eMedicine Specialties > Pediatrics: Cardiac Disease and Critical Care Medicine > Cardiology

Atrioventricular Septal Defect, Partial and Intermediate

Author: M Silvana Horenstein, MD, Consultant, Pediatric and Fetal Cardiac Diagnostic, Diagnostico Gineco-Obstetrico, PC; Associate Medical Director, Legacy Department, Best Doctors, Inc
Coauthor(s): Michael A Portman, MD, Research Director, Department of Pediatrics, Division of Cardiology, Associate Professor, Childrens' Hospital
Contributor Information and Disclosures

Updated: Oct 3, 2008

Introduction

Background

Atrioventricular septal defects (AVSDs) refer to a broad spectrum of malformations characterized by a deficiency of the atrioventricular septum. These malformations are presumed to result from abnormal or inadequate fusion of the superior and inferior endocardial cushions with the mid portion of the atrial septum and the muscular (trabecular) portion of the ventricular septum.

Several methods of classification and nomenclature are recognized, causing considerable confusion. The term partial AVSD (also called partial common atrioventricular canal) generally refers to endocardial cushion defects, which have an interatrial communication but lack an interventricular communication. In these types of defects the mitral and tricuspid annuli are separate. In addition, certain anatomic features should be present, alone or in combination: primum atrial septal defect (ASD), inlet ventricular septal defect (VSD), cleft of the anterior mitral valve leaflet, and wide anteroseptal tricuspid valve commissure or cleft septal tricuspid leaflet (see Media file 1). The most frequently encountered abnormality in patients with partial AVSD is the combination of primum ASD and cleft of the anterior mitral valve leaflet.

The term intermediate AVSD (also called transitional common atrioventricular canal) is variably defined; however, it most commonly refers to the combination of a partial AVSD with a small interventricular communication. This is an infrequent form of AVSD. A single valvar annulus is usually present where the anterior and posterior bridging leaflets fuse overlying the ventricular septum. Because of the leaflets' fusion, two distinct valvar components are observed (see Media file 2).

A thorough description of associated atrioventricular valve abnormalities should be included when classifying these defects.

This article considers AVSDs that demonstrate minimal or no shunting through an interventricular communication.

Pathophysiology

In the absence of obstruction of the right ventricular outflow tract, such as in pulmonary stenosis or pulmonary vascular obstructive disease, predominant left-to-right shunting occurs. The clinical presentation is determined by the degree of interatrial shunting, atrioventricular regurgitation, or both. The most inferior portion of the atrial septum is deficient. The resulting ostium primum defect varies in size and may occur in association with more superior ostium secundum–type ASDs. In some of the latter cases, only a small strand of the atrial septum remains, leading to the appearance of a common atrium. Some observers reserve the term common atrium for those cases with an additional sinus venosus deficiency.

The degree of left-to-right shunting through the atrial defect is determined by the size of the communication and the relative compliance of the 2 atria and ventricles. Ventricular compliance is affected by the level of pulmonary vascular resistance (PVR). In the newborn with a less compliant right ventricle (RV) and relatively high PVR, little left-to-right shunting occurs. If the defect is extremely large, obligatory mixing in a common, or near-common, atrium creates a component of right-to-left shunting. Left-to-right shunting increases with age as PVR decreases and RV compliance increases. This results in progressive RV enlargement and pulmonary vascular engorgement.

The atrioventricular valves are abnormal, even in a partial AVSD. Fusion failure of the endocardial cushions usually results in a separation or cleft in the anterior mitral valve leaflet. The degree of regurgitation through the cleft depends on its size and, occasionally, on the presence of left ventricular outflow tract (LVOT) obstruction or coarctation of the aorta. Typically, the cleft directs regurgitant blood through the atrial defect, creating an LV-to-RA (right atrium) shunt. RA enlargement, rather than left atrial (LA) enlargement, may occur. In addition, mitral regurgitation (MR) contributes to LA and LV enlargement.

Frequency

United States

Prevalence estimates of cardiovascular malformations in large cohorts vary from 4-8 cases per 1000 births. AVSD constitutes 5-8% of these defects. Incidence of AVSD in fetuses is 17%; however, occurrence of partial AVSD has not been separated from this general classification.

