Atrioventricular Septal Defect

Etiology

In almost all patients, the atrioventricular septal defect is caused by genetic mutations, and most of the time, it is associated with syndromes. Every six patients with Down syndrome have associated atrioventricular septal defect, and Down syndrome cell adhesion molecule (DSCAM) gene has been described to be associated with an atrioventricular septal defect and other congenital heart diseases in these patients.[4][5] 

The other syndromes associated with the atrioventricular septal defect may include CHARGE, Ellis-van-Creveld, Smith-Lemli-Opitz, and 3p. Other than its association with syndromes, gene mutations associated with the atrioventricular septal defect can also be inherited as an autosomal dominant trait. Gestational diabetes and maternal obesity have also been reported to increase the risk of non-syndromic atrioventricular septal defects.[6]

Epidemiology

The incidence of an atrioventricular septal defect in the general population has been reported to be 0.24 to 0.31 per 1000 live births.[3] It accounts for 3% of all congenital cardiac malformations.[2] Although both male and female genders are affected equally, one of the studies suggests a female to male ratio of 1.3 to 1.0, especially in patients with Down syndrome.[7]

Pathophysiology

Endocardial cushions are paired (superior and inferior) mesenchymal structures located in the common atrioventricular canal in the early embryonic period, and there growth is of prime importance in the development of atrioventricular septum and atrioventricular valves.[8] These endocardial cushions fuse at the end of 4 weeks of development and form two atria and two ventricles. Failure of fusion results in a variable degree of the atrioventricular septal defect.

Based on atrioventricular valve morphology and its development, atrioventricular septal defects are classified into complete and partial.

A complete atrioventricular septal defect is characterized by a common atrioventricular valve, ostium primum atrial septal defect and ventricular septal defect of inlet type. It is caused by the complete failure of the endocardial cushions to fuse.[9] 

A partial atrioventricular septal defect is characterized by separate atrioventricular valves, an ostium primum atrial septal defect, a ventricular septal defect of inlet type, and cleft mitral valve. It is caused by incomplete fusion of endocardial cushions.[10] 

In the complete form of the atrioventricular septal defect, a common atrioventricular valve has five leaflets, including superior bridging, interior bridging, left mural, right mural, and anterosuperior. Rastelli divided complete atrioventricular septal defect into three anatomical subgroups.[11]

In type A, the superior bridging leaflet of the common atrioventricular valve is attached to the left ventricular surface of the interventricular septum with the help of chordae.

In type B, the superior bridging leaflet of common atrioventricular valve overhangs the interventricular septum and attached to the right ventricle with the help of chordae.

In type C, the superior bridging leaflet does not have an attachment to the interventricular septum and floats freely, thus provides a large unrestricted ventricular septal defect.

Rastelli type A is associated with left-sided obstruction, type C is associated with tetralogy of Fallot and other complex congenital heart diseases, and type B is the least common form of the complete atrioventricular septal defect.

History and Physical

Clinical presentation of atrioventricular septal defects is influenced by the type of atrioventricular septal defect, the magnitude of the intracardiac shunt, and other associated cardiac malformations.[3] In patients with complete atrioventricular septal defect, signs of pulmonary congestion, and right heart failure develop in early infancy due to significant left to right shunt as pulmonary vascular resistance drops after birth. Heart failure and Eisenminger may develop even earlier if these patients have associated atrioventricular valve regurgitation, ventricular imbalance, or coarctation of the aorta.[12] 

Patients with the partial atrioventricular septal defect without other complex congenital cardiac malformations and minimal atrioventricular valve regurgitation, usually remain asymptomatic in infancy and early childhood. They are diagnosed on the bases of incidental findings, including pulmonary or tricuspid flow murmur and a fixed splitting due to atrial septal defect.

Symptoms of heart failure in infants may include difficulty in feeding, sleepiness, lethargy, and failure to thrive, while children may have a complaint of dyspnea.

Signs of heart failure include:

• Tachypnea, tachycardia
• S3 gallop
• Rales on chest auscultation
• Raised jugular venous pressure
• Tender hepatomegaly
• Wide fixed splitting due to atrial septal defect
• Pansystolic murmur due to atrioventricular valve regurgitation
• Pulmonary flow murmur due to increased flow through the pulmonary valve
• Mid diastolic flow murmur due to increased flow through the tricuspid valve

General physical examination may reveal cyanosis when there is a reversal of shunt (Eisenmenger syndrome) and dysmorphic features in case of associated syndromes.

