6.3 Complete, Partial & Transitional Atrioventricular Canal (AV Septal) Defects

6.3 Complete, Partial & Transitional Atrioventricular Canal (AV Septal) Defects

Clinical Core: Atrioventricular Septal Defects (AVSDs), historically termed endocardial cushion defects or AV canal anomalies, represent a complex morphological spectrum encompassing deficient septation at the cardiac crux. The pathognomonic diagnostic hallmarks—absence of normal AV valve offsetting, a common AV junction, and the "goose-neck" LVOT deformity—must be thoroughly understood by pediatric cardiac sonographers.

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Embryological Mechanisms & Foundational Morphological Hallmarks

Between the fourth and fifth embryonic weeks, the superior and inferior endocardial cushions expand within the common atrioventricular canal and fuse along the midline. This fusion accomplishes four developmental milestones:

1. Partitions the common AV canal into separate right (tricuspid) and left (mitral) valve orifices.
2. Contributes mesenchymal tissue to the basal portion of the anterior mitral valve leaflet and septal tricuspid leaflet.
3. Contributes to the inferior closure of the septum primum (closing the ostium primum).
4. Seals the crest of the muscular inlet ventricular septum.

Failure of cushion fusion results in an invariant cluster of four primary anatomical hallmarks:

                    [Common AV Junction]
                             │
        ┌────────────────────┼────────────────────┐
        ▼                    ▼                    ▼
 [Loss of Normal]    [Scooped-Out Inlet]   [Anterior Unwedged]
 [AV Valve Offset]   [Ventricular Septum]   [Aortic Valve]
 (Co-planar Valves)   (Crescentic Crest)   ("Goose-Neck" LVOT)

1. Common Atrioventricular Junction

In the normal heart, the tricuspid and mitral valves possess distinct, separate fibrous annuli. In AVSD, there is a single, common atrioventricular junctional ring shared by both ventricles, even when the valve orifice itself is partitioned into two functional components.

2. Loss of Normal Atrioventricular Valve Offsetting

In a normal heart, the septal leaflet of the tricuspid valve inserts into the ventricular septum more apically than the anterior mitral valve leaflet, creating a normal apical displacement (offsetting) of 5 to 8 mm/m². In all variants of AVSD, this offset is completely lost: both the right and left AV valve components insert into the crest of the ventricular septum at exactly the same horizontal level (co-planar insertion). The presence of co-planar AV valve insertion in the apical four-chamber view is a definitive diagnostic hallmark.

3. "Scooped-Out" Inlet Ventricular Septum

The crest of the inlet ventricular septum fails to develop fully, leaving a deep, smooth, crescentic deficiency. The distance from the cardiac apex to the crest of the inlet septum is significantly shortened relative to normal anatomy.

4. Elongated, Narrowed LVOT ("Goose-Neck" Deformity)

In normal cardiac development, the aortic valve is "wedged" deeply between the mitral and tricuspid valve rings. In AVSD, the common AV junction displaces posteriorly and inferiorly, forcing the aortic root to be displaced anteriorly and superiorly (unwedged). Consequently:

• The distance from the ventricular apex to the aortic valve is significantly greater than the distance from the apex to the AV valve junction.
• The left ventricular outflow tract (LVOT) becomes elongated, narrow, and tubular.
• On subcostal long-axis, apical five-chamber, and angiographic projections, this narrowed, elongated tract produces the pathognomonic "goose-neck" deformity.
• This anatomy creates a lifelong substrate for subaortic stenosis, which can develop preoperatively or appear as a late complication after surgical patch repair.

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Classification Spectrum of AVSD

AVSD spans a wide anatomical continuum depending on the presence of a ventricular defect and the degree of bridging leaflet fusion:

1. Partial AVSD (Partial AV Canal)

• Primum Atrial Septal Defect: A large interatrial communication at the base of the atrial septum.
• Intact Ventricular Septum: The inlet ventricular septum is scooped out, but the bridging leaflets are firmly attached to the crest of the septum by a dense fibrous raphe or short chordae, completely preventing ventricular shunting.
• Two Separate AV Valve Orifices: Right and left valve orifices are distinct, partitioned by bridging tissue.
• Cleft in the Left AV Valve (LAVV): The anterior (mitral-like) leaflet has a complete slit or cleft. Crucially, this cleft is an embryologic commissure between the anterior and posterior bridging leaflets that points directly toward the ventricular septum. In contrast, an isolated congenital cleft of a normal mitral valve points anteriorly toward the aortic valve (LVOT). The cleft in partial AVSD almost invariably causes left AV valve regurgitation directed into the left or right atrium.

