The entities to be considered in this chapter differ from all those others considered in this section of our book devoted to specific lesions. This is because hearts with straddling and overriding atrioventricular valves represent a series of anatomical stages between the extremes of double inlet and biventricular atrioventricular connections. As such, therefore, straddling and overriding can involve the right or left atrioventricular valve, or a common atrioventricular valve, in the setting of either double inlet right or double inlet left ventricle. The other end of these various spectrums will then depend upon the particular valve involved, and on the topological arrangement of the ventricular mass. The structure of the individual heart itself will reflect the precise degree of straddling, as opposed to overriding of the valvar structures. Because of this, it is not possible to approach the lesions in the same fashion as all others discussed in this section of our book. The clinical features will not only reflect the variations already discussed above, but will also differ markedly according to the specific ventriculo-arterial connections present, and the associated malformations. The understanding of the malformations, and their correct diagnosis, depends on a thorough appreciation of their anatomical features. In this chapter, therefore, we describe the different patterns on the basis of their structure. This in itself requires that we define precisely our understanding of the nature of straddling and overriding, since the definition of these features, and the way in which they are described, has varied markedly.
DEFINITIONS
In the normally structured heart, each ventricle, which functions as the muscular pump driving its circulation, has competent valves guarding its inlet and its outlet. The functional components of these valves are the leaflets. Of necessity, if the heart is normal, these leaflets are attached exclusively within their own ventricle. There are subtle differences between the nature of the valvar attachments which are not strictly relevant to this chapter, but worthy of emphasis. Thus, the leaflets of the atrioventricular valves are normally attached in annular fashion, and their line of attachment is coincident with the anatomical atrioventricular junction ( Fig. 33-1 ). The leaflets of the arterial valves,in contrast, are attached in semilunar fashion, with their lineof attachment crossing the anatomical ventriculo-arterial junction ( Fig. 33-2 ). Much more significant from the stance of our definitions for straddling and overriding is the nature of the free edge of the valvar leaflets. The atrioventricular valves close against the full force of ventricular systole. So as to retain valvar competence, therefore, the free edges of the leaflets are furnished with tension apparatus (see Fig. 33-1 ). In the normal ventricles, the entire tension apparatus for each valve, like the attachments of the leaflets at the atrioventricular junctions, is exclusively contained within its own ventricle. The arterial valves, in contrast, close in ventricular diastole. It is the force of the column of blood supported by the leaflets which ensures their competence. Arterial valves, therefore, lack any tension apparatus.
When hearts are congenitally malformed, either the atrioventricular or the ventriculo-arterial junctions, or on occasion both junctions, can be shared between the ventricles. It is the sharing of an atrioventricular junction between the ventricles that is usually the essential feature of the entities to be discussed in this chapter. In the abnormal hearts to be discussed, it is also the case that, in the majority of cases, the tension apparatus is attached not in one, but in both of the ventricles. In order to distinguish between the sharing of the junctions, and the abnormal location of the tension apparatus, and to produce a terminology which is applicable to both atrioventricular and ventriculo-arterial valves, we define separately the nature of these two independent features. 1
Overriding, therefore, describes the situation in which a junction, either atrioventricular ( Fig. 33-3 ) or ventriculo-arterial ( Fig. 33-4 ), is shared between the ventricles. This permits us to reserve straddling to describe the arrangement in which the valvar tension apparatus is attached to either side of the ventricular septum and, hence, tethers the valvar leaflets in both ventricles ( Fig. 33-5 ). Using these definitions, it follows that an atrioventricular valve can straddle and override. Such an arrangement ( Fig. 33-6 ) is by far the commonest encountered. But it is also possible for an atrioventricular valve to straddle without overriding ( Fig. 33-7 ), or to override in the absence of straddling of the tensor apparatus ( Fig. 33-8 ). It also follows that, within our definitions, it is possible for an arterial valve to override, but not to straddle, since arterial valves lack any tension apparatus.
These chosen definitions have major implications for the description of other features of the hearts that are malformed because they contain straddling and overriding atrioventricular valves. The degree of overriding of the abnormal valve determines the precise atrioventricular connections, which can vary from double inlet to concordant or discordant. A spectrum exists between these extremes. There are two ways of coping with this situation. One way is to consider straddling in itself as a special case. The second approach is to emphasise the effect of the degree of override on the atrioventricular connection, dividing the spectrum at its midpoint when categorising the segmental arrangement (see Chapter 1 ). Thus, considering as an example the spectrum of straddling of the tricuspid valve, as illustrated in Figure 33-3 , the hearts containing the straddling valve can be categorised as exhibiting either double inlet ventricle or concordant atrioventricular connections, depending of the proportion of the overriding junction supported within the right as opposed to the left ventricles. This is the principle used throughout this chapter. In the past, it was much easier to divide this spectrum in theory than it was in practise. The development of tomographic diagnostic techniques, and the ability to reconstruct individual anatomy in three dimensions, now means that it is possible also to make these distinctions in the clinical setting.
