Common arterial trunk is a rare congenital malformation of the heart. As its name implies, its essential anatomical feature is the presence of an arterial trunk, which arises from the ventricular mass through a common ventriculo-arterial junction, giving rise directly to the systemic, pulmonary and coronary circulations. In most centres, surgical treatment, the only appropriate current therapeutic approach, is now performed routinely following only non-invasive pre-operative investigations in early infancy. Operative mortality is now low, although it may be increased in patients in whom there are associated abnormalities, for example, interruption of the aortic arch, or stenosis or regurgitation of the truncal valve. In most cases, the technique used for surgical correction includes placement of a conduit as part of the reconstructed pulmonary outflow tract. Because of this, after this form of surgery patients must be followed indefinitely in a centre which specialises in the care of those having congenital cardiac disease, as most will require procedures to address stenosis of the conduit in later life.
INCIDENCE
Common arterial trunk is a rare anomaly, occurring in 0.03 per 1000 live births according to data assembled from the New England Regional Infant Cardiac Program. 1 Among 906,646 live-born infants collected in the Baltimore-Washington Study, 4390 had congenitally malformed hearts, with 51 having common arterial trunk, so that this malformation occurred in 0.056 of every 1000 live births, and represented just over 1% of all instances of congenital cardiac disease. 2 A recent study showed that surgical treatment of common arterial trunk accounted for almost 1% of open heart procedures performed at the Royal Liverpool Children’s Hospital between 1993 and 2005. The incidence of congenital malformations of the heart is known to be increased in the offspring of patients with common arterial trunk, being 6.6% in the offspring of those with uncomplicated lesions, and 13.6% of those with complex forms of the condition.4
ANATOMY
Of all the congenital lesions that benefit from being described in straightforward fashion, common arterial trunk is the most obvious. When described in terms of truncus, or various expansions of this term that implicate persistence of an embryonic condition, it is always necessary to provide a definition of the lesion as seen in the postnatal heart. Describing the entity as a common arterial trunk negates the need for further definition. A common trunk is one that arises from the ventricular mass through a common ventriculo-arterial junction, and gives rise directly to the systemic, pulmonary and coronary circulations ( Fig. 41-1 ).
Almost without exception, the common ventriculo-arterial junction is guarded by a common arterial valve, 4 but by analogy to atrioventricular septal defect with common junction (see Chapter 27 ), it might be anticipated that the arterial valvar leaflets could be divided into separate orifices for the right and left ventricles, whilst still guarding a common junction The latter situation is seen in the setting of the doubly committed and juxta-arterial ventricular septal defect (see Chapter 28 ), but such patients do not possess common arterial trunks. The pattern of branching of the common trunk itself can be complicated by various arrangements of the individual arteries within the systemic, pulmonary, and coronary arterial circulations. The presence of the common trunk, nonetheless, distinguishes the entity not only from those with doubly committed ventricular septal defects with separate aortic and pulmonary valves, but also from the close cousins in which there is a window between the intrapericardial components of the aorta and pulmonary trunk, and also those patients in which a large patent trunk leaves the base of the heart in company with an atretic trunk, which can also be traced from its origin at the ventricular mass ( Fig. 41-2 ).
One variant that can still give problems in terms of description, however, is when there is complete absence of the intrapericardial pulmonary arteries. In their time-honoured classification, Collett and Edwards 5 included this variant as the type IV within their overall grouping. The trunk arising from the ventricular mass in this setting ( Fig. 41-3 ), however, is best described as a solitary, rather than a common, trunk. This is because there is no way of knowing, had they been present, whether the intrapericardial pulmonary arteries would have originated from the arterial trunk or from the right ventricular outflow tract ( Fig. 41-4 ). In terms of clinical presentation and treatment, such patients with a solitary arterial trunk have more affinities with tetralogy and pulmonary atresia (see Chapter 37 ) than with common arterial trunk.
