Case of the Week #632
UT Southwestern Medical Center, Plano, Texas, United States of America
A 39-year-old patient, G1P0, presented to our maternal fetal medicine unit at 30 weeks of gestation for a growth scan. The fetus was male with low-risk noninvasive prenatal testing. The following findings were observed.
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Answer
We present an isolated case of Tetralogy of Fallot (TOF). The diagnosis was confirmed by pediatric cardiology.
Our images demonstrate the following:
- Image 1: Transverse view of the upper abdomen at the level of the abdominal circumference.
- Image 2: Axial plane of the fetal chest at the level of the four-chamber view. Note the normal appearance of the four-chamber view.
- Images 3 and 4: The five-chamber view shows the ventricular septal defect and the dilated overriding aorta.
- Image 5: The right ventricular outflow tract view with color Doppler demonstrates a small main pulmonary artery when compared to the dilated aorta due to pulmonary stenosis.
- Image 6: The three-vessel-trachea view with color Doppler shows the discrepant size of the great arteries with the pulmonary artery smaller than the aorta. Note the presence of antegrade blood flow in both arteries.
In the images below, the following abbreviations are used: RV (right ventricle); LV (left ventricle); Ao (Aorta), VSD (ventricular septal defect), Ao V (Aortic valve); PA (pulmonary artery)
Discussion
Tetralogy of Fallot (TOF) is characterized by a malaligned ventricular septal defect (VSD), an aortic root that overrides the ventricular septal defect, and an infundibular pulmonary stenosis. Right ventricular hypertrophy, the fourth anatomic feature of the tetralogy, is not present prenatally. Tetralogy of Fallot, one of the most common forms of cyanotic congenital heart disease, is found in about 1 in 3,600 live births and accounts for 3 to 7% of infants with congenital heart disease. There are several variations of Tetralogy of Fallot, the most common of which is with pulmonary stenosis and accounts for about 80% of all newborns with the condition. The spectrum of disease also includes severe forms such as TOF with pulmonary atresia and TOF with absent pulmonary valve.
In Tetralogy of Fallot, the four-chamber view typically appears normal unless a large ventricular septal defect is visible in this plane. TOF is usually detected in the left ventricular outflow tract view, which demonstrates a perimembranous, subaortic ventricular septal defect with an overriding aorta. The overriding aorta is due to the discontinuity between the interventricular septum and medial wall of the aorta (malalignment VSD). This results in a partial connection of the aorta to the right ventricle, with an aorta that is slightly shifted to the right, referred to as aortic dextroposition. The aortic root appears dilated, especially in the third trimester, because it receives blood from both the right and left ventricles. The overriding aorta has a parallel course to the interventricular septum in contrast to the ascending aorta in a normal heart.
The demonstration of a narrow but patent main pulmonary artery is also required for the diagnosis of Tetralogy of Fallot. This is best demonstrated at the level of the short-axis or the three-vessel view. The discrepancy between the pulmonary trunk and the aorta can be subtle in midgestation, however it becomes more obvious with advancing gestation.
Color Doppler confirms the presence of an overriding aorta with blood draining from both ventricles, through the ventricular septal defect into the aortic root. Due to high perfusion, inflow into the aorta appears aliased. At the level of the three-vessel-trachea view, color Doppler can also demonstrate a small pulmonary artery. Flow is antegrade across the ductus arteriosus in mild Tetralogy of Fallot and can be reversed in severe cases. Color Doppler can help differentiate various subgroups of TOF as postnatal ductal dependency of the pulmonary circulation can be associated with cyanosis of the newborn.
On prenatal ultrasound, associated cardiac abnormalities include a right aortic arch in 25% of cases and occasionally an atrioventricular septal defect which increases the risk of chromosomal abnormalities. A patent foramen ovale or an atrial septal defect and a persistent left superior vena cava have been reported in 83% and 11%, respectively, of newborns with Tetralogy of Fallot.
Associated extracardiac anomalies are common. Chromosomal abnormalities, mainly trisomies 21, 13 and 18, are found in 30% of cases. Deletion 22q11.2 occurs in 10 to 15% of fetuses and newborns with Tetralogy of Fallot, and is more likely in cases with a hypoplastic thymus, right-sided aortic arch, presence of extracardiac anomalies, or polyhydramnios. TOF can also be associated with syndromic conditions such as Alagille syndrome and CHARGE syndrome.
Differential diagnosis includes pulmonary atresia with ventricular septal defect, absent pulmonary valve syndrome, common arterial trunk, double outlet right ventricle, and malaligned VSD without abnormalities of the great vessels. All of these conditions present with an overriding aorta.
Pulmonary artery growth has been shown to be variable and unpredictable. Therefore, serial prenatal ultrasound examinations to assess fetal pulmonary artery growth and flow across the ductus arteriosus are crucial for counseling and appropriate care of the newborn. Findings that have been associated with poor prognosis include decelerated growth of the pulmonary artery, accelerated growth of the ascending aorta and cessation of forward flow through the pulmonic valve with reversed flow through the ductus arteriosus. Pulmonary atresia with ventricular septal defect and absent pulmonary valve syndrome are known to have a worse prognosis.
References
Abuhamad, A, et al. "Tetralogy of Fallot". A Practical Guide to Fetal Echocardiography: Normal and Abnormal Hearts (4th edition). Singapore: Wolters Kluwer, 2022. pgs 410-425.
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