Case of the Week # 513

Case of the Week # 513

Javier Cortejoso
 on 04/02/2023
 on Sunday, April 2nd, 2023
The five-chamber view of the fetal heart shows a dilated outflow tract overriding a defective ventricular septum. The last video shows antegrade (red) and retrograde (blue) flows, indicating that the valve is insufficient or there is an aliasing phenomenon due to the high perfusion. Something similar is identified in the sagittal sections, in which a single outflow tract is seen with flow in both directions and from which the brachiocephalic trunks emerge. In the axial sections, there is a pulmonary vein flowing to the left atrium and, next to it, two vessels emerging from the descending aorta (pulmonary arteries? major aortopulmonary collateral arteries?).

On fetal echocardiographic examination, a single great artery with ventricular septal defect (VSD) and relatively normal four-chamber view is caused most commonly by truncus arteriosus (TA), pulmonary atresia with VSD (PA-VSD), or rarely, by aortic atresia with VSD. While aortic atresia with VSD may present similar features, it can be differentiated from truncus arteriosus or PA-VSD by visualization of a hypoplastic aortic arch with reversed flow from the ductus arteriosus to supply the head and neck.

Truncus arteriosus (TA), also called common arterial trunk, is a conotruncal cardiac lesion in which a single artery arises from the heart, which gives origin to the systemic, coronary, and pulmonary circulations. The fetal prevalence of truncus arteriosus is unknown, as the birth prevalence (9.4 per 100,000 live-born infants) likely significantly underestimates fetal prevalence given a high rate of miscarriage, elective termination, and in utero fetal demise.

There are at least three commonly used classifications of truncus arteriosus: Anderson, Collett and Edwards, and Van Praagh. Anderson's nomenclature describes each variant sequentially rather than using an alphanumeric classification. Collett and Edwards recognizes four types of truncus arteriosus based on the anatomic origin of the pulmonary arteries. In type I, a short pulmonary trunk originating from the truncus arteriosus gives rise to both pulmonary arteries. When both pulmonary arteries separate from the truncus arteriosus, with no vestige of a main pulmonary artery, they may arise close to one another (type II) or at some distance from one another (type III). The type IV truncus arteriosus (previously termed pseudotruncus) is now considered to represent a different diagnosis: pulmonary atresia with ventricular septal defect, which is a form of tetralogy of Fallot with pulmonary atresia rather than truncus arteriosus. Van Praagh proposed an expanded classification system that also includes two commonly associated abnormalities of the great arteries. Type Al corresponds to type I of Collett and Edwards, and type A2 corresponds to types II and III. Type A3 includes cases in which one pulmonary artery, usually the right, arises from the proximal trunk, and the contralateral lung is supplied by a pulmonary artery arising more distally either from the aortic arch or from a collateral vessel. Type A4 is associated with underdevelopment of the aortic arch, including tubular hypoplasia, discrete coarctation, or complete interruption. Types A1 and A2 are the most common variants, followed by type A4 then A3. Van Praagh also described a very rare “type B” subset of truncus arteriosus lesions without VSD.

The four-chamber view is normal in a patient with truncus arteriosus. There may be only deviation from the cardiac axis to the left. When performing a sweep with the probe to the outflow tract, we will first identify the five-chamber view with only one semilunar valve located over a ventricular septal defect and leading to the truncus arteriosus. The truncal valve is often dysplastic, and it may be regurgitant or stenotic. Next comes the most critical part of the fetal echocardiographic examination of truncus arteriosus: the identification of the origin of the pulmonary arteries. Sweeping more cephalad, one can see that the three-vessel view is abnormal with only two vessels present: the truncus and the superior vena cava. Finishing the sweep, only one arch (typically the aortic arch) will be seen. When the origin of the pulmonary arteries emerging from the truncus arteriosus is not clearly identified, one strategy is to look for the distal branch pulmonary arteries within the lung by color Doppler and trace them back to their origin. Coronary artery abnormalities are frequent, with a single coronary artery being the most common variant. Other cardiac associations include aberrant origin of the arch vessels, right aortic arch, and a left superior vena cava.

Pulmonary atresia with ventricular septal defect (PA-VSD) is characterized by atresia of the pulmonary valve, hypoplasia of the pulmonary tract, membranous or infundibular ventricular septal defect, and an overriding aorta. Its antenatal diagnosis is primarily dependent on echocardiography. The 4-chamber view can be misleading due to the relatively symmetric atriums and ventricles. The primary features include a VSD and overriding aorta. The pulmonary artery may be absent, or extremely dysplastic without any connection to the right ventricular outflow tract. Pulmonary blood flow is via retrograde flow in the ductus arteriosus or collateral vessels arising from the descending aorta, called “major aortopulmonary collateral arteries” (MAPCAs). MAPCAs are defined as vessels connecting the descending aorta with pulmonary terminations either at the distal branch of the pulmonary arteries or within the lung parenchyma. Less frequently, individual collateral arteries can also originate from the brachiocephalic arteries or even from the coronary arteries. Commonly they are between two and six in number. PA-VSD is classified into three types: in type A, the pulmonary artery is present without MAPCAs, and the lungs are supplied by the arterial duct; in type B, pulmonary artery dysplasia is accompanied by MAPCAs, and the lungs are supplied by the pulmonary artery and MAPCAs; and in type C, the pulmonary artery is absent, and the lungs are completely supplied by MAPCAs.