A common arterial trunk, as we have defined it, is one form of single outlet from the heart. As a malformation involving the ventriculo-arterial junctions, it must be anticipated to co-exist with all possible segmental combinations. In almost all instances, nonetheless, there will be usual atrial arrangement, with concordant atrioventricular connections. Examples can be found in combination with discordant atrioventricular connections, or with absence of the right atrioventricular connection. 6 While the atrioventricular junctions themselves are usually separate, and guarded by mitral and tricuspid valves, a common trunk can rarely be found in association with an atrioventricular septal defect and a common atrioventricular valve. 7
In the presence of the common trunk, almost always the truncal valve is connected across the ventriculo-arterial junctions with both ventricles, the valvar orifice overriding the ventricular septal crest, and typically with its leaflets in fibrous continuity with the mitral valve in the left ventricle ( Fig. 41-5 ). Such a biventricular connection necessitates the presence of a juxta-arterial interventricular communication. The defect is generally large. Its floor is the crest of the ventricular septum, reinforced on the right ventricular aspect by the limbs of the septomarginal trabeculation, or septal band, and its roof is the leaflets of the truncal valve. The cone of space subtended by the truncal valve has right and left ventricular margins, but it is usually the right ventricular margin that is considered to represent the ventricular septal defect, and it is this space which is closed by the surgeon during repair.
In the majority of cases, fusion of the inferior limb of the septomarginal trabeculation with the ventriculo-infundibular fold along this right ventricular margin produces muscular discontinuity between the leaflets of the tricuspid and the truncal valves ( Fig. 41-6 ).
In the absence of such fusion, there is continuity between the leaflets of the tricuspid and truncal valves, making the ventricular septal defect perimembranous ( Fig. 41-7 ). When present, this muscular bar in the postero-inferior margin protects the specialised axis responsible for atrioventricular conduction. In most instances, there is a large distance between the coapting arterial valvar leaflets and the crest of the septum during ventricular diastole when the leaflets are closed. This space, however, may sometimes be reduced, or the leaflets may close directly on the septal crest ( Fig. 41-8 ).
Some have described this latter arrangement as representing an intact ventricular septum. 8 This is somewhat misleading because, even in this arrangement, a septal deficiency is seen at ventricular level when the truncal valve opens during ventricular systole. Furthermore, hearts are found when the ventricular septum is truly intact, the common trunk then arising in most instances exclusively from the right ventricle ( Fig. 41-9 ). The ventricular septal defect can also be restrictive when the common trunk takes an exclusive origin from one or other ventricle. Such a restrictive ventricular septal defect is more likely to produce problems when the trunk arises exclusively from the right ventricle ( Fig. 41-10 ).
The truncal valve has three leaflets in approximately two-thirds of patients. In most of the remaining patients, either two or four leaflets are seen guarding the common arterial orifice. The leaflets are almost always in fibrous continuity with the antero-superior leaflet of the mitral valve (see Fig. 41-5 ), but there can be a completely muscular subtruncal infundibulum, particularly when the common trunk arises exclusively from the right ventricle (see Fig. 41-10 ). Insufficiency of the truncal valve is not uncommon, and can be caused by thickened and dysplastic leaflets, or by prolapse of unsupported leaflets as a result of dilation of the ventriculo-arterial junction. Truncal valvar stenosis is relatively uncommon. When present, it is usually because the valvar leaflets are dysplastic.
The greatest anatomic variability is found in the pattern of the branching of the common trunk. The presence of a right-sided aortic arch, with mirror-imaged branching of the brachiocephalic arteries, is associated more often with common trunk, occurring in up to one-third of patients, than with any other congenital cardiac malformation. 4,9 Hypoplasia of the aortic arch, with or without coarctation, is a particularly important associated finding, but is less frequent than complete interruption of the arch ( Fig. 41-11 ).
Such interruption is one of the major subgroups recognised in the alpha-numeric system that was suggested for classification.10 When the arch is interrupted, the persistently patent arterial duct feeds the descending thoracic aorta and part of the brachiocephalic circulation, the precise proportion depending on the site of interruption. As with other forms of interruption of the aortic arch (see Chapter 46 ), retro-oesophageal origin of the right subclavian artery is frequently seen. Apart from those hearts with severe coarctation or interruption, or in which the pulmonary arteries are discontinuous, and one is fed through a patent duct, it is rare to find ductal patency co-existing with common arterial trunk, although it does exist (see Fig. 41-9 ). 11 While the state of the aortic arch is possibly the most significant clinical associated malformation, it has been the arrangement of origin of the pulmonary arteries, following the system proposed by Collett and Edwards 5 that was traditionally used most frequently for numeric classification. The pulmonary arteries typically arise from the left posterolateral aspect of the common trunk, taking origin a short distance above the truncal valve, albeit that very rarely they can take their origin directly from a truncal arterial valvar sinus ( Fig. 41-12 ).
