Atrioventricular Canal Defect

AVC defects include a spectrum of lesions ranging from an isolated primum to complete defects of the atrioventricular canal, commonly referred to as endocardial cushion defect.

The defect causes a spectrum of atrioventricular outlet arrangements. In the complete form, persistent common atrioventricular canal, the tricuspid and mitral valve are fused in a large single atrioventricular valve that opens above the bridges of the two ventricles. This valve has an anterior and a posterior leaflet. In the incomplete form, various amounts of tethering of one or both leaflets to the crest of the interventricular septum lead to a connection that creates functional atrial or ventricular septal defects, ventriculoatrial septal defects or valvular abnormalities.

Incidence: AVC accounts for 7% of structural heart defects.

Sonographic findings:

 Fig 1

  • Complete defect is usually easy to diagnose by demonstrating an obvious deficiency of the central core structures of the heart on FCV.
  • Increased nuchal translucency thickness at 10-14 weeks of gestation and nuchal translucency can be a useful early screening for CHD.
  • The atria may be dilated as a consequence of atrioventricular insufficiency.
  • The incomplete forms are more difficult to recognize. The tricuspid and mitral valve can attach at the same level at the crest of the septum. This apical displacement of the mitral valve elongates the left ventricular outflow tract.
  • Color Doppler facilitates the visualization of the central opening of the single atrioventricular valve; the outcome is adversely affected when the AV valve is regurgitant.
  • The atrial septal defect is of the ostium primum type (because the septum secundum is not affected) and thus is close to the crest of the interventricular septum.
  • Though it is detectable by FCV, the detection rate is currently <50%. Live 3D ultrasound may be helpful.
  • Usually diagnosable after 16-18 weeks but possible after 10-14 weeks with transvaginal ultrasound.
  • Pitfalls:
    • False dropout can occur from apical FCV, therefore, it is important to image the defects in a scan plane perpendicular to the septum.
    • A dilated coronary sinus may be mistaken for primum ASD.

Fig 1:  Atrioventricular canal  Four-chamber view: common atrium and ventricle, no visible central crux (arrow)

Video clips of Atrial Septal Defect (ASD)

Endocaridal cushion defect:  Four-chamber view: atrial and ventricular septal defect, arrowhead = spine

Endocardial cushion defect :  Four-chamber view: abnormal cardiac axis, atrial septal and ventricular septal defect (arrowhead = spine)

Associations: Chromosome abnormalities in more than 50% of cases with 60% being trisomy 21 and 25% being trisomy 18, also including extracardiac defects and heterotaxy syndrome in particular. Isolated AVC is a higher risk for trisomy 21 than that with associated anomalies.

Management: Careful prenatal and postnatal search for associated anomalies is required. All continuing pregnancies should be karyotyped. Serial sonography should be performed.

Prognosis: Poor when associated with chromosome abnormalities or other anomalies. Isolated AVC can be repaired with a survival rate of more than 80%.