Tetralogy of Fallot
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Tetralogy of Fallot (TOF) is the most common cyanotic congenital heart condition with many cases presenting after the newborn period. It has been classically characterized by the combination of ventricular septal defect (VSD), right ventricular outflow tract obstruction (RVOTO), overriding aorta, and late right ventricular hypertrophy.
Tetralogy of Fallot accounts for 5 to 10% of all congenital heart disease and has an estimated prevalence of 1 in 2000 births 10.
right-sided aortic arch: seen in ~25% of cases
coronary artery anomalies: seen in 3% of cases 6
persistent left-sided superior vena cava
extracardiovascular associations: may be present in ~15% of cases 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 ventricular septal defect. In cases where outflow obstruction is minimal, cyanosis may be inapparent (pink tetralogy) resulting in delayed presentation, even into adulthood, although this is rare.
Tetralogy of Fallot is classically characterized by four features which are:
maybe multiple in ~5% of cases 6
right ventricular hypertrophy (only develops after birth)
The right ventricular hypertrophy is a result of the ventricular septal defect 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 22q11 13.
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 tetralogy of Fallot, however, may not show this finding 2.
Pulmonary oligemia occurs due to decreased pulmonary arterial flow. A right-sided aortic arch is seen in 25%.
Echocardiography allows direct visualization of the abnormal anatomy and remains the primary modality for the diagnosis of tetralogy of Fallot. It has limitations on assessing associated extracardiac anomalies (e.g. peripheral pulmonary stenosis and atresia). Salient features which may be assessed during a standard sequence of transthoracic echocardiographic views include 16:
parasternal long-axis view
left and right ventricular size/function
degree of aortic override
override of the aortic root over the ventricular septal defect should be less than half of the aortic diameter 17
analysis of the ventricular septal defect
confirmation of aorto-mitral continuity
parasternal short-axis view
location of the ventricular septal defect
anatomy of the right ventricular outflow tract
dilation of the RVOT may be observed
dynamic obstruction of the RVOT may be noted, resulting from incursion of the infundibulum into the outflow tract
continuous-wave Doppler through the RVOT may demonstrate a late-peaking, high-velocity envelope is consistent with dynamic obstruction
right ventricular systolic pressure (RVSP) may also be quantified using the velocity of the tricuspid regurgitant jet if visible
presence or absence of associated coronary artery anomalies
apical 4 chamber view
assess right and left ventricular size and function
right ventricular hypertrophy may be assessed by measurement of the free wall of the right ventricle
CT is useful in demonstrating the complex cardiovascular morphology of tetralogy of Fallot, especially the anatomy of the pulmonary and coronary arteries as well as identification of major aortopulmonary collateral arteries (MAPCAs). CT 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 detail and functional information without ionizing radiation. A detailed assessment of the pulmonary artery is of particular importance because the repair of the cardiac defects without addressing pulmonary artery hypoplasia or 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 tetralogy of Fallot 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
Blalock-Taussig shunt: still performed in selected cases
Primary repair (total correction (a.k.a. repair) of tetralogy of Fallot) is now the preferred treatment and is usually performed at the time of diagnosis.
Common post-surgical complications include 6:
right bundle branch block (RBBB): 80-90% of cases
bifascicular block: 15% of cases
premature ventricular contractions (PVC): ~50% of cases
sustained ventricular tachycardias (VT): ~5% of cases
atrial arrhythmias: common
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 reoperation within 20 years 6.
History and etymology
It is named after Étienne-Louis Arthur Fallot (1850-1911) 19, a French physician, who described the tetralogy of defects in 1888 14.
Findings on a chest radiograph are commonly non-specific, and other cyanotic congenital heart diseases should be considered.
Other conditions involving an outflow tract overriding a VSD are 15:
pulmonary atresia with ventricular septal defect: considered an extreme form of tetralogy of Fallot, the pulmonary atresia is obstructed with anterograde flow; the ductus arteriosus is tortuous with retrograde flow in the pulmonary arteries; surgery and prognosis is related to the anatomy of the pulmonary arteries
type 1 features a simple diaphragm at the pulmonary annulus
type 2 has no pulmonary trunk but the pulmonary arteries are preserved
type 3 is like type 2 but also with atretic pulmonary arteries
type 4 does not feature any pulmonary arteries but a systemic-pulmonary vascularization by major aortopulmonary collateral arteries (MAPCAs)
can also be an associated feature with tetralogy of Fallot
aortic root is overriding but not enlarged; PA trunk is, conversely, very large with to-and-fro flow due to both stenosis and insufficiency; ductus arteriosus is classically absent or atretic
common arterial trunk: pulmonary is arising from the overriding vessel, the common valve often regurgitates
double outlet right ventricle: mimics transposition of the great vessels, combined with a ventricular septal defect
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