In the more usual situation, it is the presence of a short confluent arterial segment that produces the type I identified by Collett and Edwards ( Fig. 41-13 ). Alternatively, the right and left arteries can take separate origin from the posterior aspect of the trunk, producing the type II variant ( Fig. 41-14 ), or very rarely from the right and left sides of the posterior aspect of the intrapericardial segment of the common trunk, this being the type III variant. In most instances, however, the morphology is intermediate, and many have suggested that this pattern should be recognised as type 1½. It is also possible to find examples in which only one pulmonary artery arises from the common trunk, the other being supplied initially through a duct that became ligamentous. In the clinical setting, this can present as unilateral absence of one pulmonary artery, but almost always the artery presumed to be absent is identified within the hilum of the lung. When this arrangement is found with common trunk, the discontinuous pulmonary artery initially fed by the duct is most frequently on the same side as the aortic arch. This is in contrast to the finding in patients with tetralogy of Fallot when one pulmonary artery is seemingly absent, since in this setting, the discontinuous artery is more frequently on the side opposite the aortic arch.
In some circumstances, the pulmonary artery feeding the right lung is to the left at its origin from the common trunk relative to the artery running to the left lung ( Fig. 41-15 ). The two arteries then spiral as they extend to the pulmonary hilums. This entity is called crossed pulmonary arteries . As we have already discussed, in the original categorisation of Collett and Edwards 5 there was also a type IV variant, which exists when the intrapericardial pulmonary arteries are completely absent. As we have described (see Fig. 41-2 ), this arrangement is better described as a solitary arterial trunk, since in the absence of the intrapericardial pulmonary arteries, it cannot be determined whether, had they been present, they would have arisen from the heart, giving solitary aortic trunk with pulmonary atresia, or from the arterial trunk itself, producing common trunk with pulmonary atresia. Irrespective of such speculative considerations, in clinical terms patients with absence of the intrapericardial pulmonary arteries present as a subset of those with tetralogy of Fallot with pulmonary atresia, rather than common arterial trunk. It is also possible for one pulmonary artery to arise directly from the ascending aorta, while the other takes its origin from the right ventricle. Some call this malformation hemitruncus. This is incorrect since, of necessity, the hearts have separate ventriculo-arterial junctions guarded by separate aortic and pulmonary arterial valves. They cannot, therefore, be examples of common arterial trunk.
Anomalies of the origin and distribution of the coronary arteries are frequent. 10 Unlike the situation when there are separate aortic and pulmonary valves, in which virtually without exception the coronary arteries arise from one or other of the aortic valvar sinuses adjacent to the pulmonary trunk, and usually both sinuses, there is no constant pattern of sinusal origin in the presence of a common ventricular outflow tract. Instead, the coronary arteries can arise from any of the truncal valvar sinuses, albeit that in most instances there are still two coronary arteries, with the left artery giving rise to anterior interventricular and circumflex branches. The arteries often arise close to a zone of apposition between the valvar leaflets, and origin above the sinutubular junction is quite common ( Fig. 41-16 ). This can produce potential difficulties during surgical correction should the high origin be adjacent to the origin of the pulmonary arteries.
