AVCD Morphological Classification
Atrioventricular canal defects (AVCD) are classified into three main morphological types: partial AVCD (with only a primum ASD and typically a cleft left AV valve), complete AVCD (with both primum ASD and inlet VSD plus a common AV valve), and transitional/intermediate AVCD (with incomplete atrial and ventricular septal defects and/or incomplete abnormalities of the common AV valve). 1
Primary Anatomic Classification
The fundamental morphological categorization divides AVSDs into:
Partial AVSD: Contains only a primum ASD component with a cleft left atrioventricular valve (also called ostium primum defect) 1, 2
Complete AVSD: Includes both primum ASD and inlet VSD with a common atrioventricular valve 1, 3
Transitional/Intermediate AVSD: Features incomplete atrial and ventricular septal defects and/or incomplete abnormalities of the common atrioventricular valve 1, 2
Rastelli Classification System
For complete AVSDs, the Rastelli classification (Types A, B, and C) describes anatomic variations of the superior bridging leaflet of the atrioventricular valve, which is critical for surgical planning 1, 3, 4. This classification focuses on how the anterior bridging leaflet divides and attaches to the ventricular septum.
Ventricular Balance Assessment
Beyond the basic morphological types, AVSDs must be characterized as balanced or unbalanced based on the relative sizes of the ventricles, as this determines whether biventricular or single ventricle repair is feasible 1:
- Balanced AVSD: Both ventricles are adequately sized for biventricular repair
- Unbalanced AVSD: The common AV valve leads primarily into either the right ventricle (RV-dominant, more common at 10.7%) or left ventricle (LV-dominant, less common at 3.4%) 5
Key Anatomic Components
All AVSDs share these fundamental morphological features 1, 3:
- Primum atrial septal defect
- Inlet ventricular septal defect (variable in extent)
- Common atrioventricular valve (or cleft left AV valve in partial forms)
Clinical Distribution
In large series, complete AVSD is most common (68.4%), presenting earlier in infancy (mean 1.6 months), while partial AVCD accounts for 20.4% and presents later (mean 9.2 months), with transitional forms comprising 11.2% 5. There is a strong association with Down syndrome (34% overall, 79% of complete forms) 5.
Associated Anomalies
AVSD frequently occurs with other congenital lesions (46% of cases), most commonly conotruncal anomalies (18%), secundum ASD (14.1%), anomalous pulmonary venous connection (11.2%), and heterotaxy syndrome (11.2%) 5. It can also be associated with tetralogy of Fallot and coarctation of the aorta 1.