2. Complete AVSD (Complete AV Canal)

• Primum Atrial Septal Defect: Extensive low interatrial communication.
• Large Inlet Ventricular Septal Defect: Large, unrestrictive communication beneath the bridging leaflets, allowing free interventricular shunting.
• Single Common AV Valve Orifice: A solitary valve opening that spans both the right and left ventricles.
• Five-Leaflet Common Valve Architecture:
  1. Anterior Bridging Leaflet (ABL)
  2. Posterior Bridging Leaflet (PBL)
  3. Left Lateral (Mural) Leaflet
  4. Right Lateral Leaflet
  5. Right Anterior Leaflet

3. Transitional / Intermediate AVSD

• Primum ASD: Always present.
• Restrictive Inlet VSD: The ventricular defect is divided into tiny, hemodynamically restrictive fenestrations by dense, anomalous chordal attachments that tether the anterior and posterior bridging leaflets directly to the crest of the inlet septum.
• Two Functional Orifices: Dense bridging connections create two partially separated functional valve orifices. Hemodynamically, it behaves similarly to a partial AVSD with a small, high-velocity VSD shunt.

4. Unbalanced AVSD

In balanced AVSD, the common AV valve orifice is distributed roughly equally (50:50) between the two ventricles. In unbalanced AVSD, the common junction is committed predominantly to one ventricle:

• Right Ventricle-Dominant AVSD: The common valve is committed primarily to the RV (>60%), resulting in hypoplasia of the left ventricle and the left ventricular outflow tract. Often associated with aortic arch hypoplasia or coarctation.
• Left Ventricle-Dominant AVSD: The common valve is committed primarily to the LV, resulting in hypoplasia of the right ventricle and the right ventricular outflow tract. Often associated with pulmonary stenosis or pulmonary atresia.
• Surgical Consequence: Severe unbalance prohibits standard biventricular two-patch repair. Patients must be routed into a single ventricle staged palliation track (Norwood or Damus-Kaye-Stansel, bidirectional Glenn, and Fontan completion).

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Rastelli Classification of Complete AVSD

The Rastelli classification stratifies complete AVSD into three distinct categories based on the anatomical configuration, division, and chordal attachment of the Anterior Bridging Leaflet (ABL):

       [Rastelli Type A]              [Rastelli Type B]              [Rastelli Type C]
┌─────────────────────────────┐┌─────────────────────────────┐┌─────────────────────────────┐
│ • ABL divided               ││ • ABL divided               ││ • ABL undivided & floating  │
│ • Chords attach to crest of ││ • Chords attach to abnormal ││ • NO septal chordal attach  │
│   ventricular septum        ││   RV papillary muscle       ││ • Free-floating across VSD  │
│ • 70% of cases (most common)││ • <5% of cases (rarest)     ││ • 25% (Linked to Down & TOF)│
└─────────────────────────────┘└─────────────────────────────┘└─────────────────────────────┘

Detailed Rastelli Subtypes

• Rastelli Type A (~70% of complete AVSDs):
  • The anterior bridging leaflet is divided into distinct left and right components.
  • The medial aspect of the leaflet is firmly tethered to the crest of the ventricular septum by multiple short, dense chordae tendineae.
  • The leaflet minimally bridges into the right ventricle.
  • Surgical significance: Most common anatomical variant and technically favorable for surgical division and two-patch repair.
• Rastelli Type B (<5% of complete AVSDs):
  • The anterior bridging leaflet is divided, but its medial chordae do not attach to the ventricular septum.
  • Instead, chordae attach to an abnormal, ectopic papillary muscle in the right ventricle located near the apical moderator band.
  • Surgical significance: The rarest type; challenging to divide surgically without compromising right-sided AV valve competence.
• Rastelli Type C (~25% of complete AVSDs):
  • The anterior bridging leaflet is undivided and completely free-floating across the inlet VSD.
  • It has zero chordal attachments to the ventricular septum.
  • The leaflet extends freely into the RV, anchoring solely to the anterior right ventricular papillary muscle.
  • Surgical significance: Highly associated with Trisomy 21 (Down syndrome) and conotruncal malformations, especially Tetralogy of Fallot (AVSD with TOF occurs almost exclusively with Rastelli Type C).