Truncus arteriosus can be easily confused with several other congenital heart lesions, including PA-VSD, aortic atresia with VSD, tetralogy of Fallot, double-outlet right ventricle, and aorto-pulmonary window. The last three can be correctly identified by the presence of a second semilunar valve. Differentiating between truncus arteriosus and PA-VSD should be based on the characteristics of the arterial trunk valve, identification of the atretic pulmonary valve, the origin of the common pulmonary trunk and its branches, and the identification of a patent ductus arteriosus. Ductus arteriosus is absent in Van Praagh types A1 and A2, but is always present in type A4, and can be present in type A3 because a pulmonary artery can originate from the ductus arteriosus. Ductus arteriosus is also missing in a significant proportion of PA-VSD cases with MAPCAs. The atretic pulmonary valve is difficult to detect in PA-VSD but if demonstrated in those infrequent cases in which the pulmonary trunk is not extremely hypoplastic, truncus arteriosus can be excluded. According to Traisrisilp et al, the antenatal sonographic features that help to differentiate these two anomalies are:
• abnormal morphology of the semilunar valve; thickening or increased echogenicity are found in most cases with truncus arteriosus but not in PA-VSD
• semilunar valve incompetence or regurgitation is found in most cases of truncus arteriosus but in no case of PA-VSD
• in most cases of truncus arteriosus, the root of the single great artery originates predominantly from the right ventricle, while in PA-VSD the great artery typically equally overrides the VSD
• vascular branching to supply the lungs is proximal in truncus arteriosus and more distal in most cases with PA-VSD
• demonstration of the MAPCA originating from the descending aorta to supply the lungs is specific to PA-VSD
• in some cases, it is possible to identify a confluent main pulmonary artery with no antegrade flow from the right ventricle, which is a specific finding of PA-VSD.
The identification of a patent ductus arteriosus is a key criterion for establishing a correct differential diagnosis and to define perinatal management. It is generally accepted that a fetus with truncus arteriosus can be delivered locally, whereas PA-VSD is a ductus-dependent cardiac defect and the affected fetus should ideally be born in a tertiary care center where treatment with prostaglandins can be started immediately after birth.

Suggested readings
• Abuhamad A, Chaoui R. Tetralogy of Fallot, pulmonary atresia with ventricular septal defect, and absent pulmonary valve syndrome. In: Abuhamad A and Chaoui R, ed. A practical guide to fetal echocardiography: normal and abnormal hearts, 3rd ed. Wolters Kluver, Philadelphia, PA, 2016; pg 396-421
• Abuhamad A, Chaoui R. Common arterial trunk. In: Abuhamad A and Chaoui R, ed. A practical guide to fetal echocardiography: normal and abnormal hearts, 3rd ed. Wolters Kluver, Philadelphia, PA, 2016; pg 422-434
• Anderson RH, Thiene G. Categorization and description of hearts with a common arterial trunk. Eur J Cardiothorac Surg. 1989;3(6):481–487. doi:10.1016/1010-7940(89)90105-x
• Collett RW, Edwards JE. Persistent truncus arteriosus; a classification according to anatomic types. Surg Clin North Am. 1949;29(4):1245–1270. doi:10.1016/s0039-6109(16)32803-1
• Gómez O, Soveral I, Bennasar M, et al. Accuracy of Fetal Echocardiography in the Differential Diagnosis between Truncus Arteriosus and Pulmonary Atresia with Ventricular Septal Defect. Fetal Diagn Ther. 2016;39(2):90–99. doi:10.1159/000433430
• Miyashita S, Chiba Y. Prenatal demonstration of major aortopulmonary collateral arteries with tetralogy of Fallot and pulmonary atresia. Fetal Diagn Ther. 2004;19(1):100–105. doi:10.1159/000074270
• Morris SA and Lara DA. Truncus arteriosus. In: Yagel S, Silverman NH and Gembruch U, ed. Fetal Cardiology. Embryology, Genetics, Physiology, Echocardiographic Evaluation, Diagnosis, and Perinatal Management of Cardiac Diseases, 3rd ed. CRC Press, Boca Raton, FL, USA, 2019; pg. 359-371
• Traisrisilp K, Tongprasert F, Srisupundit K, et al. Prenatal differentiation between truncus arteriosus (Types II and III) and pulmonary atresia with ventricular septal defect. Ultrasound Obstet Gynecol. 2015;46(5):564–570. doi:10.1002/uog.14788
• Van Praagh R, Van Praagh S. The anatomy of common aorticopulmonary trunk (truncus arteriosus communis) and its embryologic implications. A study of 57 necropsy cases. Am J Cardiol. 1965;16(3):406–425. doi:10.1016/0002-9149(65)90732-0
• Yang SH, Luo PH, Tian XX, et al. Prenatal diagnosis of pulmonary atresia with ventricular septal defect. J Med Ultrason (2001). 2018;45(2):341–344. doi:10.1007/s10396-017-0809-2

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