6.2 Atrioventricular Septal Defects (AVSD / Endocardial Cushion Defects)

Embryology and Pathophysiology

Atrioventricular septal defects (AVSD), historically referred to as endocardial cushion defects, arise from a developmental failure during the 4th to 5th week of gestation. Normally, the superior and inferior endocardial cushions fuse to form the central core of the heart—the crux. This critical junction separates the atria from the ventricles and forms the lower portion of the atrial septum (ostium primum), the upper/posterior portion of the ventricular septum (inlet VSD), and contributes heavily to the formation of the mitral and tricuspid valves.

When the endocardial cushions fail to fuse properly, the result is a spectrum of defects ranging from partial to complete forms. The most defining hemodynamic consequence postnatal is a massive left-to-right shunt leading to heart failure and a severe risk of early, irreversible pulmonary hypertension (Eisenmenger syndrome) if left uncorrected.

Classification of AVSD

The severity and anatomical arrangement of an AVSD depend on the extent of the endocardial cushion deficiency.

1. Complete AVSD

The most severe and classic presentation. A complete AVSD encompasses a common atrioventricular junction characterized by:

• An ostium primum ASD.
• An inlet VSD.
• A common, single atrioventricular valve spanning across both the right and left ventricles, replacing the distinct mitral and tricuspid valves. The common valve typically consists of five leaflets (anterior bridging, posterior bridging, left mural, right mural, and right anterosuperior).

2. Partial AVSD

In a partial AVSD, the endocardial cushion defect is less extensive. The central structures fuse sufficiently to create two separate AV valve orifices (mitral and tricuspid), but the valves themselves are abnormal.

• It features an ostium primum ASD.
• Crucially, there is no VSD. The ventricular septum is intact.
• The mitral valve typically exhibits a "cleft" in its anterior leaflet, leading to variable degrees of mitral regurgitation.

3. Intermediate AVSD

An intermediate or transitional AVSD is a rarer variant that shares features of both. There are two distinct AV valve orifices, but a small inlet VSD is present alongside the ostium primum ASD.

The Rastelli Classification

For complete AVSDs, the Rastelli classification provides surgical relevance by describing the morphology and chordal attachments of the anterior bridging leaflet of the common AV valve:

• Type A: The anterior bridging leaflet is divided over the ventricular septum and its chordae attach specifically to the crest of the ventricular septum. This is the most common form and is highly associated with Trisomy 21.
• Type B: The anterior bridging leaflet is undivided and its chordae attach to an anomalous papillary muscle situated on the right side of the ventricular septum. This is the least common type.
• Type C: The anterior bridging leaflet is undivided, "free-floating," and unattached to the ventricular septum, inserting entirely onto the anterior papillary muscle of the right ventricle. This form is frequently associated with other complex heterotaxy syndromes and presents significant surgical challenges.

Sonographic Evaluation in Utero

The diagnosis of an AVSD is primarily made using the apical four-chamber view. The defining sonographic features include:

1. Loss of the Cardiac Crux: Normally, the tricuspid valve inserts slightly more apically than the mitral valve (normal offset). In an AVSD, this offset is completely lost. The AV valves insert at the exact same level, creating a linear, "T-shaped" or horizontal line across the heart instead of the normal "cross" appearance.
2. Common Valve in Diastole: During ventricular diastole, the single common AV valve opens widely, creating a massive, central empty space in the middle of the heart, connecting all four chambers simultaneously.
3. Color Doppler Flow: Color Doppler will clearly demonstrate flow crossing a single large annulus, often with a large central jet of regurgitation extending back into the atria during systole.
4. Disproportionate Ventricles: While AVSDs are typically "balanced" (equal-sized ventricles), an "unbalanced" AVSD occurs when the common valve opens predominantly into one ventricle, resulting in hypoplasia of the other (e.g., right-dominant or left-dominant AVSD).

Genetic and Clinical Associations

The association between a complete AVSD and chromosomal abnormalities is exceptionally strong.

• Trisomy 21 (Down Syndrome): More than 50% of fetuses diagnosed with a complete AVSD have Trisomy 21. Conversely, nearly 40-50% of individuals with Trisomy 21 have congenital heart disease, with AVSD being the most frequent.
• Heterotaxy Syndromes: Complete AVSD, particularly Rastelli Type C, is a hallmark of isomerism (both right and left atrial isomerism).
• Fetal ECG: In postnatal and specialized fetal electrocardiography, AVSD classically presents with a left axis deviation (superior QRS axis) and a prolonged PR interval, reflecting the displacement of the AV node due to the absent crux.