The cleft in CAVCD is located between the left components of the superior and inferior bridging leaflets. This “cleft” in CAVCD is more accurately called as the “zone of apposition” between the left ventricular components of the superior and inferior bridging leaflets, because it is embryologically and morphologically distinct from an isolated cleft of a normal mitral valve. Left-sided superior bridging leaflet forms the anterior-superior component of the left atrio ventricular (AV) valve?. Left-sided inferior bridging leaflet forms the posterior-inferior component of the left AV valve?. The gap or the deficient zone between these two left-sided components? opening into the left atrium constitutes the zone of apposition, which points towards the ventricular septum (in contrast to an isolated mitral cleft, which typically points towards the aortic root/ anteriorly).¹ Clefts are present in all patients with complete CAVCD. The cleft prevents adequate coaptation of the left-sided leaflet components? and creates an anatomical defect where mitral regurgitation occurs?. Thus, this zone is the most important source of left AV valve regurgitation in CAVCD?. The commissure is anchored to the crest of the interventricular septum by chordae or a “tongue” of tissue in the Rastelli classification, and the degree of bridging of the superior leaflet defines Rastelli types A, B, and C.

A cleft between the left and right bridging leaflets would be in the midline between right and left ventricles? and at the ventricular level, not the atrial level?.² Thus, the cleft does not separate the left and right superior bridging leaflets (option “a”). The plane of apposition between the right and left components of the bridging leaflets lies over the ventricular septum and does not create the left-sided regurgitant gap.

CAVCD does NOT have a typical anterior and posterior mitral leaflet like isolated mitral valve disease.² CAVCD has a common AV valve with bridging structures? that cross the septum, not separate mitral and tricuspid valves?. A cleft between "anterior" and "posterior" mitral leaflets would describe isolated cleft mitral valve (non-CAVCD), with normal mitral commissure anatomy?, not the pathology of CAVCD?. Thus, CAVCD cleft involves bridging leaflet components, not traditional mitral leaflet nomenclature? (option “c”). Further, the normal left AV valve in CAVCD is trifoliate, so the “anterior/posterior” mitral nomenclature does not apply.

The inferior bridging leaflet is generally less involved in the mitral regurgitation compared to the left superior bridging leaflet. The cleft in CAVCD does NOT separate the inferior bridging leaflets between right and left sides. This would place the cleft at a midline/ventricular level, not the characteristic left atrial location?. The inferior bridging leaflet cleft (between left and right components) would not be the primary mitral regurgitation mechanism?.² The characteristic CAVCD pathology, therefore, involves the left-sided zone of apposition between the superior and inferior bridging leaflets, not a right-left-sided division (option “d”).?

REFERENCES

1. Calabrò R, Limongelli G. Complete atrioventricular canal. Orphanet J Rare Dis 2006;1:8.
2. Adachi I, Uemura H, McCarthy KP, et al. Surgical anatomy of atrioventricular septal defect. Asian Cardiovasc Thorac Ann 2008;16:497-502.