Congenitally Corrected Transposition

Basic Morphology

As far as we are aware, the lesion was first described by the Baron von Rokitansky. 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. 38.1 ). In the majority of cases, the atrial chambers are usually in their expected position. In approximately 10% of all cases, the atrial chambers are mirror-imaged, along with the remaining thoracic and abdominal organs. In both instances, by virtue of the discordant AV connections, the morphologically right atrium is connected to the morphologically left ventricle (mLV) through a mitral valve, while the left atrium is joined to the morphologically right ventricle (mRV) through a tricuspid valve ( Figs. 38.2 and 38.3 ). However, with usual atrial arrangement, the ventricular mass shows left-sided topology, whereas the right-sided topology anticipated for the normal heart is found when the atrial chambers are mirror-imaged ( Fig. 38.4 ).

In both instances, if the ventricular septum is intact, there is reversed offsetting of the septal attachments of the leaflets of the AV valves. However, the apically displaced valve will be left sided with usual atrial arrangement but right sided when the atrial chambers are mirror-imaged. Irrespective of atrial arrangement, the mLV will give rise to the pulmonary trunk, almost always with fibrous continuity between the leaflets of the pulmonary and mitral valves (see Fig. 38.3A ). 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 AV conduction axis. The mRV, receiving the pulmonary venous return ( Fig. 38.5A ), empties into the aorta, which almost always is supported by a completely muscular infundibulum (see Fig. 38.5B ). The aortic valve is usually anterior and to the left relative to the pulmonary trunk but only in the setting of usual atrial arrangement ( Fig. 38.6 ).

When the atrial chambers are mirror-imaged and there is right-sided ventricular topology, it is the rule in patients with congenitally corrected transposition to find the aorta in right-sided and anterior position (see Fig. 38.4B ). Even in some of the 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. This shows that “l-transposition” is not the same thing as congenitally corrected transposition, the more so because left-sided aortas can be found in patients with regular transposition (see Chapter 37 ).

Within the ventricles, the morphologically mitral valve is usually supported by paired papillary muscles located in inferomedial and superolateral positions (see Fig. 38.3A ). The superolateral papillary muscle is potentially vulnerable during surgical ventriculotomy, albeit that in most instances nowadays the surgeon approaches the ventricles either through an AV or arterial valve. In hearts with intact ventricular septal structures, the attachment of the morphologically tricuspid valve to the extensive membranous septum divides it into interventricular and AV portions. However, unlike the normal situation, by virtue of the discordant connections, the AV part of the membranous septum separates the morphologically left atrium from the mLV. There is also a prominent recess seen anterosuperiorly within the mLV (see Fig. 38.3B ). The ventricles tend to be positioned side by side, often with an added superoinferior obliquity. It is also possible to find rotational abnormalities. Such abnormal rotation produces the additional malformations known as criss-cross relationships, better described as twisted AV connections. Excessive tilting of the ventricular mass produces superoinferior ventricles. 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. Therefore finding the heart abnormally positioned or an unusual orientation of the apex should always raise the suspicion of the congenitally corrected transposition. The heart is located in the middle of the chest, so-called mesocardia, in up to 20% of patients. In this situation, with the apex of the ventricular mass pointing toward the midline, the ventricular mass is very much anterior to the atriums. In consequence, the AV valves face forward, making surgical access to them more difficult.

The coronary arteries usually originate from the two aortic sinuses that are adjacent to the pulmonary trunk. Both of the arteries, nonetheless, can arise from the same sinus, often with a common stem. Their precise position relative to the heart will vary according to the precise position of the aortic root, but the specific arrangement, as in hearts with concordant AV and discordant VA connections, is well described by taking note of their origin from either sinus 1 or sinus 2. The epicardial distribution of the arteries is reasonably constant and is determined by the ventricular topology. Thus, in persons with usual atrial arrangement, 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. 38.7A ). 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 themselves mirror-imaged when the atrial chambers are also mirror-imaged but with right-sided ventricular topology so as to produce congenitally corrected transposition. The epicardial pattern of the coronary arteries in this setting resembles that expected for the normal heart (see Fig. 38.7B ).