It was also the case that, when we first used this 50% rule for the purpose of assigning patients with straddling valves as having double inlet or biventricular atrioventricular connections, we had problems in moving from 49% to 51%. This was because, depending on our decision, a patient considered to have a biventricular heart in the setting of biventricular connections would suddenly possess a univentricular heart if we designated the connection as double inlet. At that time, we were also promoting the concept of univentricular hearts as discrete from hearts with two ventricles, illogically disqualifying as ventricles all those chambers which lacked an inlet component or, more specifically, half an inlet component. 2 Once the lack of logic in this approach was identified, 3 and we realised that it was the atrioventricular connection, rather than the ventricular mass, which was effectively biventricular or univentricular, then the semantic problems disappeared. 4 The procedural problems, nonetheless, remain. It is still difficult precisely to adjudicate between 49% and 51% of overriding of an atrioventricular junction. Since the decision will no longer affect the description of the ventricles, the distinction loses much of its force. As we will see, in clinical practise it is more important in this setting to describe the size of the ventricles. While this, to some extent, reflects the precise atrioventricular connection, it is certainlynot the only determining feature. The degree of override, therefore, is determined with as much accuracy as is possible, dividing the spectrum on the basis of the proportion of the overriding junction attached within the two ventricles relative to the location of the ventricular septum (see Fig. 33-3 ).
There is then a second major point to consider in terms of definitions. This is when the valve which straddlesand overrides is guarding a common atrioventricular junction, rather than a separate right or left atrioventricular junction. The first problem here is in defining a common atrioventricular junction, and distinguishing the common junction from the arrangement seen when one atrioventricular connection is absent and the other junction is itself straddling and overriding ( Fig. 33-9 ). We use the atrial septum as our defining feature. We define a common atrioventricular junction as one which is common to both atriums and both ventricles, although the sharing of this junction does not have to be uniform between the four chambers. The exemplar of hearts with such a common junction is the entity usually described simply as an atrioventricular septal defect (see Chapter 27 ). In these hearts, the presence of the septal defect between the leading edge of the atrial septum, and the crest of the ventricular septum ( Fig. 33-10 ), serves to emphasise the common nature of the junction. In all hearts having such an atrioventricular septal defect and common atrioventricular junction, the common atrioventricular valve of necessity both straddles and overrides. According to conventional wisdom, 5 such hearts are not discussed under the heading of straddling and overriding. There is no morphological reason why they should not be thus described, since the feature of leaflets being tethered in both ventricles is found in both settings. Other hearts, nonetheless, can have a common atrioventricular junction guarded by a common atrioventricular valve, and not usually be described in terms of atrioventricular septal defects. These are the hearts with double inlet ventricle, but with both atrioventricular junctions guarded by a common atrioventricular valve. In these hearts ( Fig. 33-11 ), the common atrioventricular valve does not straddle and override. In between these extremes, there are further spectrums of malformations that reflect the precise degree of override of the common valve. These series of anomalies parallel the spectrums of overriding of the right or left valves, to be described in the body of this chapter. In such hearts with eccentric commitment of the common atrioventricular valve, it is again hard to make the distinction between double inlet and biventricular atrioventricular connections. Here, the decision reached probably will affect clinical decision-making, since it will determine whether to proceed to univentricular as opposed to biventricular repair. From the morphological stance, the decision is also harder to make than when either the right or left valve is straddling, since it requires the use of a 75% rather than a 50% rule. In clinical terms, it is almost certainly the size of the ventricle which is most important in determining the feasibility of biventricular versus univentricular repair. The precise atrioventricular connection, however, is by no means insignificant. This is again made, therefore, on the basis of the perceived attachments of the overriding junction, using the best information available to reach this decision. We will not consider further in this chapter those hearts with common valve which are intermediate between the extremes of atrioventricular septal defect and double inlet ventricle, but they exist in the same patterns of right hand and left hand ventricular topology which will be discussed for straddling and overriding of those having separate right and left atrioventricular valves.
