The essence of congenitally corrected transposition is the presence of discordant connections at both atrioventricular and ventriculo-arterial junctions. This segmental combination, like transposition itself (see Chapter 38 ), can be found in patients with either usual or mirror-imaged atrial arrangement ( Fig. 39-1 ), but not in the presence of isomeric atrial appendages. Strictly speaking, congenitally correct transposition exists only when the ventriculo-arterial connections are also discordant. Discordant connections across the atrioventricular junctions, nonetheless, can also be found with double outlet from either ventricle, usually the morphologically right, with pulmonary or aortic atresia, and rarely with concordant ventriculo-arterial connections. These entities, therefore, are close cousins of the main malformation, and we will discuss their diagnosis and treatment in this chapter. The combination of discordant atrioventricular with concordant ventriculo-arterial connections, of course, produces the clinical picture of transposition, since the circulations are corrected only when there is double discordance. Even with double discordance, however, the purportedly corrected pattern of the circulation is usually perturbed by the associated anomalies, of which an interventricular communication, obstruction of the outflow tract from the morphologically left ventricle, and abnormalities of the morphologically tricuspid valve are sufficiently frequent to be considered part and parcel of the malformation. 1,2 The other complication occurring with sufficient frequency to be considered as almost part of the malformation is problems with the atrioventricular conduction axis. 3 We will discuss all of these anatomical combinations, emphasising their clinical significance.
ANATOMY AND MORPHOGENESIS
Basic Morphology
As far as we are aware, the lesion was first described by the Baron Von Rokitansky. 4 It was with great prescience that he provided an exquisite illustration of the morphology as seen by the echocardiographer cutting the heart in the short axis of the ventricular mass ( Fig. 39-2 ). When the atrial chambers are in their usual position, which is typically the situation, with only about one-tenth of all cases exhibiting mirror-imaged atrial arrangement, then by virtue of the discordant atrioventricular connections, the right-sided morphologically right atrium is connected to the right-sided morphologically left ventricle through a mitral valve, while the left atrium is joined to the left-sided morphologically right ventricle through a tricuspid valve. When the ventricular septum is intact, there is reversed off-setting of the septal attachments of the leaflets of the atrioventricular valves ( Fig. 39-3 ). The morphologically left ventricle then gives rise to the pulmonary trunk, and almost always there is fibrous continuity between the leaflets of the pulmonary and mitral valves ( Fig. 39-4 ). The wedging of the pulmonary valve between the septum and the mitral valve deviates the atrial septum away from the ventricular septum, this having crucial significance for the disposition of the atrioventricular conduction axis. 4 The morphologically right ventricle, receiving the pulmonary venous return, empties into the aorta, which almost always in those with usual atrial arrangement is supported by a complete muscular infundibulum, the aortic valve typically being anterior and to the left relative to the pulmonary trunk ( Figs. 39-5 and 39-6 ).
In a small proportion of hearts from patients with usually arranged atriums, and with the segmental arrangement of congenitally corrected transposition, the aorta can be found in right-sided position, or directly anterior to the pulmonary trunk. Anterior and right-sided positioning of the aorta is the rule when congenitally corrected transposition is found in the mirror-imaged arrangement. L-transposition, therefore, is not the same thing as congenitally corrected transposition, the more so since left-sided aortas can be found in patients with regular transposition. Within the ventricles, the morphologically mitral valve is usually supported by paired papillary muscles located in infero-medial and supero-lateral positions. The supero-lateral papillary muscle is potentially vulnerable during surgical ventriculotomy, albeit that in most instances nowadays the surgeon approaches the ventricles either through an atrioventricular or arterial valve. In hearts with intact ventricular septal structures, the attachment of the morphologically tricuspid valve to the extensive membranous septum creates an interventricular part, as in hearts with concordant atrioventricular connections, along with an atrioventricular portion. Unlike the normal situation, the atrioventricular part of the membranous septum separates the morphologically left atrium from the morphologically left ventricle, the latter chamber being right-sided in the setting of usual atrial arrangement. There is also a prominent recess seen antero-superiorly within the morphologically left ventricle. This was a useful feature for recognition of the abnormality in the days when diagnosis depended on angiography.