Knowledge of location of the atrioventricular conduction axis is also important when planning surgical repair. The sinus node and the atrioventricular node are normal in their location and structure. Having taken origin from the atrioventricular node, the penetrating atrioventricular bundle pierces through the central fibrous body, and the left bundle branch originates along the left ventricular septal endocardium (see Chapter 28 ). The right bundle branch travels within the myocardium of the ventricular septal crest, attaining a subendocardial course at the level of the moderator band. In those hearts in which a muscular bar interposes between the attachments of the truncal and tricuspid valves in the postero-inferior margin of the septal defect, the membranous septum is intact behind the muscular tissue, and the atrioventricular conduction tissues are somewhat distant from the rim of the defect (see Fig. 41-5 ). In patients in whom the ventricular septal defect is perimembranous, in contrast, the conduction tissue passes directly along the left aspect of the fibrous postero-inferior rim of the defect (see Fig. 41-6 ). It is then at greater surgical risk. We have already mentioned most of the common associated cardiovascular anomalies found in the setting of common arterial trunk, including a right aortic arch, interrupted aortic arch, patency of the arterial duct, discontinuity of one pulmonary artery, coronary arterial anomalies, and incompetent truncal valve. A defect within the oval fossa has been noted in up to one-fifth of patients, persistence of the left superior caval vein draining to the coronary sinus in up to one-tenth, and an aberrant subclavian artery in between one-tenth and one-twentieth. 12 Partially anomalous pulmonary venous connection has also been reported. 13
AETIOLOGY AND MORPHOGENESIS
Over the past decade, the evidence has accumulated that many cases of common arterial trunk result from a genetic defect. The evidence comes from interpretation of morphology, experiments in animals, studies on the role of cells migrating from the neural crest in the development of the outlet components of the heart and the arterial trunks, 14 and the discovery of deletions in chromosome 22q11 in patients with malformations involving the outflow tracts, often described as so-called conotruncal defects. 15,16
The morphology of common arterial trunk supports very strongly the notion that, during development, there has been failure of septation of the ventricular outlets and the outflow segment of the heart tube. It had been suggested that the entity represents failure of formation of the subpulmonary infundibulum, with the common arterial trunk in essence representing the aorta. 12 No evidence has accrued over the last decades to support this latter notion, but much has appeared to contradict it. From the morphological stance, specimens with common arterial trunk show no evidence of a blind-ending subpulmonary outflow tract, such as is seen in tetralogy with pulmonary atresia. It is this latter entity that provides the paradigm for underdevelopment of the subpulmonary outflow tract. In such hearts with tetralogy and pulmonary atresia, four-fifths of specimens have perimembranous ventricular septal defects. In contrast, in the setting of common arterial trunk, four-fifths of hearts have a muscular postero-inferior rim to the ventricular septal defect. Additionally, if the arterial root truly represented the aorta, then coronary arteries would be anticipated to arise in patterns comparable to those seen in the normal heart. This is rarely the case in the setting of common arterial trunk, where the origins and course of the coronary arteries are frequently bizarre. 11
Direct studies of cardiac development, on both normal and abnormal hearts, have always shown that the initially common ventricular outflow tract is separated by endocardial cushions, or ridges, to produce the intrapericardial arterial trunks, along with the arterial valvar leaflets and sinuses and their supporting ventricular outflow tracts. 17 Failure of such separation during embryological development was demonstrated conclusively as producing common arterial trunk in an elegant study using Keeshond dogs published as long ago as 1978. This study 18 showed that the cushions which normally divided the outflow segment of the heart failed to fuse in the setting of common arterial trunk, the concept subsequently being endorsed by further studies carried out by Bartelings and colleagues. 19 Much work over the past 15 years has provided further evidence of the importance of the outflow cushions in dividing the initially common ventricular outflow tract, and has demonstrated an important role for cells migrating from the neural crest in populating these cushions ( Fig. 41-17 ).
Other studies have shown that, when the migration of the cells from the neural crest is perturbed, the cushions do not develop properly, and one of the lesions produced is common arterial trunk.20 This is in keeping with the results of selective inbreeding of the Keeshond dogs, which were used to study normal development.18 Using the same colony of inbred animals, a similar spectrum of malformations to that obtained subsequent to perturbation of the neural crest was observed. 21 Many animals had common arterial trunk, but others had ventricular septal defects, or tetralogy of Fallot. Significantly, the pattern of inheritance was consistent with a defect at a single autosomal locus. The genetic basis for such malformations is further supported by observations made in the homozygous mutant Splotch mouse, in which a common arterial trunk arises exclusively from the right ventricle in many of the malformed embryos.20 Common arterial trunk has been produced when there is deficiency of sox4 , a gene which also normally populates the endocardial cushions of the developing outflow tracts. 22,23 Some of the afflicted embryos in these experiments, however, had doubly committed ventricular septal defects rather than common arterial trunk. This is of particular interest, since as we have already discussed, the morphology of the outflow tracts is almost identical in the setting of a common arterial trunk (see Fig. 41-17 ) and in doubly committed defects, apart from the finding of separate aortic and pulmonary valvar orifices in the latter malformations. All of this is pertinent to findings in humans with microdeletions of chromosome 22q11.
