Atrioventricular Canal Defects

Etiology

Most of the AV canal defects are almost always associated with a syndromic abnormality. The syndromes that carry an association with AV canal defects are CHARGE, Down, Ellis-van-Creveld, Ivemark, Kaufman McKusick, Ritscher-Schinzel, Smith-Lemli-Opitz, and 3p. Down syndrome has a very strong association with AV canal defects.[2]

Epidemiology

The incidence of AV canal defects obtained from several studies ranges from 0.24 to 0.31 per 1000 live births.[3] It also comprises 3% to 5% of the total CHDs.[4][5] There is no gender preponderance, although some studies state that there is a slight female predilection (1:1.3), especially in cases associated with Down syndrome.[6] This may, in turn, be seen as gender as an effect modifier in the association of Down syndrome with CHDs. Certain studies have proved that factors such as maternal gestational diabetes mellitus (GDM), pregestational diabetes mellitus, and obesity carry a significant association with the development of non-syndromic AV canal defects.[7]

Pathophysiology

Forming a four-chambered heart involves coordination and organized fusion of several mesenchymal tissues. Mesenchymal tissue deposition on these endocardial cushions aids in the process of fusion, which in turn leads to the separation of the common AV canal. AV canal has superior and inferior endocardial cushions that fuse around 4 to 5 weeks of gestation. The failure of fusion of this endocardial cushion at various levels leads to septal defects ranging from partial to complete defects.[8] The abnormal fusion of the interatrial septum leads to communication in the inferior part of the septum called ostium primum defect.

The AV canal defects can primarily be classified into complete vs. partial defects based on the anatomical abnormalities.

1. Complete: Complete failure of fusion of endocardial cushions leading to single AV valve, ostium primum septal defect, variable deficiency of IVS.[9]
2. Partial (intermediate/transitional): Incomplete fusion causing mitral valve abnormalities, usually a cleft in the anterior leaflet leading to MV regurgitation.[10]

Further down, the complete AV canal defect may further be classified based on the AV valve morphology (Rastelli classification). The Rastelli classification is based on the insertion of the chordae and the morphology of the superior bridging leaflet.[11]

• Type A: This is the most common type associated with Down syndrome. The superior bridging leaflets by means of chordal attachments are inserted onto the left ventricle.
• Type B: This is the least common. The superior bridging leaflets by means of chordal insertion are extended onto the body of the right ventricle.
• Type C: This is associated with the tetralogy of Fallot (TOF), transposition of great arteries (TGA), etc.[12] There are free-floating superior bridging leaflets with no chordal insertions.

History and Physical

In patients with complete atrioventricular canal defects, mostly symptoms of pulmonary over circulation develop before the age of 6 months, and the severity of symptoms depends on the size and the type of the defects.[4] 

Tachypnea and difficulty to gain weight are the first signs noticeable in patients with AV canal defects. The occurrence and severity of symptoms depend on the degree of AV valve regurgitation and other associated CHD, which can contribute to the early development of congestive heart failure (CHF).

In patients with partial AV canal defect, symptoms might not be detected in the first few years of life and might manifest later in childhood. Of note, ostium primum atrial septal defect (ASD) seen in AV canal defects presents earlier than ostium secundum ASD. The most common presentation is for the evaluation of a murmur, heard secondary to increased flow across the pulmonary valve, which is heard best at the upper left sternal border. 

Symptoms of Congestive Heart Failure: These include increased work of breathing, sweating while feeding, poor feeding, lethargy, or increased sleepiness.

Signs of Congestive Heart Failure: These include tachypnea, tachycardia, failure to thrive (fall across two major centiles on the growth chart), wheezing or rales on lung auscultation, S3 gallop rhythm, apical displacement of the apical impulse, hepatomegaly, or increased jugular venous pressure (JVP).

A careful and complete cardiac exam should be done to look for:

1. Wide and fixed split S2 heard due to left to right shunting across the ASD causing increased blood flow in the right side of the hear irrespective of the phase of respiration.
2. S3 due to increased flow of blood splashing across the left ventricular walls which are still compliant.
3. Additional murmurs such as:

• Holosystolic murmur secondary to left AV valve regurgitation is heard best at the apex.
• Mid diastolic murmur if the shunt is large or if there is significant AV valve regurgitation.

Signs on General Physical Exam

Dysmorphic features, such as flat facies, upslanting palpebral fissures, single palmar crease, saddle toe are commonly seen in children with Down syndrome and should be evaluated for especially in a patient whose karyotype is not known (40% to 45% of patients with Down syndrome have AV canal defect).