In the past, it was frequently the practise to describe the ventricles as being inverted in the setting of usual atrial arrangement. In that they are not upside down, this is a less than accurate description of the ventricular arrangement. In terms of topology, nonetheless, the ventricular mass is the mirror image of the usual arrangement, exhibiting a left hand pattern when the atrial appendages are usually arranged, but showing right hand topology when the atrial chambers are mirror-imaged. Even with right hand topology, the ventricular mass is not the same as is seen in the normal heart, nor does the left hand topological arrangement show perfect mirror-imagery of the normal arrangement. This is largely due to the fact that the outflow tracts are parallel to each other, rather than crossing, as in the normal situation. The ventricles, therefore, tend to be side by side, often with an added supero-inferior obliquity. It is also possible to find rotational abnormalities. It is marked abnormal rotation that produces the additional malformations known as criss‑cross relationships, 5,6 while excessive tilting produces supero-inferior ventricles. 6 In addition to the ventricles occupying a side-by-side relationship, it is also frequent for the entire ventricular mass to be abnormally located within the thorax, and for the apex of the ventricular mass to point in unexpected directions. Finding the heart abnormally positioned, therefore, or an unusual orientation of the apex should always raise the suspicion of the congenitally corrected transposition.
As is always the case, the coronary arteries originate from the two aortic sinuses which are adjacent to the pulmonary trunk. It is usually the case that the right coronary artery arises from one of the adjacent sinuses, and the main stem of the left coronary artery from the other adjacent sinus, but sometimes both of the arteries arise from one or the other sinus, often with the arteries having a common stem. 7 Their precise position relative to the heart will vary according to the precise position of the aortic root, but the specific arrangement is well described by taking note of their origin from either sinus #1 or #2 ( Fig. 39-7 ). Origin of all three main coronary arteries from one or the other sinus is a frequent finding, often with the arteries having a common stem. The epicardial distribution of the arteries is reasonably constant, and is determined by the ventricular topology. Thus, in persons with atrial chambers in their expected position, the overall pattern is mirror‑imaged relative to the arrangement seen in the normal heart. The right‑sided coronary artery exhibits the pattern of a morphologically left coronary artery, with its short main stem dividing into anterior interventricular and circumflex branches ( Fig. 39-8 ). The circumflex artery encircles the mitral orifice, which is right‑sided when there is usual atrial arrangement. The position of the anterior interventricular artery is an excellent guide to the location of the ventricular septum. The left‑sided coronary artery in the setting of usual atrial arrangement is a morphologically right coronary artery. It gives off infundibular and marginal branches while encircling the left‑sided tricuspid orifice. In most instances, the inferior interventricular branch arises from this artery. These relationships are all mirror-imaged when the atrial chambers themselves are mirror-imaged, since this arrangement is associated with right hand ventricular topology so as to produce congenitally corrected transposition. The epicardial pattern of the coronary arteries, therefore, resembles that expected for the normal heart ( Fig. 39-9 ).
As we have already emphasised, because of the wedged position of the subpulmonary outflow tract in the morphologically left ventricle, there is gross malalignment between the atrial septum and the inlet part of the ventricular septum ( Fig. 39-10 ). When the septal structures are intact, the septal aspect of the malalignment gap is filled by the extensive membranous septum. Not surprisingly, deficiency of this structure is common, producing a perimembranous ventricular septal defect. Because of the gross septal malalignment, it is impossible for the penetrating atrioventricular bundle to take its origin from the regular atrioventricular node, located at the apex of the triangle of Koch in the base of the atrial septum. Instead, the atrioventricular conduction axis originates from a second anomalously located atrioventricular node, located beneath the opening of the right atrial appendage at the lateral margin of the area of pulmonary-to-mitral valvar fibrous continuity ( Fig. 39-11 ). The atrioventricular conduction axis, having penetrated through the fibrous trigone, comes to lie immediately underneath the pulmonary valvar leaflets. An extensive non-branching bundle then runs superficially underneath the right anterior facing leaflet of the pulmonary valve, descends for some distance down the anterior septal surface of the subpulmonary outflow tract, and branches into a cord‑like right bundle branch, which extends leftwards to reach the morphologically right ventricle, and a fan‑like left bundle branch, which cascades down the smooth left ventricular septal surface.