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Trisomy 21 (Down Syndrome) Association & Accelerated PVOD

• Genetic Incidence: Approximately 40% to 50% of children with Down syndrome (Trisomy 21) have congenital heart disease. Of these, AVSD is the single most common defect, accounting for roughly 60% of cases.
• Accelerated Pulmonary Vascular Obstructive Disease (PVOD): In non-syndromic infants with a large left-to-right shunt, irreversible pulmonary vascular disease typically develops after 12 to 24 months of age. In infants with Down syndrome, however, fixed pulmonary arteriolar changes (medial hypertrophy, intimal proliferation, and plexiform lesions) can develop rapidly as early as 6 to 12 months of age.
• Underlying Mechanisms: Trisomy 21 infants exhibit intrinsic endothelial dysfunction, reduced numbers of pulmonary vascular resistance units, diminished alveolar multiplication, chronic alveolar hypoventilation due to midface hypoplasia/macroglossia, and generalized muscular hypotonia.
• Surgical Timing Rule: Because of this risk of rapid progression to irreversible pulmonary hypertension, infants with Down syndrome and complete AVSD undergo elective complete surgical repair early in infancy, typically between 3 and 6 months of age.

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Comprehensive Echocardiographic Assessment & Ventricular Balance

Complete echocardiographic assessment of AVSD requires a systematic evaluation protocol:

1. AV Valve Regurgitation Severity & Mechanism

• Regurgitation can emerge from the cleft between the anterior and posterior bridging leaflets, the commissure between bridging leaflets and the lateral mural leaflet, or from annular dilatation.
• Color Doppler must assess vena contracta width, proximal isovelocity surface area (PISA), and jet direction (central vs. eccentric into the wall of the LA).

2. Quantification of Ventricular Balance

Determining whether an AVSD is balanced or unbalanced is essential in selecting between biventricular repair and single ventricle palliation:

• Modified Atrioventricular Valve Index (AVVI): Measured in diastole from subcostal en-face or apical four-chamber views:

$\text{AVVI} = \frac{\text{Left-Sided AV Valve Annular Area}}{\text{Total Common AV Valve Annular Area}}$

• Balanced AVSD: $\text{AVVI} = 0.40 - 0.60$ (Predicts successful biventricular repair).
• RV-Dominant Unbalanced AVSD: $\text{AVVI} < 0.33$ (Left ventricle is severely hypoplastic; single ventricle track required).
• LV-Dominant Unbalanced AVSD: $\text{AVVI} > 0.67$ (Right ventricle is severely hypoplastic; single ventricle track required).
• Ventricular Dimension Ratio: Compares left ventricular end-diastolic inflow width and length to the right ventricle. An LV end-diastolic volume index $<15-20\text{ mL/m}^2$ strongly argues against biventricular tolerance.

3. Left Ventricular Outflow Tract Surveillance

• Continuous-wave Doppler interrogation of the LVOT must be performed in every sweep. The elongated "goose-neck" LVOT can develop progressive subaortic narrowing due to accessory bridging leaflet tissue attachments or an abnormal anteriorly displaced muscle bar.

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AVSD Classification Spectrum & Rastelli Anatomy

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Clinical Pearls & Sonographic Traps

[!WARNING] The Isolated Cleft Mitral Valve vs. Partial AVSD Cleft Pitfall: A crucial board-exam distinction:

• In Partial AVSD, the cleft is an embryological space between bridging leaflets that points directly toward the ventricular septum. It is accompanied by a primum ASD, loss of normal AV valve offset, and a scooped-out inlet septum.
• In an Isolated Cleft of the Mitral Valve, the cleft affects a morphologically true anterior mitral leaflet and points anteriorly toward the aortic valve/LVOT. The interatrial septum is completely intact, and normal apical offsetting of the tricuspid valve is fully preserved.

[!TIP] Post-Repair LVOT Obstruction Monitoring: Because all AVSD patients possess an elongated, narrowed "goose-neck" LVOT, surgical patch placement and cleft closure pull the left AV valve closer to the septum. This exacerbates LVOT narrowing. Follow-up pediatric echocardiograms must routinely utilize continuous-wave Doppler across the LVOT from the apical five-chamber and three-chamber views to detect late, progressive subaortic stenosis.

[!NOTE] Timing of Postnatal PVR Drop in AVSD with Down Syndrome: Because infants with Trisomy 21 maintain elevated pulmonary vascular resistance longer than normal infants, clinical signs of heart failure (and murmur intensity) may be delayed until 4 to 8 weeks of age. Lack of overt early distress does not indicate a mild defect; complete echocardiography must be performed promptly at birth.