Before leaving the topic of definitions, we should address the problems of distinguishing a common valve from a straddling solitary valve associated with absence of one atrioventricular connection (see Fig. 33-9 ). We use the arrangement of the atrial septum as our arbiter. In almost all hearts in which one atrioventricular connection is absent, the atrial septum fuses with the parietal atrial wall so as to separate the blind-ending atrial chamber from the patent atrioventricular junction. This arrangement is seen even when the valve guarding the right or left atrioventricular junction is itself straddling and overriding between the ventricles (see Fig. 33-9 ). This morphological pattern serves to distinguish these hearts in which one atrium is connected to both ventricles, in other words, those with a uniatrial but biventricular atrioventricular connection, from other hearts with comparable connection of one atrium to both ventricles, so-called double outlet atrium, but in the setting of a common atrioventricular junction. In these latter hearts, which unequivocally have straddling and overriding of an atrioventricular valve, the valve itself is common to both the atriums because the atrial septum has failed to fuse with the parietal atrioventricular junction. The overall lesion, in the setting of common atrioventricular junction, is the consequence of malalignment between the atrial and ventricular septums, but with retention of the septal deficiency. This space, between the malaligned atrial septum in these hearts and the atrioventricular junction, is the so-called ostium primum defect. It can be argued that, on morphogenetic terms, this interatrial communication can become gradually smaller until it disappears, and therefore that the entities should be grouped together. From the stance of strict morphology, nonetheless, the junction remains a common structure until the primary foramen, or the ostium primum, has closed. The hearts with common atrioventricular valves and double outlet atrium are discussed further in Chapter 27 . Those with a uniatrial but biventricular atrioventricular connection 6 will receive further attention in this chapter, and are also discussed in Chapter 31 .
INCIDENCE AND AETIOLOGY
In that all atrioventricular septal defects with common orifice have a straddling valve as here defined, they should strictly be included in statistics concerning incidence. Because of problems of this kind, and because recognition of straddling and overriding is a recent event, it is difficult, if not impossible, to give precise figures. In our experience, straddling valves are recognised with more frequency now that they are specifically sought, particularly in anomalies such as congenitally corrected transposition (see Chapter 39 ), hearts with double inlet ventricle (see Chapter 31 ), and in double outlet right ventricle with subpulmonary ventricular septal defect (see Chapter 40 and Fig. 33-5 ). Clinical recognition of straddling, as opposed to overriding, of atrioventricular valves was not possible prior to the advent of cross sectional echocardiography. Until recently, straddling tended to be inferred when angiography demonstrated that the valvar orifice was overriding the ventricular septum. As overriding or straddling can exist in isolation, there were obvious diagnostic limitations. Straddling valves in the context of the congenitally malformed heart, therefore, should be considered relatively rare, but highly significant, malformations. In terms of aetiology, straddling mitral and tricuspid valves have been produced experimentally in rat fetuses exposed to teratogens, 7 while straddling tricuspid valve was produced by preventing expansion of the right atrioventricular junction. 8
MORPHOLOGY AND DIAGNOSIS
Although straddling right or left atrioventricular valves can be found with any segmental combination, 1,6,9 description and diagnosis are simplified if they are considered in five series of malformations. In four of these series, either the right or left atrioventricular valve is straddling and overriding in the setting of right hand or left hand ventricular topology, respectively. As we have discussed, a common valve can also straddle and override with either right hand or left hand ventricular topology, and can show all the extremes between double inlet and biventricular atrioventricular connections. As explained, these variants with common valve will not be further discussed in this chapter, although it would be entirely appropriate to include such details. The fifth series is the uniatrial but biventricular atrioventricular connection. 6,9 As we will see, there are also several morphological variants to be found within this prototype, depending on ventricular topology and the side of absence of the atrioventricular connection. First, we account for the four patterns in which either the right or left atrioventricular valve is straddling.
While it might appear self-evident that straddling of an atrioventricular valve implies the presence of a ventricular septal defect, hearts are found in which an overriding atrioventricular valve straddles the septum, but the valvar leaflets are firmly adherent to the septal crest as they straddle. In such cases, a bridging tongue between the straddling leaflets usually creates two orifices within the overriding valve, with one valvar orifice then opening into each ventricle. This situation is comparable to atrioventricular septal defect with common atrioventricular junction guarded by separate right and left atrioventricular valves (see Chapter 27 ).
Straddling Morphologically Tricuspid Valve in the Setting of Right Hand Ventricular Topology
As is shown in Figure 33-12 , the extremes of this spectrum are concordant atrioventricular connections (see Fig. 33-12A ) and double inlet left ventricle with right-sided incomplete right ventricle (see Fig. 33-12B ). The essence of this series is that the valve usually overrides the postero-inferior part of a malaligned ventricular septum which does not extend to the crux (see Fig. 33-3 ). When the atrioventricular connections are concordant, the septum joins the atrioventricular junction in its postero-inferior quadrant, whereas when the connection is basically double inlet, the septum joins the junction more or less at the acute margin. When the degree of override is approximately equal, the septum joins the junction halfway between the crux and the acute margin ( Fig. 33-13 ). Irrespective of the precise atrioventricular connection, the septum does not reach the crux, and is malaligned relative to the atrial septum. Because of this, there is an abnormal atrioventricular conduction system ( Fig. 33-14 ). The atrioventricular node is formed at the site where the ventricular septum joins the atrioventricular junction, the bundle branches being disposed astride the postero-inferior part of the septum. We have, on one occasion, seen straddling of the tricuspid valve through a muscular inlet defect, in the absence of override of the atrioventricular junction. In this setting, the atrioventricular connections were concordant, and the atrioventricular bundle arose from a regular atrioventricular node located in the atrial septum at the apex of the triangle of Koch.
