Atrio-Ventricular Canal Defect (AVC)
also known as endocardial cushion defect

Sixty-nine per cent (69%) of all AV canal have Down syndrome. Holosystolic murmur at the left lower sternal border and apex is auscultated. In addition, the S2 is accentuated with an S3.

The atrioventricular junction is guarded by two masses of endocardial cushions, a superior and inferior cushion.  These two masses would meet in the middle, thus dividing the common atrioventricular canal into a right and left atrioventricular orifices.  The process through which these two cushions fuse is not clear and the role of apoptosis in this process is debatable.  The fusion of the two endocardial cushions resulting in the formation of two atrioventricular orifices.  In addition, the atrioventricular cushion appear to play a role in the closure of the inter-atrial communication at the edge of the primum atrial septum.  This septum grows towards the atrioventricular endocardial cushion and fuse with it.

The formation of the atrioventricular valve starts when the atria and inlet portion of the ventricle enlarge, while the atrioventricular junction (or canal) lags behind.  Such a process will cause the sulcus tissue to invaginate into the ventricular cavity, forming a hanging flap.  The endocardial cushion tissue is located at the tip of this flap, which is formed from three layers at this point:  The outer layer from atrial tissue, the inner layer formed by ventricular tissue and the inner layer by invaginated sulcus tissue.  The inlet portion of the ventricles then become undermined forming the tethering cords holding the newly formed valve leaflets.  The inner sulcus tissue will eventually come in contact with the cushion tissue at the tip of valve leaflets, thus interrupting the muscular continuity between the atria and ventricles.

Pathology

The ventricular septal defect can be very large extending beyond what endocardial cushion contributes to the ventricular septum. It is rare for a ventricular septal defect to be present with small primum atrial septal defect and even more rare without an atrial septal defect. The atrial septal defect is very variable in size. Primum atrial septal defect is seen with no VSD or a tiny inlet VSD. Cleft mitral valves are seen with primum atrial septal defect. A double inlet mitral valve is sometime noted with primum atrial septal defects. Tricuspid valve may override the ventricular septum when chordea insert into the left ventricle causing right atrial output to be directed into the left ventricle which may lead to hypoplasia of the right ventricle. The left ventricular papillary muscles may be single or close to each other causing mitral stenosis. AV canal defect is present in 50% of all children with Downs syndrome and congenital heart disease. It is rare to have ventricular size discrepancy in children with AV canal and Down syndrome in comparison to other children with AV canal defect. Tetralogy of Fallot or pulmonary stenosis could also be present with AV canal defect. Other types of congenital heart disease are common with AV canal defects except in those with Down syndrome that tend to have either a straight forward AV canal defect or AV canal defect with Tetralogy of Fallot or pulmonary stenosis.

Types of AV canal:

• Type A: The anterior leaflet is divided and attached to the crest of the VSD.
• Type B: Anterior leaflet is divided and chordae from left sided leaflet crosses and inserts in the RV.
• Type C: Anterior leaflet is ridging (single) with no chordal attachment (free floating).

Pathophysiology

Clinical Manifestations

 Left to right shunting at the atrial and ventricular septal defects will cause increase pulmonary blood flow (PBF).  This will eventually lead to congestive heart failure (CHF) with dilation of all cardiac chambers, congested lungs as well as liver and GI system due to elevated right atrial pressure.
In the neonatal period symptoms are not present due to the normally elevated pulmonary vascular resistance (PVR) which would prohibit excessive pulmonary blood flow.  As this drops, PBF will increase resulting in CHF.
CHF will cause tachypnea and failure to thrive.
On examination, CHF signs will be present, such as hepatomegaly, prominent RV and LV impulses.  In severe cases capillary refill becomes sluggish and peripheral pulses are diminished due to decreased systemic cardiac output.  Lungs may exhibit wheezing on auscultation.
Cardiac auscultation reveals a holosystolic murmur at left lower sternal border due to left to right shunting at the VSD, there may also be a mid-diastolic murmur due to increase blood flow across the AVC.  AVC regurgitation is variable and will also present as holosystolic murmur, heard best at cardiac apex.

Electrocardiography

This shows a northwest axis as well as RVH and LVH.

Chest Radiography

Echocardiography

AVC defect can be visualized echocardiographically from many views.
The subxiphoid view shows the common AV valve as well as the primum ASD and inlet VSD.  The ventricles tend to be foreshortened in this view, therefore, it may be difficult to assess the VSD size.
Apical 4-chamber view is an excellent view to assess the common AV valve, the inlet VSD and primum ASD.  The extent of AV valve regurgitation is also well appreciated in this view.  Any AV valve tissue obstruction to the LVOT cold be assessed in the "5-Chamber" modification of this view.
Additional defects should be sought, such as patent ductus arteriosus and tetralogy of Fallot.

Cardiac Catheterization

Assessment of the PVR may be needed in children older than one year of age with AVC defect, particularly if CHF symptoms and signs are not present despite large septal lesion.

Treatment

Surgical repair will be necessary. This is performed at 6-12 months of age, because the likelihood of developing irreversible pulmonary vascular obstructive disease is higher after one year of age. When repairing AVC defect, one or two patches are used to close the ASD and VSD, then the common AV valve is fashioned to make two functional AV valves.

Pulmonary artery banding may be associated with increased AV valve insufficiency and should be avoided if possible.

Univentricular repair is considered when one of the ventricles is hypoplastic due to mal-alignment of the AV valve orifice and the ventricles.

Factors influencing mortality:

AV canal anatomy

Ventricular hypoplasia

Mal alignment of AV valves and ventricles.

Additional VSD’s

Single papillary muscle

Double orifice mitral valve

Down syndrome patients do not have a higher mortality than the rest of the population with AV canal defect.

Primum ASD

Infants are usually asymptomatic. Symptoms when develop are due to left-to-right shunting at the atrial level causing increased pulmonary blood flow. There may or may not be mitral regurgitation due to the mitral valve cleft which will further cause pulmonary venous congestion and increase in pulmonary arterial pressure. The signs are those of increased pulmonary blood flow where there is a hyperactive precordium, a systolic murmur due to pulmonary blood flow or MR could be auscultated. There is wide splitting of S₂ . Also, if MR is present there will be a holosystolic murmur at the apex.

Electrocardiography

Superior axis deviation with or without incomplete right bundle branch.

Chest X-ray

Cardiomegaly with increased pulmonary blood flow. Cardiomegaly may be out of proportion to left-to-right shunting secondary to mitral insufficiency.

Echocardiography

Echocardiography shows the primum ASD as well as a cleft mitral valve and mitral insufficiency. Mitral regurgitation may be directly to the RA through the atrial septal defect. The mitral regurgitation when its directed to the right atrium may explain why the left atrium may not be dilated in these circumstances. The attachment of valve leaflets to the septum and the number and spacing of papillary muscles could also evaluated by echocardiography.

Cardiac catheterization is not necessary in such cases.

Management

This is best delayed if possible since AV valve replacement may be necessary and therefore the older the child the better unless symptoms are excessive secondary to large left-to-right shunting with or without mitral insufficiency. Mitral insufficiency may worsen even after surgery but generally postop results are very good