Truncus Arteriosus

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CHAPTER 45 Truncus Arteriosus

Truncus arteriosus is an uncommon but potentially lethal congenital heart disease that manifests during the neonatal period or early infancy. It is defined by a common origin of the aorta and the pulmonary arteries, resulting from an incomplete embryologic septation and separation of the aorta and the pulmonary trunk. This congenital cardiac anomaly was first described in 1798 by Wilson.¹ In 1942, Lev and Saphir² proposed the anatomic criteria that defined truncus arteriosus. Since then, different classifications had been proposed by Collett and Edwards³ in 1949, by Van Praagh and Van Praagh⁴ in 1965, and by the Society of Thoracic Surgeons in 2000.⁵

DEFINITION

In common usage, the term truncus arteriosus implies persistent truncus arteriosus at birth. Synonyms are common arterial trunk, truncus arteriosus communis, and common aorticopulmonary trunk. Truncus arteriosus is defined by the anatomy: the aorta and the pulmonary arteries arise from a common trunk above a single truncal valve.

CLASSIFICATIONS

Historically, two major classification schemes for anatomic variations of the truncus arteriosus were proposed by Collett and Edwards ³ and by Van Praagh and Van Praagh.⁴ The Collett and Edwards classification is strictly defined by the anatomic origins of the pulmonary arteries. It divides truncus arteriosus into four types, I to IV (Fig. 45-1). In Collett and Edwards type I, a common arterial trunk divides into an aorta and a short pulmonary trunk, which divides into the left and right branch pulmonary arteries. This is the most common configuration (48% to 68% of cases). In Collett and Edwards type II (29% to 48% of cases), the left and right branch pulmonary arteries arise close together from the common arterial trunk without a distinct pulmonary trunk. Collett and Edwards type III (6% to 10% of cases) is similar to type II except that the origins of the pulmonary arteries are far apart. Collett and Edwards described a type IV configuration in which the pulmonary arteries arise from the descending aorta. These arteries are now known to be major aorticopulmonary collateral arteries associated with pulmonary atresia with ventricular septal defect (VSD) and not truncus arteriosus.

FIGURE 45-1 Collett and Edwards classification: type I, a pulmonary trunk connects the common arterial trunk to the branch pulmonary arteries; type II, branch pulmonary arteries arise closely from the common arterial trunk; type III, branch pulmonary arteries arise far apart from the common arterial trunk; type IV, branch pulmonary arteries arise from the descending aorta. LPA, left pulmonary artery; RPA, right pulmonary artery.

The Collett and Edwards classification developed in 1949 did not consider the roles of patent ductus arteriosus (PDA) and interrupted aortic arch, which are associated with truncus arteriosus and have prognostic and surgical implications. Subsequently, Van Praagh and Van Praagh ⁴ proposed a different classification with four types, 1 to 4 (Fig. 45-2). Each number may have a prefix letter A if a VSD is present (common) or B if a VSD is absent (rare). Van Praagh and Van Praagh type 1 is identical to Collett and Edwards type I, describing a short pulmonary trunk arising from the common arterial trunk. Van Praagh and Van Praagh type 2 describes separate origins of the left and right branch pulmonary arteries arising from the common arterial trunk, regardless of the distance separating their origins (Fig. 45-3). It corresponds to Collett and Edwards type II and type III. Van Praagh and Van Praagh type 3 describes one branch pulmonary artery arising from the common arterial trunk and the other connected to a ductus arteriosus or an aorticopulmonary collateral artery (Fig. 45-4). Van Praagh and Van Praagh type 4 describes the coexistence of a common arterial trunk and an interrupted or a severely hypoplastic aortic arch. The descending aorta is supplied by a PDA. Van Praagh and Van Praagh type 3 and type 4 have no correspondence in the Collett and Edwards classification.

FIGURE 45-2 Van Praagh and Van Praagh classification: type 1, a pulmonary trunk connects the common arterial trunk to the branch pulmonary arteries; type 2, branch pulmonary arteries arise anywhere from the common arterial trunk; type 3, one branch pulmonary artery connects to a PDA or major aorticopulmonary collateral artery (MAPCA); type 4, interrupted aortic arch. LPA, left pulmonary artery; RPA, right pulmonary artery.

FIGURE 45-3 ECG gated CT scan of a Van Praagh and Van Praagh type 2 truncus arteriosus. Volume rendered image shows the left pulmonary artery (LPA) and the right pulmonary artery (RPA) arising separately from the common arterial trunk (TA). The LPA is typically positioned above the RPA. The topmost branch is the left carotid artery (LCA).

FIGURE 45-4 ECG gated CT scan of a Van Praagh and Van Praagh type 3 truncus arteriosus. Volume rendered image shows the right pulmonary artery (RPA) arising from the common arterial trunk (TA). The left pulmonary artery is connected by a major aorticopulmonary collateral artery (MAPCA) to the descending aorta (DA).

