Developmental considerations

Prior to the formation of the endocardial cushions, cardiac jelly expands in the region between the atria and the primitive ventricles, the so‐called atrioventricular canal. The endocardial cushions are the primordia of the valves and septa of the developed heart which form as a result of epithelial to mesenchymal transformation [7]. The process of normal atrioventricular septation requires: (i) fusion of four mesenchymal masses or “cushions” in the middle of an initially common atrioventricular junction or canal; (ii) a mesenchymal cap on the leading edge of the (primary) atrial septum; and (iii) normal formation of the dorsal mesenchymal protrusion or vestibular spine (Figure 16.1). The process of fusion needs to take place along with rightward expansion of the atrioventricular junction and perforation of the superior margin of the primary atrial septum in order to create both normal atrioventricular connections and alignments as well as access of oxygenated blood from the ductus venosus to the left heart through the oval fossa. Once fused, the four mesenchymal cushions form an anchor point around which separate right and left atrioventricular junctions can expand, off‐setting of the atrioventricular valves can develop and the aortic root can move into a central “wedged” position. The cushions themselves form the fibrous or membranous components of the atrioventricular septum, including the central fibrous body, as well as the septal leaflet of the tricuspid and anterior leaflet of the mitral valves. Abnormal development of these components results in CAVC. Detailed description of development of the endocardial cushions can be found in several publications [7–9].

Anatomy

Common atrioventricular canal defects are categorized in several ways. The deficiency in the atrioventricular septum is typically the same in all forms, with the relationship of the atrioventricular valve within that defect being variable. Indeed, if the atrioventricular valve leaflets are removed from a heart specimen with CAVC, it is not possible to distinguish between the various subtypes. The complete form of CAVC is characterized by an interatrial communication anteroinferior to the margin of the fossa ovalis and adjacent to the atrioventricular valves, along with a large posterior VSD along the septal leaflet of the atrioventricular valve extending into the membranous septum, giving the ventricular septum a scooped‐out appearance (Figures 16.2 and 16.3). The atrial and ventricular communications are contiguous with each other and only divided by the atrioventricular valve as a result of absence of the canal portion of the septum. Instead of two distinct atrioventricular valves with separate annuli or junctions, there is a common atrioventricular valve with a common annulus. Typically there are two mural or lateral leaflets positioned exclusively over the right ventricle and one mural leaflet positioned exclusively over the left ventricle that can be variable in size [10].

In the normal heart, the aorta sits wedged between the mitral valve and the tricuspid valve annuli (Figure 16.4a). In CAVC, there are two leaflets that bridge the crest of the ventricular septum in the superior and inferior position (Figure 16.4b). The common annulus results in anterior displacement (“unwedging”) of the aortic outflow tract. This displacement elongates the left ventricular outflow tract, making it vulnerable to obstruction (Figure 16.5) [11]. The inferior bridging leaflet has extensive chordal attachments to the crest of the ventricular septum while the chordal attachments of the superior bridging leaflet are variable and have been subclassified by Rastelli and colleagues (Figure 16.6) [12]. In Rastelli type A, the superior bridging leaflet is divided at the crest of the ventricular septum; some describe this as minimal bridging (Figure 16.7a and c). In type B arrangement, the rarest form, division of the superior bridging leaflet occurs at a right ventricular papillary muscle which is often displaced onto the septal surface of the right ventricle. In type C, the superior bridging leaflet is undivided or “free floating;” it can also be described as extreme bridging. This is the most rudimentary form of CAVC and common in Down syndrome (Figure 16.7b and d). Rastelli type A is more commonly associated with preoperative left ventricular outflow obstruction in part because of increased elongation of the outflow tract [13].

The complete form of CAVC is typically seen in children with Down syndrome but can be seen in children with normal chromosomes as well. In a more unusual form of complete CAVC, the atrioventricular valve sits superiorly in the atrioventricular septal defect attached to the atrial septum such that the atrial communication is trivial or absent and the ventricular communication is large (Figure 16.2). Even rarer is the CAVC defect where atrioventricular valve tissue fills in the atrial and ventricular communications and there is no shunting whatsoever.

