Tetralogy of Fallot (TOF) is the most common form of congenital cyanotic heart disease (~8-10%)
Worldwide reported prevalence of TOF is 0.34 per 1,000 live births
TOF is a constellation of four findings:
The degree of pulmonary stenosis is variable. Infundibular (subvalvar) stenosis is common and is often dynamic, but stenosis may also include hypoplasia and dysplasia of the pulmonary valve as well as main pulmonary artery hypoplasia (supravalvar). Right ventricular outflow tract obstruction causes increased right ventricular pressure and can lead to right to left shunting across the VSD in addition to decreased pulmonary blood flow leading to cyanosis. In the setting of mild pulmonary stenosis, shunting across the VSD is predominantly left to right and patients are acyanotic. This is clinically referred to as a “pink Tet”
The typical ventricular septal defect in TOF is a conoventricular septal defect with anterior malalignment, resulting in anterior, superior and leftward deviation of the infundibular septum causing narrowing of the right ventricular outflow tract.
Right ventricular hypertrophy component is the consequence of right ventricular pressure rather than a primary morphologic feature but adds to the right ventricular outflow obstruction
The aorta overrides the VSD and native ventricular septum
Right aortic arch (25%)
Clockwise rotation of the aortic root resultant rotation of the origin of the coronary arteries
Coronary artery anomalies (~ 5%)
Left SVC to coronary sinus (~11%)
Atrioventricular septal defect (AVSD) (~5%)
Subtypes: TOF with atrioventricular septal defect, TOF with absent pulmonary valve
Fetal Imaging Pearls
Degree of aortic override best seen from fetal 5 chamber view
Typically ~50% override of the native ventricular septum
Presence of mitral-aortic continuity (differentiates TOF from double outlet right ventricle where there is mitral-aortic discontinuity).
Conoventricular VSD: Assess size and direction of shunting by 2D and color Doppler
Degree of pulmonary valvar hypoplasia with pulmonary valve stenosis (2D, color and spectral Doppler, fetal z-scores)
Degree of subvalvar stenosis (assess degree of anterior deviation of the infundibulum)
Findings consistent with severe pulmonary stenosis include retrograde aortic-to-pulmonary flow in PDA with severe pulmonary hypoplasia with flow turbulence or minimal flow across the pulmonary outflow
Patent Ductus Arteriosus
Assess for presence of PDA: Small percentage of TOF patients will have absent PDA
Assess flow pattern in PDA: normally right to left shunting, however, retrograde aortic-to-pulmonary flow in PDA consistent with severe pulmonary stenosis and likely ductal dependency postnatally
Assess aortic arch size, flow (2D, color spectral Doppler) and arch sidedness (~25% right aortic arch)
Chromosome anomalies occur in ~25-30% of fetuses with TOF and include:
Trisomy 21: Can be associated with a specific variant of TOF/AVSD
22q11 Deletion Syndrome (DiGeorge Syndrome): ~10-15% of cases with increased risk in the setting of right aortic arch, hypoplastic thymus or extracardiac abnormalities
Other associated syndromes:
Aneuplodies: Trisomy 13,18
Assess interval growth of right ventricular outflow tract, pulmonary valve and pulmonary artery and for any change in ductal flow pattern (flow reversal in PDA concerning for postnatal ductal dependent pulmonary blood flow).
Depends on severity of pulmonary stenosis. Patients with severe pulmonary stenosis/hypoplasia may require PGE following birth.
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