6.4 Tetralogy of Fallot & Truncus Arteriosus
Key Takeaways
- Tetralogy of Fallot involves an anterior, superior, and rightward deviation of the conal septum, leading to an overriding aorta, VSD, RVOT obstruction, and subsequent RV hypertrophy.
- An overriding aorta must arise >50% from the left ventricle; if it arises >50% from the right ventricle, the diagnosis shifts to Double Outlet Right Ventricle (DORV).
- Absent Pulmonary Valve Syndrome is a severe TOF variant characterized by massive aneurysmal dilation of the pulmonary arteries causing critical airway compression.
- Truncus Arteriosus features a single common great artery supplying systemic, pulmonary, and coronary circulations, strongly linked to a 22q11.2 microdeletion (DiGeorge syndrome).
- Differentiation between severe TOF and Truncus Arteriosus in utero relies heavily on identifying the presence or absence of a main pulmonary artery on the 3VT view.
Tetralogy of Fallot (TOF)
Tetralogy of Fallot is the most common cyanotic congenital heart defect. The fundamental embryological error in TOF is a single defect: the anterior, superior, and rightward deviation of the conal (infundibular) septum. This singular deviation directly causes the classic four features of the tetralogy:
- Overriding Aorta: The aorta is shifted anteriorly and to the right, positioning it directly over the ventricular septum.
- Ventricular Septal Defect (VSD): The malalignment of the conal septum creates a large, subaortic malalignment VSD.
- Right Ventricular Outflow Tract (RVOT) Obstruction: The anterior displacement of the septum crowds the RVOT, resulting in infundibular pulmonary stenosis or hypoplasia of the pulmonary valve and main pulmonary artery.
- Right Ventricular Hypertrophy (RVH): A postnatal consequence. Because the RV faces high resistance from the pulmonary stenosis and the systemic pressure of the overriding aorta, it hypertrophies over time. (Note: RVH is rarely seen in utero because both ventricles pump at equal pressures systemically).
Sonographic Evaluation of TOF
The 4-chamber view is typically normal. The diagnostic clues emerge in the Left Ventricular Outflow Tract (LVOT) view and the 3-Vessel Trachea (3VT) view.
- The Y-Sign: In the LVOT (5-chamber) view, the hallmark finding is the large aorta overriding the ventricular septum. The septum appears to split the aortic root into a "Y" shape. To diagnose TOF, the aorta must be connected more than 50% to the left ventricle. If the overriding aorta is connected more than 50% to the right ventricle, the diagnosis shifts to Double Outlet Right Ventricle (DORV).
- Small Pulmonary Artery: In the 3VT view, the pulmonary artery will appear significantly smaller than the enlarged aorta. Forward flow is usually present across the pulmonary valve, but at elevated velocities due to stenosis.
- TOF with Pulmonary Atresia: The extreme end of the TOF spectrum where there is no forward flow across the pulmonary valve. The pulmonary artery is supplied retrograde via a patent ductus arteriosus (PDA) or through major aortopulmonary collateral arteries (MAPCAs).
Absent Pulmonary Valve Syndrome
A rare, severe variant of TOF (accounting for 3-6% of cases). The pulmonary valve leaflets are rudimentary or completely absent. This leads to severe pulmonary regurgitation in utero. The constant volume overload causes massive, aneurysmal dilation of the main and branch pulmonary arteries. These massive arteries compress the developing fetal tracheobronchial tree, leading to severe, often lethal bronchomalacia and respiratory failure at birth.
Truncus Arteriosus
Truncus Arteriosus is a rare conotruncal anomaly where the embryological conotruncal ridges completely fail to septate the primitive truncus arteriosus into a separate aorta and pulmonary artery.
The result is a single, massive great artery that arises from the base of the heart, overriding a large VSD, and supplying all three major circulations: systemic, pulmonary, and coronary.
Sonographic Diagnosis
Like TOF, the 4-chamber view is normal, and an overriding great vessel is seen on the LVOT view. The critical differentiation between TOF and Truncus Arteriosus relies on finding the pulmonary arteries:
- In Truncus, there is NO separate right ventricular outflow tract or pulmonary valve.
- The Collett and Edwards Classification defines how the pulmonary arteries arise from the common trunk:
- Type I: A short main pulmonary artery arises from the trunk, then bifurcates into left and right branches.
- Type II: Left and right pulmonary arteries arise separately but adjacent to each other from the posterior aspect of the trunk.
- Type III: Left and right pulmonary arteries arise separately and laterally from the trunk.
- Type IV: Now classified as TOF with pulmonary atresia and MAPCAs, not true truncus.
- The Truncal Valve: The single valve is often severely dysplastic, thickened, and may have 2, 3, 4, or even 5 leaflets. Truncal valve regurgitation or stenosis is common and significantly impacts prognosis.
- 3VT View: The 3VT view is highly abnormal. It will show only two vessels (a large single trunk and the SVC), rather than the normal three. The ductus arteriosus is usually absent.
Genetic Link
Both TOF and especially Truncus Arteriosus have a remarkably high association with a 22q11.2 microdeletion (DiGeorge Syndrome). Roughly 30-40% of patients with Truncus Arteriosus will carry this deletion, making genetic amniocentesis strongly recommended upon diagnosis.
| Feature | Tetralogy of Fallot (TOF) | Truncus Arteriosus |
|---|---|---|
| Primary Defect | Anterior deviation of conal septum | Failure of conotruncal septation |
| Overriding Vessel | Aorta | Common Truncal Artery |
| Pulmonary Artery | Small, stenotic, arises from RV | Arises directly from the common trunk |
| 3VT View | 3 vessels seen (small PA, large Ao, SVC) | 2 vessels seen (massive Trunk, SVC), ductus absent |
| Genetics | High association with 22q11.2 | Very high association with 22q11.2 (34-40%) |
Which singular embryological event is responsible for all four classic anatomical components of Tetralogy of Fallot?
A fetus is diagnosed with an overriding aorta and an absent pulmonary valve. Ultrasound reveals massively dilated pulmonary arteries. What is the most critical postnatal complication this infant faces?
During a fetal echocardiogram, you identify a single large vessel overriding a VSD. In the 3VT view, only two vessels are seen, and the ductus arteriosus is absent. The pulmonary arteries are seen branching directly off the posterior aspect of the single great vessel. What is the diagnosis?