Evaluation

Antenatal Evaluation

Antenatal ultrasonography with a four-chamber view is the commonly used diagnostic test for the atrioventricular septal defect. The most common findings include a common atrioventricular valve and a defect in the atrial or ventricular septum. However, the sensitivity of antenatal ultrasound for the atrioventricular septal defect is very low.[13]

Postnatal Evaluation

Chest Radiograph: It demonstrates cardiomegaly and pulmonary plethora, especially in those cases with associated atrioventricular valve regurgitation.

Electrocardiogram: The characteristic electrocardiographic findings include a superior axis in the frontal plan, right ventricular hypertrophy, and atrioventricular block. Other findings may include superior p wave axis and partial right bundle branch block.

Echocardiogram: Echocardiographic findings include:[14]

• Abnormal configuration of atrioventricular valve
• Loss of normal offset of atrioventricular valve
• Abnormal position of papillary muscles
• Disproportion in left ventricular inlet and outlet
• Ostium primum atrial septal defect
• Ventricular septal defect of inlet type
• And other associated cardiac malformations.

Cardiac Magnetic Resonance Imaging: Magnetic resonance imaging reveals findings similar to an echocardiogram. It is more accurate in measuring the size of defects and regurgitation fraction through the atrioventricular valve.

Treatment / Management

The management of atrioventricular septal defects can be divided into medical and surgical treatment.[15]

Medical Treatment

It includes diuretics and vasodilators to reduce the preload and afterload to relieve the symptoms associated with pulmonary congestion and heart failure. Associated feeding problems and failure to thrive are managed by tube feeding and providing extra calories. In atrioventricular septal defects, medical treatment is usually directed at optimizing the condition of the patient for surgery.[3] 

Surgical Treatment

Surgical correction is the ultimate treatment of atrioventricular septal defect. Atrioventricular septal repair is a complex surgical procedure and carries operative mortality of more than 3% even in the contemporary era of advanced surgical techniques.[16] It also carries significant postoperative mortality and morbidity due to residual intracardiac shunts, atrioventricular valve regurgitation, left ventricular outflow tract obstruction, and arrhythmias.[17]

Assessment of pre-operative imaging and hemodynamic data is essential for the optimal selection of surgical procedures to reduce the requirement of recurrent surgery and postoperative complications.[18] In previous studies, the requirement of the recurrent procedure is reported as high as 18.2% at 15 years after surgical correction and left atrioventricular valve dysplasia, absence of cleft closure and associated cardiac malformations are found to increase the rate of recurrent procedures.[19](B2)

In complete AVSD, surgical closure should be performed in early infancy to reduce the pulmonary vascular disease, whereas, in incomplete atrioventricular septal defect, a repair can be slightly delayed if the patient is not symptomatic.

For partial AVSD, the primary repair is preferred with patch closure and atrioventricular valvuloplasty.

For balanced complete AVSD, early primary repair with two patch closure techniques is preferred over one patch closure, as one patch closure is associated with an increased rate of recurrent procedures due to patch dehiscence and residual shunt. Pulmonary artery banding is no longer used as a routine procedure in complete AVSD repair.[20] 

For unbalanced complete AVSD, repair technique may include single ventricle palliation with the staged biventricular repair or primary biventricular repair.[17]

Differential Diagnosis

Common differential diagnoses of AVSD include ostium secondum atrial septal defect, isolated ventricular septal defect and tetralogy of Fallot. The symptoms of heart failure and enlargement of cardiac chambers are common in these malformations, and echocardiogram plays a major role in differentiating AVSD from the aforementioned conditions.

Prognosis

The prognosis of untreated atrioventricular septal defect is dismal. Around 50% of the patients die during infancy, either due to heart failure or pulmonary infections.[2] Those who survive beyond one year, they develop the irreversible pulmonary vascular disease and later on the reversal of the shunt.

Patients undergoing surgical repair have 15 years of survival of around 90%, and 9% to 10% of those require reoperation within 15 years.[21]

Complications

Most of the complications of AVSD are related to intracardiac shunts or atrioventricular valve regurgitation. In complete AVSD, shunting of blood from left to right leads to right-sided overload and signs of heart failure and pulmonary congestion at a very early age, which contributes to significant mortality during infancy. If the shunt is not corrected, it causes an irreversible pulmonary vascular disease that leads to pulmonary hypertension and Eisenmenger syndrome.

Regurgitation of blood from the ventricle to atria through the atrioventricular valve leads to pulmonary congestion and enlargement of the atrium. Enlargement of the atrium can lead to supraventricular arrhythmias. Other complications are related to poor feeding, which may include malnutrition and failure to thrive.