The precise anatomy of the atrioventricular conduction axis has far‑reaching surgical significance in the presence of associated malformations. The close relationship between the non‑branching bundle and the pulmonary valvar orifice complicates both closure of ventricular septal defects and relief of obstruction within the morphologically left ventricular outflow tract. Thus, in patients with perimembranous ventricular septal defects, the bundle has a grossly abnormal position when compared to the regular position of the conduction axis in hearts with ventricular septal defects in the setting of concordant atrioventricular connections. When viewed by the surgeon, it is to the right hand side ( Fig. 39-12 ) rather than to the left hand side as anticipated in a heart with normal junctional anatomy.
In hearts with less malalignment between the atrial and ventricular septums, the regular node may be positioned so as to make contact posteriorly with the ventricular septum. This is produced by such lesions as double outlet from the right ventricle 8 and severe pulmonary stenosis or atresia. 9 In the past, it was often stated that the frequent origin of the conduction axis from a regular node in the setting of mirror-imaged atrial arrangement was because of the right-handed ventricular topology. As was shown by Hosseinpour and colleagues, 9 all cases described with this pattern had better septal alignment because of the presence of severe pulmonary stenosis or pulmonary atresia. Thus, the same rules apply for prediction of the location of the conduction axis in congenitally corrected transposition irrespective of whether there is usual or mirror-imaged atrial arrangement. The key is the extent of septal malalignment. When there is good septal alignment, then both the regular and the anterior nodes can give rise to penetrating bundles, both of which can join with the branching bundle. Should there be a ventricular septal defect, which is usually the case, this can produce a sling of conduction tissues, as initially described by Monkeberg. 10 The vulnerable position of the long non‑branching bundle almost certainly explains why heart block remains a frequent complication after surgical closure of associated malformations. Atrioventricular dissociation, nonetheless, may sometimes be present at birth. More frequently, there is progressive acquired atrioventricular dissociation, often culminating in complete heart block. 4
Associated Malformations
In the majority of cases encountered clinically, the potential correction of the circulatory pattern produced by the double discordance is uncorrected by the presence of one or more associated malformations. Of these, three are so typical as to be considered almost part of the segmental combination, namely, an interventricular communication, obstruction of the subpulmonary outflow tract, and anomalies of the morphologically tricuspid valve. 1,2 The morphology of each of these lesions can be variable.
Ventricular Septal Defect
An interventricular communication, if present, and such holes are expected in at least half the patients, is usually perimembranous. It occupies a subpulmonary position, with the diagnostic feature being fibrous continuity between the leaflets of the pulmonary valve and, in usual arrangement, the left-sided tricuspid valve ( Fig. 39-13 ). Such perimembranous defects typically extend posteriorly and inferiorly towards the crux of the heart, opening primarily into the inlet of the morphologically left ventricle. The posterior margin of the defect is then formed by an extensive area of fibrous continuity between the leaflets of the pulmonary, mitral, and tricuspid valves (see Fig. 39-13 ), with this feature removing the anticipated reversed off-setting of the attachments of the atrioventricular valves. As already explained, the atrioventricular conduction axis runs antero-superiorly relative to these defects (see Fig. 39-13 ), the opposite of that expected for perimembranous defects in hearts with concordant atrioventricular connections. In rare instances, the defect can be subpulmonary but with exclusively muscular rims. In this setting, the atrioventricular conduction axis will continue to run antero-superiorly. Muscular defects, however, can be found in any other part of the ventricular septum. Should a muscular defect open between the outlets, then the bundle may occupy a postero-inferior position ( Fig. 39-14 ). Defects can also be found in doubly committed and juxta-arterial position, roofed by continuity between the leaflets of the aortic and pulmonary valves, with absence of the septal component of the infundibulum. These defects are particularly common in Asian populations. 11
Obstruction of the Morphologically Left Ventricular Outflow Tract
Excluding hearts with pulmonary atresia, stenosis of the outflow tract from the morphologically left ventricle occurs in approximately one-third to one-half of patients with usual atrial arrangement. The stenosis is isolated in less than one‑fifth of these cases, being combined in about four-fifths with a ventricular septal defect, and in about one‑third also with abnormalities of the morphologically tricuspid valve. The anatomical nature of the stenosis varies. 12 Valvar stenosis is usually accompanied by one or another variety of subpulmonary obstruction. The latter may take the form of muscular hypertrophy of the septum and the ventricular free wall, a fibrous diaphragm, or else an aneurysmal dilation of fibrous tissue derived from the interventricular component of the membranous septum ( Fig. 39-15 ). More rarely, tags may originate from either of the atrioventricular valves, or even from the leaflets of the pulmonary valve. 12 Subvalvar pulmonary obstructions, when present, are intimately related to the non-branching atrioventricular bundle ( Fig. 39-16 ).
