What are the different types of Tetralogy of Fallot (TOF)?

Types of Tetralogy of Fallot (TOF)

Tetralogy of Fallot (TOF) is primarily classified into classic TOF and several anatomical variants, with the classic form characterized by four components: ventricular septal defect, pulmonary stenosis, overriding aorta, and right ventricular hypertrophy. 1, 2

Classic Tetralogy of Fallot

The classic form of TOF consists of four key components resulting from anterosuperior deviation of the conal or outlet septum:

1. Ventricular Septal Defect (VSD) - Typically a malalignment-type VSD
2. Pulmonary Stenosis - Narrowing or atresia of the right ventricular outflow tract (RVOT)
3. Overriding Aorta - Biventricular origin of the aorta (aortic override)
4. Right Ventricular Hypertrophy - Secondary to RVOT obstruction 1, 2, 3

Anatomical Variants of TOF

1. Pentalogy of Fallot

• Classic TOF with an additional atrial septal defect (ASD)
• The ASD creates an additional shunt at the atrial level
• May increase right-to-left shunting and exacerbate cyanosis 2

2. TOF with Absent Pulmonary Valve

• Characterized by rudimentary or absent pulmonary valve tissue
• Typically presents with hugely dilated pulmonary arteries
• Associated with significant airway compression due to the dilated pulmonary arteries 4

3. TOF with Double-Outlet Right Ventricle (DORV)

• Variable degrees of aortic override
• Both great arteries (aorta and pulmonary artery) arise predominantly from the right ventricle 4

4. TOF with Atrioventricular Septal Defect (AVSD)

• Occurs in approximately 13% of TOF cases
• Typically associated with Down syndrome
• Features an inlet VSD as part of the AVSD 4

5. TOF with Pulmonary Atresia

• Complete atresia of the pulmonary valve and often portions of the pulmonary arteries
• May be associated with major aortopulmonary collateral arteries (MAPCAs)
• Considered one of the most severe forms of TOF 1, 4

Associated Anomalies in TOF

Several important associated anomalies can occur with TOF that affect management and outcomes:

Coronary Artery Anomalies

• Major coronary artery crossing the RVOT
• Left anterior descending artery arising from right coronary artery
• Dual left anterior descending artery 4

Additional Cardiac Anomalies

• Multiple VSDs
• Right aortic arch (occurs in approximately 25% of cases)
• Atrial septal defects
• Aortopulmonary collaterals 4, 5

Genetic Associations

• Microdeletions of chromosome 22 (22q11.2 deletion)
• Trisomies 21,18, and 13
• Various genetic syndromes 5

Clinical Significance of TOF Variants

The specific type of TOF has important implications for:

1. Surgical approach - Different variants require tailored surgical techniques
2. Timing of intervention - More severe forms may require earlier intervention
3. Long-term outcomes - Complications vary based on the specific anatomical variant
4. Need for additional procedures - Some variants require staged repairs or additional interventions 1

Post-Repair Complications

After TOF repair, several complications can develop that require monitoring:

• Pulmonary regurgitation
• Residual RVOT obstruction
• RV volume overload and dysfunction
• Ventricular arrhythmias
• Aortic root dilation
• Conduit stenosis or RVOT patch aneurysm 1, 4

Understanding the specific type of TOF is critical for appropriate surgical planning, risk stratification, and long-term management of these patients. Cardiac magnetic resonance imaging (CMR) plays a central role in the evaluation and follow-up of TOF patients, providing detailed anatomical and functional information 1, 6.