As already emphasized, because of the wedged position of the subpulmonary outflow tract in the mLV, there is gross malalignment between the atrial septum and the inlet part of the ventricular septum. When the septal structures are intact, the septal aspect of the malalignment gap is filled by the extensive membranous septum. And, as emphasized, the AV component of this septum interposes between the mLV and the morphologically left atrium. Not surprisingly, deficiency of this structure is frequent, producing a perimembranous ventricular septal defect. Because of the gross septal malalignment, it is impossible for the penetrating AV bundle to take its origin from the regular AV node, located at the apex of the triangle of Koch in the base of the atrial septum. Instead, the AV conduction axis originates from an anomalous AV 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. 38.8A ).

The AV conduction axis, having penetrated through the fibrous trigone, comes to lie immediately underneath the pulmonary valvar leaflets. An extensive nonbranching bundle then runs superficially underneath the right anterior facing leaflet of the pulmonary valve and descends for some distance down the anterior septal surface of the subpulmonary outflow tract. It then branches into a cordlike right bundle branch, which extends leftward to reach the mRV, and a fanlike left bundle branch, which cascades down the smooth left ventricular septal surface (see Fig. 38.8B ). The precise anatomy of the AV conduction axis has far-reaching surgical significance in the presence of associated malformations. The close relationship between the nonbranching 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 with the regular position of the conduction axis in hearts with ventricular septal defects in the setting of concordant AV connections. When viewed by the surgeon, it is to the left side (see Fig. 38.8B ), as opposed to the right side as anticipated in hearts 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, or severe pulmonary stenosis or atresia. In the past, it was often stated that the observed origin of the conduction axis from a regular node in the setting of mirror-imaged atrial arrangement was because of the right-sided ventricular topology. However, as was subsequently shown, 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. The vulnerable position of the long nonbranching bundle almost certainly explains why heart block remains a frequent complication after surgical closure of associated malformations. Nonetheless, AV dissociation may sometimes be present at birth. More frequently, there is progressive acquired AV dissociation, often culminating in complete heart block.

Associated Malformations

In the majority of cases encountered clinically, the potential congenital correction of the circulatory pattern 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. The morphology of each of these lesions can be variable.

Ventricular Septal Defect

An interventricular communication is found in 50% to 70% of patients and is usually perimembranous (see Fig. 38.4A ). 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. Such perimembranous defects typically extend posteriorly and inferiorly toward the crux of the heart, opening primarily into the inlet of the mLV. 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, with this feature removing the anticipated reversed offsetting of the attachments of the AV valves. The AV conduction axis runs anterosuperiorly relative to these defects ( Fig. 38.9 ), the opposite of that expected for perimembranous defects in hearts with concordant AV connections. In rare instances, the defect can be subpulmonary but with exclusively muscular rims. In this setting, the AV conduction axis will continue to run anterosuperiorly. However, muscular defects 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 posteroinferior position (see Fig. 38.4B ). Defects can also be found in doubly committed and juxtaarterial position, roofed by continuity between the leaflets of the aortic and pulmonary valves, with absence of the septal component of the infundibulum. These defects, as is usually the case, are particularly common in Asian populations.

Obstruction of the Morphologically Left Ventricular Outflow Tract

Excluding hearts with pulmonary atresia, stenosis of the outflow tract from the mLV occurs in approximately 30% to 50% of patients presenting with usual atrial arrangement. The stenosis is isolated in less than 20% of these cases, being combined in approximately 80% with a ventricular septal defect, and in approximately 30% also with abnormalities of the morphologically tricuspid valve. The anatomic nature of the stenosis varies. Valvar stenosis is usually accompanied by one or more varieties 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. 38.10 ). More rarely, tags may originate from either of the AV valves or even from the leaflets of the pulmonary valve. Subvalvar pulmonary obstructions, when present, are intimately related to the nonbranching AV bundle.