Lesions of the Morphologically Tricuspid Valve
There is a marked discrepancy between the incidence of such changes found at autopsy and those recognised during life. Examination of autopsied cases reveals anomalies of the tricuspid valve in almost nine-tenths of cases, whereas only one in three patients has haemodynamic alterations due to such abnormalities. The commonest underlying pathology is valvar dysplasia, with or without apical displacement of the septal and mural leaflets, the latter of course being the essence of Ebstein’s malformation. Unlike the situation in hearts with concordant atrioventricular connections, atrialisation and thinning of the inlet portion of the morphologically right ventricle, as commonly seen in Ebstein’s malformation with concordant atrioventricular connections, are not always found in the setting of congenitally corrected transposition ( Fig. 39-17 ).
In about three-quarters of the cases, the valvar anomalies are combined with a ventricular septal defect. The tricuspid valve can also override and straddle in the setting of congenitally corrected transposition, with hypoplasia of the morphologically right ventricle increasing concomitant with the proportion of the overriding atrioventricular junction connected to the dominant left ventricle ( Fig. 39-18 ). The end-point of this spectrum of overriding is double inlet left ventricle with left-sided incomplete right ventricle, with double inlet being diagnosed when more than half of the overriding junction is connected in the left ventricle. The morphologically mitral valve can also override and straddle, often in combination with double outlet from the right ventricle. 8 As with overriding of the tricuspid valve, mitral valvar overriding is part of a spectrum of malformation, with the endpoint in this instance being double inlet right ventricle with right-sided incomplete left ventricle. The relationships of dominant and incomplete ventricles in these spectrums, of course, are reversed when congenitally corrected transposition is found in its mirror-imaged variant. The mitral valve can also prolapse with some frequency in the setting of congenitally corrected transposition. 13
Disharmonious Segmental Arrangements
In almost all instances, when congenitally corrected transposition is found with usual atrial arrangement, there is left hand ventricular topology, whereas right hand ventricular topology is found when discordant atrioventricular connections are found with mirror-imaged atrial arrangement. On exceedingly rare occasions, the atrioventricular connections can be discordant when there is usual atrial arrangement and right hand ventricular topology. 14 The hearts typically show multiple associated malformations, with rotational abnormalities of the ventricular mass, straddling valves, and juxtaposition of the atrial appendages. Providing the usual rules are followed for determining the morphology of the chambers, there should be no problems these days in diagnosing the malformations, despite their complexity.