The partial (also known as incomplete) form of CAVC is characterized by an interatrial communication in the canal portion of the atrial septum (located between the anteroinferior margin of the fossa ovalis and the atrioventricular valves) with a common atrioventricular valve annulus divided into two atrioventricular valve orifices at the ventricular septum (Figure 16.2c). This subtype is often called ostium primum atrial septal defect. However, this term does not fully describe the atrioventricular valve abnormality or the fact that the defect in the atrioventricular septum is the same as in the complete form of CAVC. The distinction is that the atrioventricular valve tissue is adherent to the crest of the ventricular septum forming a connecting tongue, thus allowing no ventricular level shunt and dividing the common annulus into two orifices (Figure 16.3). Although the two orifices are separate, the valves do not resemble normal tricuspid and mitral valves. In particular, the left atrioventricular valve has a trifoliate rather than a bifoliate appearance. Where the superior and inferior bridging leaflets meet at the ventricular septum is often called a “cleft” but is usually not a true deficit of tissue but rather a commissure (Figure 16.8). Atrioventricular valve regurgitation at the level of apposition of the superior and inferior bridging leaflets (the so‐called “cleft”) in CAVC is typical.

There is an intermediate form in the spectrum of CAVC known as transitional type typified by significant chordal attachments of the superior bridging leaflet to the ventricular septum with restrictive shunting through these chordae at the ventricular level. The VSD component is typically small enough that the right ventricle is at subsystemic pressure. Other rare intermediate types include variants where there is a large atrial and ventricular component but the superior and inferior bridging leaflets are fused together resulting in a common annulus with two atrioventricular valve orifices.

In CAVC, the relationship of the atrial septum to the common atrioventricular valve may be balanced or unbalanced. In the balanced form, the atrial septum sits over the ventricular septum with relatively equal distribution of atrial inflow to the right and left side of the atrioventricular valve. Rarely, the atrial septum is malaligned in relation to the ventricular septum (known as “double‐outlet atrium”) such that the atrial septum is deviated predominantly towards the left atrial inflow (double‐outlet right atrium) or the right atrial inflow (double‐outlet left atrium) (Figure 16.9). Inflow into the contralateral ventricular may be limited in some cases.

Maldistribution of the atrioventricular valve over the ventricles occurs much more commonly than double‐outlet atrium and accounts for 10% of all cases of CAVC [14]. When the atrioventricular valve sits more over one ventricle than the other, the contralateral ventricle is typically hypoplastic (Figures 16.9 and 16.10). Unbalance to the right (with left ventricular hypoplasia) is more common and is often associated with other levels of left‐sided obstruction such as subaortic stenosis or coarctation of the aorta. In its most extreme form, unbalance to the right is a variant of hypoplastic left heart syndrome, sometimes with no left ventricular cavity seen. Unbalance to the left (with right ventricular hypoplasia) can be associated with pulmonary outflow obstruction.

In CAVC, the left ventricular anterolateral papillary muscles are often more closely spaced than normal [15]. Left atrioventricular valve abnormalities can also be seen in approximately 5% of CAVC, most typically double‐orifice or single papillary muscle (parachute) types [16]. These lesions are more common in partial CAVC and can also be seen in association with unbalanced defects to the right with concomitant left ventricular hypoplasia. Such valve abnormalities may complicate surgical repair with possible residual left atrioventricular valve regurgitation, stenosis, or both [16,17].

Frequently associated lesions include tetralogy of Fallot (usually seen in children with Down syndrome), patent ductus arteriosus, coarctation of the aorta, and left ventricular outflow tract obstruction. Ostium secundum atrial septal defects and additional VSDs can also be seen. In heterotaxy/isomerism, CAVC is common particularly in the right isomerism (asplenia) type.

Inlet‐type VSD (otherwise known as atrioventricular canal‐type VSD) can exist as an entity without a common atrioventricular junction. These are defects along the septal leaflet of the tricuspid valve often with straddling of the tricuspid valve chordae into the left ventricle. Often the ventricular septum is malaligned from the atrial septum with concurrent right ventricular hypoplasia. These defects are frequently seen in association with complex congenital heart disease.