Lesions of the Morphologically Tricuspid Valve

There is a marked discrepancy between the incidence of such changes found at autopsy and those recognized during life. Examination of autopsied cases reveals anomalies of the tricuspid valve in almost 90% of cases, whereas only 30% of patients have hemodynamic alterations due to such abnormalities. The most common 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 malformation. Unlike the situation in hearts with concordant AV connections, when these are usually so-called arterialization and thinning of the inlet portion of the mRV, these features are not always found in the setting of congenitally corrected transposition ( Fig. 38.11 ). Because of this, the apical displacement of the hinges of the leaflets of the tricuspid valve is referred to as being “ebsteinoid,” acknowledging the fact that the arrangement does not usually exhibit all the features of Ebstein malformation as seen when the AV connections are concordant.

In approximately 75% 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 mRV increasing concomitant with the proportion of the overriding AV junction connected to the dominant left ventricle. 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 ( Fig. 38.12 ). As with overriding of the tricuspid valve, mitral valvar overriding is part of a spectrum of malformation, with the end point in this instance being double inlet right ventricle with right-sided incomplete left ventricle. The relationships of dominant and incomplete ventricles in these spectrums 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.

Disharmonious Segmental Arrangements

As emphasized, when congenitally corrected transposition is found with usual atrial arrangement, almost always there is left-sided ventricular topology, whereas right-sided ventricular topology is found when discordant AV connections are found with mirror-imaged atrial arrangement. On exceedingly rare occasions, the AV connections can be discordant when there is usual atrial arrangement and right-sided ventricular topology. 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 in diagnosing the malformations, despite their complexity (see Chapter 49 ).

Other Ventriculoarterial Connections

In strict terms, those patients having other VA connections in combination with discordant AV connections do not have congenitally corrected transposition. It is convenient, nonetheless, to discuss them in this chapter because it is the discordant AV connections that are the most important morphologic features. The most common variant in terms of the VA connections is single outlet from the heart. In this respect, there are three possibilities, namely 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 extremely rare. With pulmonary atresia, there is usually a large ventricular septal defect in the subaortic position ( Fig. 38.13A ). This lends itself to tunneling to the aorta as part of anatomic correction. The conduction tissue should be anticipated to be anterosuperiorly located relative to this defect, arising from an anterior AV node, so extreme care is needed if enlargement of the defect is attempted as part of an anatomic correction. The problem is then magnified because 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. Pulmonary atresia can also be found when the ventricular septum is intact (see Fig. 38.13B ). In these cases the atretic pulmonary artery can usually be traced to the mLV, so it is appropriate to describe them as having congenitally corrected transposition with pulmonary atresia. However, the arrangement of the mLV itself is reminiscent of hypoplastic left heart syndrome, and they will be candidates only for functionally univentricular repair. The rare but important combination of discordant AV connections with concordant AV VA connections can be found with either usual atrial arrangement ( Fig. 38.14 ) or in the setting of mirror-imaged atriums.

Several purported examples of this rare combination have been seen in patients with isomeric atrial appendages, but in this combination the AV connections will be biventricular and mixed rather than discordant (see Chapter 26 ). 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. The important feature is that, when discordant connections at AV level are combined with concordant VA connections, the combination produces the hemodynamics of transposition irrespective of the infundibular morphology. Such patients are ideal candidates for surgical correction by atrial redirection procedures because this provides both anatomic and physiologic correction of the circulations, restoring the mLV to its systemic role.

Many of the patients having discordant AV connections combined with double outlet right ventricle have right-sided hearts and pulmonary stenosis. The ventricular septal defect is usually in subpulmonary position, but subaortic, doubly committed, and noncommitted defects have all been described. All kinds of relations between the great arterial trunks can also be present. The rarest VA connection is that of double outlet left ventricle. This must also be anticipated to exist with various arterial relationships, with the interventricular communication related in varying fashion to the arterial trunks, and with the typical complicating lesions.

Morphogenesis

As 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. This leaves the AV canal connected primarily to the part of the loop from which will develop the mLV. Expansion of the canal to the right then permits the right atrium to connect directly with the developing mRV, which itself is positioned rightward relative to the mLV. 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 mRV, to the left of the mLV. In this setting, to permit the AV 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 mRV and leaving the morphologically right atrium connecting to the mLV. Therefore this process produces discordant AV connections. It is not yet known why such disharmonious looping should be associated so frequently also with discordant VA connections nor why the presence of discordant AV connections should be associated so frequently with the typical associated malformations.