Other Ventriculo-arterial Connections
In strict terms, those patients having other ventriculo-arterial connections in combination with discordant atrioventricular connections do not have congenitally corrected transposition. It is convenient, nonetheless, to discuss them in this chapter, since it is the discordant atrioventricular connections which are the most important morphological features. The commonest variant in terms of the ventriculo-arterial connections is single outlet from the heart. In this respect, there are three possibilities: a common arterial trunk, a single pulmonary trunk with aortic atresia, and a single aortic trunk with pulmonary atresia. Of these, the first two are rare. 15,16 With pulmonary atresia, there is usually a large ventricular septal defect in the subaortic position. This lends itself to tunnelling to the aorta as part of anatomical correction. The conduction tissue should be anticipated to be antero-superiorly located relative to this defect, arising from an anterior atrioventricular node, so extreme care is needed if enlargement of the defect is attempted as part of an anatomical correction. The problem is then magnified, since it is cases such as these with pulmonary atresia that should be anticipated also to have a regular node, or else a sling of conduction tissue. Thus, the entire ventricular borders of the defect are likely to be occupied by the conduction tissue axis. 9 Pulmonary atresia can also be found when the ventricular septum is intact. 17,18 In these cases, the atretic pulmonary artery can usually be traced to the morphologically left ventricle, so it is appropriate to describe these patients as having congenitally corrected transposition with pulmonary atresia. The arrangement of the morphologically left ventricle itself, however, is reminiscent of hypoplastic left heart syndrome, and these patients will be candidates only for functionally univentricular repair.
The rare but important combination of discordant atrioventricular connections with concordant atrioventricular connections can be found either with usual atrial arrangement or in the setting of mirror‑imaged atriums. Several purported examples of this rare combination have been in patients with isomeric atrial appendages, but in this combination the atrioventricular connections will be biventricular and mixed rather than discordant (see Chapter 1 ). In some cases, the aorta is posterior and to the right of the pulmonary trunk, albeit with a parallel rather than a spiral arrangement of the arterial trunks. The combination has been described as isolated ventricular inversion, not that this nomenclature contributes much to understanding. 19 The important feature is that, when discordant connections at atrioventricular level are combined with concordant ventriculo-arterial connections, the combination produces the haemodynamics of transposition irrespective of the infundibular morphology. Such patients are ideal candidates for surgical correction by atrial redirection procedures, 20 since this provides both anatomical and physiological correction of the circulations, restoring the morphologically left ventricle to its systemic role.
Many of the patients having discordant atrioventricular connections combined with double outlet right ventricle have right‑sided hearts and pulmonary stenosis. 21 It was in a heart such as this that Mönckeberg 10 first described the unusual formation of paired atrioventricular nodes joining to a common atrioventricular bundle, the so-called Mönckeberg sling. The heart described by Mönckeberg 10 was positioned in the right side of the chest, but the segmental combination can exist with any cardiac position. The ventricular septal defect is usually in subpulmonary position, but subaortic, doubly committed, and non-committed defects have all been described. All kinds of relations between the great arterial trunks can also be present. The rarest ventriculo-arterial connection is that of double outlet left ventricle. This must also be anticipated to exist with various arterial relationships, and with the interventricular communication related in varying fashion to the arterial trunks, and with the typical complicating lesions.
Morphogenesis
As we have described in Chapter 3 , the definitive chambers of the heart are produced by ballooning from the primary heart tube, with atrial appendages ballooning in parallel from the atrial component of the tube, while the apical components of the ventricles balloon in series from the inlet and outlet parts of the ventricular loop. If development proceeds in normal fashion, the primary heart tube bends to the right during early development. 22 This leaves the atrioventricular canal connected primarily to the part of the loop from which will develop the morphologically left ventricle. Expansion of the canal to the right then permits the right atrium to connect directly with the developing morphologically right ventricle, which itself is positioned rightward relative to the morphologically left ventricle. In certain circumstances, instead of bending to the right during development, the heart tube turns leftward. Such leftward looping places the outlet component of the primary tube, from which will develop the morphologically right ventricle, to the left of the morphologically left ventricle. In this setting, so as to permit the atrioventricular canal to open directly to both ventricles, it must expand leftward rather than rightward, at the same time placing the developing morphologically left atrium in communication with the morphologically right ventricle, and leaving the morphologically right atrium connecting to the morphologically left ventricle. This process, therefore, produces discordant atrioventricular connections. It is not yet known why such disharmonious looping should be associated so frequently also with discordant ventriculo-arterial connections, nor why the presence of discordant atrioventricular connections should be associated so frequently with the typical associated malformations.
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