The initial report of a family with a chromosomal translocation resulting in partial trisomy for chromosome 20 and partial monosomy for chromosome 22, with all patients having DiGeorge syndrome, and one having common arterial trunk, led to searching for microdeletions in the 22q11 region, first by high resolution banding, 24,25 and then by fluorescent in situ hybridisation. 26 These investigations revealed that a majority of patients with DiGeorge syndrome have 22q11 deletions. As yet, however, no single gene has been found to be responsible for all cases, although the Tuple gene has been implicated in some cases. It has been shown, nonetheless, that the DiGeorge syndrome is best considered as the severe end of a clinical spectrum. For a while, it was referred to as CATCH 22, with reference to the novel by Joseph Heller, and reflecting cardiac defects, abnormal facies, thymic hypoplasia, cleft palate, and hypocalcaemia produced by deletions of 22q11. Experience showed that such usage was interpreted in pejorative fashion, and the term is now frowned upon. Be that as it may, nearly one-third of patients with non-syndromic defects involving the ventricular outflow tracts, and one-third of those with common arterial trunk, have been shown to have microdeletions in the DiGeorge critical region. 27 An association between common arterial trunk, and other anomalies of the outflow tracts, and CHARGE syndrome (coloboma, heart disease, choanal atresia, retardation [mental and physical] genital hypoplasia, and ear anomalies) has also long been recognised. The anomalies of the outflow tracts found with CHARGE syndrome are much the same as in DiGeorge syndrome, except that tetralogy of Fallot and double outlet right ventricle predominate in the former, whereas, in the latter, common arterial trunk and interruption of the aortic arch proximal to the left subclavian artery are more common. It seems likely therefore, that chromosomal damage leads to deletions of 22q11, which, in some way, interferes with the migration of cells from the neural crest, and thereby causes damage to the third and fourth pharyngeal pouches. We have come a long way in understanding the morphogenesis of these lesions since the late lamented Robert Freedom and his colleagues 28 drew the attention of paediatric cardiologists to the association between anomalies of these pharyngeal pouches and congenital cardiac malformations.
It should not be thought, however, that all is now resolved. One-fifth of patients with terminal deletions of the long arm of chromosome 7 have been reported to have cardiac anomalies, including common arterial trunk. 29 In patients identified in the Baltimore-Washington Infant Study, 2 those with malformations of the outflow tracts showed no recurrences from 109 parents or siblings, in keeping with the findings of Nora and Nora, 30 who suggested a lower than usual recurrence rate of 1% for common arterial trunk, although subsequent studies were less convincing in this respect. 31 Other studies have suggested an autosomal recessive pattern of inheritance.32 There are also other reliably documented reports of common arterial trunk in siblings, 31,33,34 while two sets of three siblings have been documented with the lesion. 35,36 Dizygotic twins concordant for common arterial trunk have also been described. 37 Patients with common arterial trunk, therefore, continue to be a fertile population in which to study the genetic background of congenital cardiac malformations.
DIAGNOSTIC FEATURES
Antenatal Diagnosis
It might be expect that common arterial trunk would be detected during the mid-trimester scan for fetal anomalies, which is routine in many countries. In practice, population-based studies demonstrate that the diagnosis may often be missed, 38,39 presumably because the four-chamber view of the heart may appear to be normal on superficial examination. More detailed echocardiographic examination of the fetal heart, nonetheless, should reveal a ventricular septal defect. An arterial trunk that overrides the defect is the only outlet which arises from the ventricular mass. 40 Under ideal circumstances it will be possible to document the degree of regurgitation or stenosis of the truncal valve, and the course and integrity of the aortic arch can be delineated 40 ( Table 41-1 ).
