 12 months of age and involves:

• Closure of the ventricular septal defect using a patch.
• Widening of the right ventricular outflow tract and, if necessary, reconstruction or repair of the pulmonary valve.

Palliative Procedures

In very small or hemodynamically unstable infants, a Blalock-Taussig-Thomas shunt may first be placed as a temporary measure. This is an artificial connection between a systemic artery and the pulmonary artery that improves blood flow to the lungs until complete surgical repair is feasible.

Medication

Medications such as propranolol (a beta-blocker) or morphine may be used to manage acute hypoxic spells (Tet spells) in the short term. However, medication does not replace surgical correction and is not a long-term solution.

Long-Term Outlook and Follow-Up

After successful surgical repair, most patients have a good long-term prognosis and can lead relatively normal lives. However, lifelong cardiology follow-up is essential. Possible long-term complications include cardiac arrhythmias, pulmonary valve regurgitation, or re-narrowing of the outflow tract, which may require repeat surgery or catheter-based intervention in some patients.

References

1. Apitz C, Webb GD, Redington AN. Tetralogy of Fallot. The Lancet. 2009;374(9699):1462-1471. doi:10.1016/S0140-6736(09)60657-7
2. Van Arsdell GS, Maharaj GS, Tom J, et al. What is the optimal age for repair of Tetralogy of Fallot? Circulation. 2000;102(19 Suppl 3):III123-129.
3. American Heart Association. Tetralogy of Fallot. Available at: https://www.heart.org (Accessed 2024).