Introduction
This group of defects is characterized by a defect in the centre of the heart (the atrio-ventricular junction) with a single valve structure straddling the centre of the heart. Thus there is no true mitral or tricuspid valve; rather there is a left and right component of the common valve which is made up a mural leaflets and bridging leaflets – so called as they bridge the ventricular septum (Figure 10.1). Overall they account for 4 to 5 per cent of all congenital heart disease. The defects are strongly associated with Down syndrome, accounting for up to 80 per cent of all complete atrio-ventricular septal defects (AVSDs) in the United Kingdom (40 per cent of all Down syndrome cases have an AVSD). These defects have also been called ‘endocardial cushion defects’, reflecting their embryological development, and ‘A-V canal defects’, which, although still used, is not morphologically correct.
Morphology.
There is a spectrum of disease that most importantly focuses on the extent of the VSD component: if the VSD is shallow or small, then the bridging leaflets of the valve become fused to the crest of the ventricular septum, obliterating the VSD – thus, the only defect is above the valve at the atrial level in the form of a primum ASD. This is referred to as a ‘partial AVSD’ (although also called ‘primum ASD’). However, with larger VSD components, there is a defect both below and above the A-V valve creating a ‘complete AVSD’, with the VSD having a characteristic semi-circular shape and involving the peri-membranous septum, extending from the inlet to the outlet. Occasionally, the result is an ‘intermediate AVSD’, where the majority of the VSD component has been sealed off by the A-V valve tissue but there is still a small shunt at the ventricular level where the bridging leaflets gape at their meeting point in the middle of the defect. The valve morphology is extremely variable, particularly in the septal attachments of the superior bridging leaflet (defined by the Rastelli classification), and the leaflets themselves are variable in both number and quality, which can lead to varying degrees of valvar incompetence. Rarely there can be associated right ventricular outflow tract (RVOT) obstruction with deviation of the outlet septum creating ‘AVSD/Fallot’.
Pathophysiology and Presentation.
These depend on the extent of the VSD component and the degree of associated A-V valve incompetence. Large defects create a significant ventricular shunt with volume loading of the circulation and congestive cardiac failure with pulmonary plethora. The volume load on the heart is exacerbated by any associated A-V valve regurgitation. These lesions have a tendency to develop early pulmonary vascular hypertension if untreated, particularly in association with Down syndrome. Thus, presentation is usually in early infancy or the neonatal period with heart failure, tachypnoea and failure to thrive. In contrast, the haemodynamic lesion in partial AVSD is an atrial shunt, and as with other ASDs, these rarely produce symptoms in childhood. However, if there is associated A-V valve regurgitation, this can cause congestive heart failure. AVSD/Fallot may be a more balanced lesion, the RVOT obstruction protecting from heart failure – but it may produce cyanosis and spelling depending on the degree of obstruction (see Chapter 11).
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