Atrioventricular Septal Defects (aka AV canal, common atrioventricular canal, endocardial cushion defect)

Betul Yilmaz Furtun, MD

 

An atrioventricular septal defect (AVSD) describes a constellation of defects defined by deficient atrioventricular septation and a common atrioventricular junction. The variability seen in atrioventricular septal defects is then determined by both the position of the common atrioventricular valve within the atrioventricular septal defect as well as the presence or absence of bridging tissue between the superior and inferior bridging leaflets.  
 

AVSD Subtypes

  • AVSD with a common AV junction and single orifice with shunting at the atrial or ventricular levels (also known as a completely common atrioventricular canal). 
  • AVSD with a common AV junction with shunting at the atrial level and restrictive shunting at the ventricular level (also known as a transitional AV canal).
  • AVSD with a common atrioventricular junction with two orifices with bridging leaflets attached to the crest of the ventricular septum with exclusively atrial level shunting with zone of apposition (also known as a partial AV canal with mitral valve cleft)

 

Secondary to deficiencies in both the atrial (primum ASD) and ventricular (inlet VSD) septae, the distance from the crux of the heart to the apex is foreshortened and the distance from the apex to the aorta is increased and results in an elongated LVOT (“goose neck” deformity) with anterior displacement of the aorta.

AVSD accounts for approximately 4% to 5% of infants with CHD and occurs in 0.19 to 0.35 per 1000 live births.  

AVSD may be associated with conotruncal anomalies such as tetralogy of Fallot (TOF) and double outlet right ventricle (DORV).  AVSD can also occur in heterotaxy syndrome and is more common in asplenia (bilateral right atrial isomerism) than in polysplenia (bilateral left atrial isomerism).

Depending on the morphology of the superior leaflet of the common atrioventricular valve, three types of AVSD have been delineated (type A, B and C, according to Rastelli's classification). The anatomic subgroups (Rastelli's type A, B and C) can be classified on the basis of the chordal insertions and morphology of the superior bridging leaflet of the common atrioventricular valve.

 

Rastelli Types

  • Type A:  The superior bridging leaflet demonstrates chordal attachments to the crest of the interventricular septum (~50-70%)
  • Type B:  The superior bridging leaflet attaches past the crest of the ventricular septum to an anomalous papillary muscle on the right side of the ventricular septum (~3%, rare)
  • Type C:  The superior bridging leaflet demonstrates no attachments to the ventricular septum and is referred to as a free floating leaflet (~30%).

 

An AVSD is classified as balanced or unbalanced. In balanced AVSD the common AV valve is equally distributed to the right and left ventricle. An unbalanced AVSD accounts for 10% of all AVSDs and occurs when the AV valve is mostly or primarily committed to one ventricle. Approximately two thirds of unbalanced AVSDs are RV dominant. RV-dominant AVSD is frequently seen with hypoplastic LV, aortic valve hypoplasia, LVOT obstruction and aortic arch anomalies such as coarctation of the aorta. Similarly, in LV-dominant AVSD, the RV is hypoplastic, and there are associated RVOT obstruction and pulmonary stenosis or atresia.

 

Genetic Considerations

  • Trisomy 21: ~45% of patients with trisomy 21 have an AVSD.  Therefore, for any fetus with an AVSD, trisomy 21 should be high on the differential
  • Ellis van Creveld: Common atrium and common AV valve (no VSD).  This disorder has an autosomal recessive inheritance
  • Heterotaxy Syndrome:  AVSD can occur as a common associated CHD lesion in this syndrome (more common in right atrial isomerism)

 

Fetal Imaging Pearls:

  • Defect must be seen at the level of the AV valves because they serve as borders of this defect and AV valve leaflets insert at the same level at the cardiac crux.
  • Four-chamber view diagnostic given that the ASD, VSD, and common AV valve are seen.
  • Carefully assess for a primum ASD, inlet VSD (four-chamber view) and common AV valve (AV valve leaflets insert at the same level at the cardiac crux (four chamber view) and en face imaging in the short-axis view).
  •   Zone of apposition (or cleft) in the left AV valve component is directed toward the ventricular septum (en face imaging in the short-axis view).
  • LV papillary muscles are rotated counterclockwise and usually closely spaced (en face imaging in the short-axis view).
  • LVOT has an elongated appearance (“goose-neck” deformity) (long-axis LVOT view).
  • If common AV is committed mostly or primarily to one ventricle; consider diagnosis of unbalanced AVSD and assess for associated outflow tract and semilunar valve obstruction (detailed above).
  • In RV dominant AVSD evaluation for coarctation of the aorta is required (3V view and/or aortic arch view).
  • Common AV valve with two separate orifices in the single AV valve annulus with presence of a primum ASD and and zone of apposition (cleft MV) and absent ventricular shunting is diagnostic for a partial AVSD (four-chamber view).
  • Transitional AVSD is demonstrates a restrictive VSD or multiple small VSDs (four-chamber view).

 

Fetal Surveillance:

  • Assess common AV valve function for worsening regurgitation or evidence of stenosis (may be underestimated in the presence of unrestrictive atrial level communication).
  • Assess left ventricular outflow tract for possible development of left ventricular outflow tract obstruction and associated arch obstruction.
  • In unbalanced AVSD cases, assess interval growth of ventricles, semilunar valve, great arteries, and ductal and aortic arches to determine whether the lesion is ductal dependent needing PGE at birth to maintain ductal patency for pulmonary or systemic blood flow.
  • If AVSD is seen as part of a more complex disease such as heterotaxy syndrome, follow-up is needed for the other associated lesions (anomalous pulmonary veins, systemic venous anomalies, outflow tract or great artery obstruction, rhythm abnormalities etc).

 

Prostaglandins (PGE)  

  • Variable. 
  • Balanced AVSD: Usually not needed in the balanced AVSD cases unless there is significant outflow tract or arch obstruction.
  • Unbalanced AVSD: May need PGE for systemic or pulmonary blood flow (depending whether is RV or LV dominant).
 
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