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 conal 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%)
Subtypes: TOF/AVSD, TOF with absent pulmonary valve
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 Stenosis
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)
Degree of supravalvar stenosis (MPA size, fetal z-scores)
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 postntally
Assess aortic arch size, flow (2D, color spectral Doppler) and arch sidedness (~25% right aortic arch)
Follow up fetal imaging every 4-6 weeks is reasonable to assess pulmonary architecture and growth.
Assess for any change in ductal flow pattern (flow reversal in PDA likely consistent with postnatal ductal dependent pulmonary blood flow).
Genetics: Chromosomal abnormalites are common (~30%). Referral to genetic councilling warranted with consideration of non-invasive testing (NIPT) +/- amniocentesis given association with genetic abnormalities such as 22q11 deletion syndrome/DiGeorge (~10-15%), trisomy 21 or VACTERL.
Surgical: For mild stenosis, council on full repair in infancy (3-6 months). For severe stenosis, council on likely intervention/surgery in neonatal period (ductal stent versus BT shunt). Consider cath or surgical referral prenatally if neonatal intervention anticipated.
Mildpulmonarystenosis: in setting of mild hypoplasia rarely ductal dependent. Postnatal monitoring of saturations with postnatal echocardiogram to assess degree of PS is warranted.
Moderatetoseverepulmonarystenosis: Fetuses with moderate to severe hypoplasia require very close postnatal monitoring of saturations and often PGEs for ductal dependent pulmonary blood flow with BT shunt or ductal stent in neonatal period followed by a complete repair in infancy (3-6 months of life).
Severepulmonarystenosis: Any fetus with severe pulmonary hypoplasia with flow reversal in the PDA should be started electively on prostaglandins (PGE) postnatally.
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