What are the differences between intermediate atrioventricular septal defect (AVSD) and complete AVSD in terms of anatomy, management, and treatment?

Anatomic and Management Differences Between Intermediate and Complete AVSD

Anatomic Distinctions

Intermediate AVSD represents a transitional form between partial and complete AVSD, with incomplete atrial and ventricular septal defects and/or incomplete abnormalities of the common atrioventricular valve, whereas complete AVSD has a septal defect extending into both the interatrial and interventricular septum with a single common atrioventricular valve orifice. 1

Key Structural Features

Partial AVSD:

• Primum ASD component only 1
• Anterior and posterior bridging leaflets are fused centrally, creating separate left and right-sided orifices 1
• Typically has a cleft left atrioventricular valve 1, 2

Intermediate AVSD:

• Incomplete atrial and ventricular septal defects 1
• Incomplete abnormalities of the common atrioventricular valve 1
• Represents a transitional anatomic variant 3

Complete AVSD:

• Both ASD and VSD components present 1
• Single common atrioventricular valve orifice without central fusion of bridging leaflets 1
• Septal defect at the crux of the heart extending into both interatrial and interventricular septum 1
• Common AV annulus guarded by five leaflets 1

Shared Anatomic Features

All AVSD variants demonstrate:

• Abnormal AV node position (posterior and inferior to coronary sinus) 1
• Displaced bundle of His and left bundle branch posteriorly 1
• Superior left-axis deviation with counterclockwise loop on ECG 1

Clinical Presentation Differences

Complete AVSD typically presents earlier and with more severe symptoms than intermediate AVSD due to the larger shunt burden and risk of early pulmonary vascular disease. 4

Complete AVSD Presentation:

• Median age at diagnosis: 7.5-8.6 months 4
• High risk of early pulmonary arterial hypertension if unrepaired 1
• Requires repair typically before 6 months of age to prevent irreversible pulmonary vascular disease 1
• May present with severe PAH, single loud second heart sound, and cyanosis/clubbing if unoperated 1

Intermediate AVSD Presentation:

• Median age at surgical intervention: 10.5-17.8 months 4
• Usually asymptomatic, presenting at preschool age 5
• Less urgent repair timeline compared to complete AVSD 5

Management and Treatment Approach

All intermediate and complete balanced AVSDs require surgical correction, with early repair essential for complete AVSD to prevent pulmonary vascular obstructive disease. 3

Surgical Indications

For Complete AVSD:

• Surgery recommended with net left-to-right shunt (Qp:Qs ≥1.5:1), PA systolic pressure <50% systemic, and pulmonary vascular resistance <1/3 systemic 1, 2
• Early repair (typically <6 months) mandatory to prevent irreversible pulmonary vascular disease 1
• Surgery contraindicated with PA systolic pressure >2/3 systemic, pulmonary vascular resistance >2/3 systemic, or net right-to-left shunt 1

For Intermediate AVSD:

• Surgical closure indicated with significant volume overload of the right ventricle 1
• Less urgent timeline allows for repair at preschool age if asymptomatic 5

Surgical Technique Differences

Complete AVSD repair:

• Requires closure of both ASD and VSD components 3
• Reconstruction of common atrioventricular valve 3
• Two-patch technique commonly used for VSD closure 6, 7
• More complex valve reconstruction due to single common valve 3

Intermediate AVSD repair:

• Closure of incomplete septal defects 1
• Repair of incomplete valve abnormalities 1
• Generally less complex valve reconstruction 5

Surgical Outcomes

Modern surgical techniques have equalized early mortality between complete and partial/intermediate AVSD:

• Complete AVSD: 2.5% early mortality, 7.5% reoperation rate 6
• Partial AVSD: 0% early mortality, 13.0% reoperation rate 6

Long-Term Complications and Follow-Up

Both intermediate and complete AVSD require lifelong follow-up at specialized adult congenital heart disease centers, with surveillance focused on residual shunts, AV valve dysfunction, LVOT obstruction, and arrhythmias. 1, 2

Common Late Complications (Both Types):

• Left AV valve regurgitation or stenosis (5-10% requiring reoperation) 1
• LVOT obstruction (5% of patients) 1
• Development of heart block (can occur up to 15 years post-surgery) 1
• Atrial arrhythmias in older patients 1

Follow-Up Intervals by Physiological Stage:

• Stage A (no hemodynamic abnormalities): Every 24-36 months with ECG and TTE 1, 2
• Stage B (mild abnormalities): Every 24 months 1
• Stage C (moderate abnormalities): Every 6-12 months 1
• Stage D (severe abnormalities): Every 3-6 months 1

Surveillance Requirements:

• Transthoracic echocardiography to assess residual shunts, AV valve function, ventricular size/function, and pulmonary artery pressures 1, 2
• ECG monitoring for conduction abnormalities and arrhythmias 1
• Exercise testing to objectively assess functional capacity 1
• Cardiac MRI when additional quantification needed or echocardiographic windows inadequate 1

Critical Management Pitfalls

The most critical error is delaying repair of complete AVSD beyond 6 months of age, which allows irreversible pulmonary vascular disease to develop and precludes surgical correction. 1

• Failure to distinguish residual LV-to-right atrial shunt from tricuspid regurgitation with RV hypertension may result in erroneous diagnosis of PAH 1
• Complete AVSD patients with Down syndrome require special attention due to increased risk of pulmonary hypertension 4
• Surgery must be avoided in patients with Eisenmenger physiology; PVR testing recommended when in doubt 1