A survey to look for other congenital anomalies including cleft lip, cleft palate, musculoskeletal abnormalities should also be done.

Evaluation

Echocardiogram

Routine antenatal screening with fetal echocardiography has been able to aid in the early diagnosis of septal defects. The cases that are diagnosed during the antenatal period generally carry a poor prognosis. When a defect is suspected in such screening, they are usually referred for a much more detailed fetal echocardiography, which can accurately depict the type of defect, associated valve morphology, presence of a shunt, and blood flow parameters. The radiological diagnosis during the postnatal period is usually performed when there are CHF signs, symptoms in a newborn, or if diagnosed (or suspected) during antenatal screening. With the recent advances in Doppler echocardiography, the diagnosis of AV canal defects is no longer dependent on cardiac catheterization.[13] Routine cardiac catheterization is done prior to surgical correction to delineate the anatomy.

Chest X-ray

In partial defect, chest X-ray shows right heart enlargement with increased pulmonary vasculature. While intermediate and complete forms show more of the diffuse enlargement of all chambers, although left atrial involvement is unusual and left ventricular enlargement is not very obvious due to the masking by the enlarged right ventricles.[1]

Electrocardiogram

The characteristic ECG finding in AV canal defects is the presence of a superior axis directed more towards the left. The next most common finding is the presence of rsR’ or rR’ in the right precordial leads caused due to the volume/pressure overload. The other findings include prolongation of the PR interval and right ventricular enlargement.[14][15]

Treatment / Management

The definitive management of complete atrioventricular canal defect (CAVCD) is the surgical correction of the defect as early as possible. However, overall management can be divided into three parts. Initial medical management, definitive surgical correction, and long-term follow-up care.[16]

The timing of correction is preferably before 6 months of age, as the risk of developing the pulmonary vascular disease is directly related to the duration of the disease.[17] The medical management is generally directed towards improving the myocardial function by means of reducing the preload and afterload using diuretics and angiotensin-converting enzyme inhibitors (ACEI) and angiotensin 2 receptor blockers (ARB). To improve contractility, inotropic agents such as digoxin are used.[3] Surgical management of CAVCD is dependent on various factors such as the type of defect, valve morphology, associated valvular and conduction abnormalities, the presence of shunt, and other vascular anomalies.[18] The intra-operative mortality rate is 3%, and the 10-year survival rate is more than 90%.

Complete AVCD 

1. Balanced lesion

Primary complete repair

• Single-patch
• Double-patch
• Modified single-patch

2. Unbalanced lesion

Palliative intervention: Single ventricle palliation (usually done for HLHS and tricuspid atresia)

Partial and Intermediate AVCD

Primary surgical repair

• Patch closure
• Mitral valvuloplasty

Long-term Follow-up Care

Annual cardiologist evaluation is recommended to check for and prevent complications in uncorrected individuals and due to surgery. Children with AVCD are at risk for neurological impairment; hence routine screening for neurological and developmental disorders is advised. The other general measures that have to be taken care of are infective endocarditis prophylaxis and risk assessment during pregnancy.

Differential Diagnosis

The differential diagnosis mainly consists of the large ventricular septal defect, ASD, surgery for VSD due to the CHF symptoms that are common with AVCD. Though ECG and chest x-ray may share some common features, echocardiography can be efficiently used for an accurate diagnosis.

Prognosis

The mortality rate of patients treated with atrioventricular canal defects is estimated to be 3%.[16] 90% of treated patients have a 10-year survival rate. The reoperation rate is found to be 10% to 20%.[19] The most common cause of reoperation, that occurs in 5-10% of patients is the worsening of mitral regurgitation.[20]

Complications

The patients will eventually start to develop left to right shunt irrespective of the type of defect, although the magnitude of the shunting depends on the type of defect. In complete AV canal defects, blood from the left to right flows through interatrial, interventricular, and AV valves, while in a partial defect, blood flows through the ostium primum. These individuals develop heart failure symptoms even before the age of one if it is not corrected. Due to the increased flow to the right heart, the pulmonary vasculature develops pulmonary hypertension. There is a slight symptomatic improvement during the initial phase of this change, which is one of the poor prognostic factors as it may lead to Eisenmenger syndrome. There is a significant regurgitation of blood in the heart chambers due to incompetent AV valves. Most of the regurgitation happens from ventricles to the atrium of the same side, although some occur from LV to RA through the cleft in the anterior mitral leaflet. This can lead to atrial dilatation or ventricular hypertrophy, depending on the defect, which can accelerate the development of congestive heart failure in children.