Tetralogy of Fallot
Tetralogy of Fallot (TOF) is the second most common cyanotic congenital heart condition and has been classically characterised by the combination of ventricular septal defect (VSD), right ventricular outflow tract obstruction (RVOTO), overriding aorta, and a late right ventricular hypertrophy.
This anomaly accounts for 5 to 10% of all congenital heart disease and has an estimated prevalence of 1 in 2000 births 10.
The presentation relies on the degree of right ventricular outflow tract obstruction (RVOTO). Typically this is significant, resulting in cyanosis evident in the neonatal period, as a consequence of the right to left shunt across the VSD. In cases where outflow obstruction is minimal, cyanosis may be unapparent (pink tetralogy) resulting in delayed presentation, even into adulthood, although this is rare.
Tetralogy of Fallot is classically characterised by four features which are:
ventricular septal defect (VSD)
- may be multiple in ~5% of cases 6
- right ventricular outflow tract obstruction (RVOTO) due to
- overriding aorta
- right ventricular hypertrophy: only develops after birth
The right ventricular hypertrophy is a result of the VSD and right ventricular outlet obstruction, both contributing to elevated resistance to right heart emptying 6.
In ~15% of cases, it is associated with a deletion on chromosome 21q11 6-7.
- right-sided aortic arch: seen in ~25% of cases
- pulmonary hypoplasia +/- atresia: particularly important in determining treatment 8, when there is pulmonary atresia it is sometimes termed pseudotruncus arteriosus 9
- atrial septal defect (ASD) or patent ductus arteriosus (PDA) (termed pentalogy of Fallot)
- coronary artery anomalies: seen in 3% of cases 6
- persistent left sided superior vena cava
- extra cardiovascular associations: may be present in ~15% of cases 10
Chest radiographs may classically show a "boot-shaped" heart with an upturned cardiac apex due to right ventricular hypertrophy and concave pulmonary arterial segment. Most infants with TOF, however, may not show this finding 2.
Pulmonary oligaemia due to decreased pulmonary arterial flow. Right-sided aortic arch is seen in 25%.
Echocardiography allows direct visualisation of the abnormal anatomy and remains the primary modality for the diagnosis of TOF. It has limitations on assessing associated extracardiac anomalies (e.g. peripheral pulmonary stenosis and atresia).
MDCT is useful in demonstrating the complex cardiovascular morphology of TOF, especially the anatomy of the pulmonary and coronary arteries as well as identification of aortopulmonary collateral vessels (MAPCA's). MDCT can be used to evaluate post-surgical changes (e.g. patency of palliative shunts) and complications.
MRI has the great advantage of providing both exquisite anatomical details and functional information without ionising radiation. The detailed assessment of the pulmonary artery is of particular importance because repair of the cardiac defects without addressing pulmonary artery hypoplasia/stenosis has a poor outcome 8.
The main pulmonary artery or right pulmonary artery diameter should be compared to that of the ascending aorta. A ratio of <0.3 usually signifies that primary repair would be unsuccessful, and a bridging shunt operation may be of benefit 8.
Assessment of coronary artery origin is also essential to surgical planning.
Treatment and prognosis
Approximately 90% of untreated TOF patients succumb by the age of 10 years 6. Over the years many surgical approaches were performed until the current primary repair was developed. Shunts are nowadays only performed as a palliative procedure in inoperable cases or to bridge patients until the repair can be carried out, typically in the setting of pulmonary arterial hypoplasia 8.
Shunt operations included 6:
- shunts: designed to reduce cyanosis
- Pott shunt
- Waterston shunt
- Blalock-Taussig shunt: still performed in selected cases
Primary repair is now the preferred treatment and is usually performed at the time of diagnosis.
Common post-surgical complications include 6:
- conduction abnormalities
- right bundle branch block (RBBB): 80-90% of cases
- bifascicular block: 15% of cases
- premature ventricular contractions: ~50% of cases
- sustained ventricular tachycardias: ~5% of cases
- atrial arrhythmias: common
- valvular dysfunction
- tricuspid regurgitation
- pulmonary regurgitation
Prognosis is largely dependent on how soon the defect is diagnosed and corrected, with the best outcome seen in patients repaired before the age of 5 6. Overall there is a 90-95% survival rate at 10 years of age, however, residual right ventricular dysfunction is common. Up to 10% of patients require re-operation within 20 years 6.
History and etymology
It is named after Etienne-Louis Arthur Fallot - French physician (1850-1911).
Findings on a chest radiograph are commonly non-specific, and other cyanotic congenital heart diseases should be considered.
Congenital heart disease
There is more than one way to present the variety of congenital heart diseases. Whichever way they are categorised, it is helpful to have a working understanding of normal and fetal circulation, as well as an understanding of the segmental approach to imaging in congenital heart disease.
congenital heart disease
- normal relationship between chambers and valves
- atrioventricular valves
- outflow tract
- great vessels
- venous inflow
- anomalous valves
- abnormal relationship of chambers and valves
- atrioventricular abnormality
- great vessel connection abnormality
- congenital heart disease - chest x-ray approach
- surgical repairs (mnemonic)
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