Although the Van Praagh and Van Praagh classification provides a more refined description of truncus arteriosus, it does not completely serve the needs of surgeons. In 2000, the Society of Thoracic Surgeons proposed a uniform reporting system with modifiers that better describe anatomic features useful for surgical outcome studies.⁵ In this system, cryptic numerical labels are replaced by descriptive phrases. Three major types are described: truncus arteriosus with confluent or near-confluent pulmonary arteries (corresponding to Van Praagh and Van Praagh type 1 and type 2), truncus arteriosus with absence of one pulmonary artery (corresponding to Van Praagh and Van Praagh type 3), and truncus arteriosus with interrupted aortic arch or coarctation (corresponding to Van Praagh and Van Praagh type 4). Additional modifiers describe the presence or absence of VSD, the number of truncal leaflets, the presence of truncal insufficiency or stenosis or both, coexisting coronary anomalies, any ventricular hypoplasia, the truncal valve position relative to the ventricles, and the presence or absence of thymus.

EPIDEMIOLOGY AND GENETICS

Truncus arteriosus occurs in 1% to 2% of infants born with congenital heart defects, or about 10 cases per 100,000 live births.⁶ Among conotruncal anomalies, truncus arteriosus has a similar rate of occurrence as congenitally corrected transposition of the great arteries and double outlet right ventricle, but it is three times less common than complete transposition of the great arteries and four times less common than tetralogy of Fallot. There is no predilection by sex. Of patients with truncus arteriosus, 34% to 41% harbor a chromosome 22q11 deletion.⁷ This deletion is also seen in a large proportion of patients with conotruncal developmental anomalies as found in DiGeorge syndrome, velocardiofacial syndrome, and interrupted aortic arch, suggesting the importance of this gene in the regulation of the embryologic development of the conotruncus. Today, this chromosomal deletion is readily detected with the fluorescence in situ hybridization technique.

Genetic screening is important because 22q11 deletion is inherited in an autosomal dominant fashion from one parent in 6% to 28% of cases.⁸ This is important information for genetic family counseling. Knowledge of this deletion also heightens clinical suspicions for associated anomalies, including athymia, hypocalcemia, and nasopalatal malformation. Truncus arteriosus is also associated with trisomy 8 and chromosomal 10p deletion.⁹

ETIOLOGY AND PATHOPHYSIOLOGY

Embryology

The embryologic truncus arteriosus is a normal vascular structure that connects the conus cordis to the branchial arches from days 25 to 50 of gestational age. The conus cordis is destined to become the ventricular outflow tracts; the truncus arteriosus separates into the ascending aorta and the pulmonary trunk; and the branchial arches evolve into the branch pulmonary arteries and the aortic arch. For this reason, some authors prefer the term persistent truncus arteriosus.

In normal embryologic development, beginning in the fifth week of gestation, fusion of the endocardial cushions separates the atrioventricular canal into right and left openings, which are destined to become the tricuspid annulus and the mitral annulus. Through these two openings, blood flow develops into two intertwined, spiraling streams that flow out the conus cordis and the truncus arteriosus, or conotruncus for short. Between the two spiraling streams is a layer of stagnant blood. There is evidence that this layer of stagnant blood promotes the development of two ridges of tissue at opposing margins inside the conotruncus.¹⁰ These two ridges grow toward each other until they meet and fuse. The fusion begins at the conotruncal junction. Similar to the closing of a zipper, the fusion proceeds upward through the truncus arteriosus, creating two spiraling lumens that become the ascending aorta and the pulmonary trunk. From this junction, the fusion also travels downward through the conus cordis until it meets the ventricular septum, separating the right and left ventricles. Separation of the systemic and the pulmonary circulations is completed by 9 weeks of gestation. Beginning in the seventh week, specialized tissue swellings from the conotruncal junction evolve into the aortic and the pulmonary semilunar valves; these are also completed by 9 weeks.

Persistent truncus arteriosus is the failure of development or fusion of the conotruncal ridges. This developmental anomaly has three major consequences. First, the incomplete septation of the conus cordis leaves a large VSD at the outflow region. Second, the failure to form separate aortic and pulmonary valves leave a single truncal valve. Third, the pulmonary and aortic blood flows are in free communication. All three defects lead to important hemodynamic derangements and their clinical manifestations.

Associated Cardiac Anomalies

Because the conus cordis is not partitioned properly below the truncal valve, a VSD usually exists at the infundibular septum beneath the truncal valve (Fig. 45-5). This VSD is often large and nonrestrictive. The right ventricle, subjected to the systemic pressure generated by the left ventricle, becomes hypertrophic. In most instances (68% to 83%), the common arterial trunk and the truncal valve straddle the ventricular septum in a manner resembling the overriding aorta in tetralogy of Fallot or pulmonary atresia with VSD. Uncommonly (11% to 29%), the truncal valve aligns exclusively with the right ventricle. Rarely (4% to 6%), it aligns with the left ventricle. In the two latter situations, the VSD may be small or absent.