Atrioventricular septal defect (AVSD)
Major etiologies of AVSD
1. Chromosomal abnormalities
- Trisomy 21 (Down syndrome)
- Most common etiology overall
- Present in ~40–50% of fetuses with AVSD
- Especially complete AVSD
- Often associated with:
- Duodenal atresia (double bubble)
- Mild ventriculomegaly
- Short long bones
- Echogenic bowel
- Increased NT in first trimester
2. Syndromic and genetic causes
- Ellis–van Creveld Syndrome
- Short limbs
- Postaxial polydactyly
- Narrow thorax
- Noonan Syndrome
- CHARGE Syndrome
- Heterotaxy Syndrome
- Especially left atrial isomerism
- Often with interrupted IVC
- Abnormal cardiac situs
- Smith–Lemli–Opitz Syndrome
3. Sporadic developmental cause
Occurs due to abnormal development of:
- Endocardial cushions
This is the fundamental embryologic basis of AVSD:
- Failure of fusion of superior and inferior endocardial cushions
- Leads to:
- Common AV valve
- Septal defects at atrial and ventricular level
Most isolated AVSDs without syndromes fall into this group.
4. Maternal/environmental factors
These are weaker associations but relevant:
- Maternal diabetes
- Maternal obesity
- Retinoic acid exposure
- Poorly controlled phenylketonuria
Types of AVSD
Complete AVSD
- Strongest association with Trisomy 21
- Often detected prenatally
- More likely syndromic
Partial AVSD
- May be isolated
- Lower chromosomal risk than complete AVSD
Transitional AVSD
- Intermediate features
- Genetic risk depends on associated findings
Important combined associations
When you see AVSD + double bubble, think strongly of:
- Trisomy 21
Prenatal risk estimates to remember
- AVSD → Trisomy 21 risk ~40–50%
- AVSD + additional anomalies → Risk increases significantly
- Isolated AVSD → Still ~30–40% chromosomal risk