Studies report the incidence of congenital heart defect (CHD) in children with Down syndrome (trisomy 21) to be 42-48%. Of those CHDs, 45% are AVSDs.

In general, when not associated with heterotaxia syndrome, AVSDs commonly occur in Down syndrome.

Partial AVSD, as opposed to complete AVSD, of the ostium primum type is more common in patients without Down syndrome.

International

International frequency of cardiovascular malformations is similar to US figures.

Mortality/Morbidity

Left-to-right shunting through the atrial communication is generally well tolerated through the first decade of life. Patients are asymptomatic if MR is mild or absent. Symptoms of left-to-right shunting may develop in adolescence and are exacerbated by atrial arrhythmia. Sinus node dysfunction may occur and contributes to exercise intolerance if the defect is not repaired.

Moderate to severe MR may lead to morbidity in infancy and early childhood. Severe MR causes congestive heart failure (CHF) and failure to thrive in infants; it may result in death if left untreated.

A large left-to-right shunt from the LV to the RA through a cleft mitral valve causes volume overload in both ventricles, with CHF early in life.

Clinical

History

In the absence of moderate to severe mitral regurgitation (MR) and other associated congenital heart disease (CHD), partial atrioventricular septal defect (AVSD) is often discovered later in childhood when the patient is referred for evaluation of a heart murmur. Also, partial AVSD is less common in Down syndrome than in complete AVSD.

• The clinical presentation of patients with partial AVSD depends on the degree of MR and on the associated cardiac defects.
• Other cardiac anomalies that may be associated with partial AVSD include secundum atrial septal defect (ASD), persistent left superior vena cava draining to the coronary sinus, pulmonary stenosis, discrete subaortic stenosis, tricuspid stenosis, tricuspid atresia, coarctation of the aorta, patent ductus arteriosus (PDA), perimembranous ventricular septal defect (VSD), and hypoplastic left ventricle (LV).
• Children with atrioventricular valve competence usually exhibit no significant symptoms. They are usually referred to a pediatric cardiologist if a heart murmur is detected during routine examination.
• Substantial left-to-right shunting may exacerbate pulmonary disease and cause frequent lower respiratory infections in some patients. These patients may present with tachypnea, respiratory distress, and inadequate weight gain.
• Infants with severe MR often demonstrate poor feeding, tachypnea, and labored breathing. Rarely, respiratory distress may be so severe as to require mechanical ventilation.
• Progressive cardiac enlargement and LV dysfunction cause shocklike symptoms and eventually lead to mortality.
• Adolescents and young adults may note progressive exercise intolerance.
• Palpitations caused by atrial arrhythmia become more common in young adulthood, and sustained supraventricular tachycardia, atrial flutter, or atrial fibrillation may trigger the onset of congestive heart failure (CHF) in older patients with AVSD.
• Hypervolemia of pregnancy may trigger CHF symptoms and complicate pregnancy.

Physical

• General appearance
  • Most children with partial AVSD and minimal MR appear healthy. Patients who have Down syndrome exhibit features typical of the condition.
  • Patients with severe MR in infancy can manifest tachypnea, retractions, and diaphoresis, especially during and immediately after feeding. Poor caloric intake and excessive metabolic demands lead to growth failure. Older children and adolescents with severe MR may display a prominent left chest as well as a slim (asthenic) build.
• Pulmonary and cardiovascular examination
  • Palpation and auscultatory findings depend on the severity of the left-to-right shunt, the presence of MR, and associated defects (eg, LV outflow obstruction, PDA).
  • Fine rales or rhonchi, or both, may be heard in the lung fields of older patients with severe MR but are rare in infants.
  • The partial AVSD provides auscultatory findings that are indistinguishable from those created by any other large ASD. A prominent impulse along the right sternal border, consistent with a right ventricle (RV) lift, may be present. Alternatively, severe MR can cause a prominent apical impulse or thrill.
  • The classic auscultatory finding associated with an ASD is a constant or fixed splitting of the second heart sound (S₂), frequently accompanied by a pulmonary ejection murmur audible at the upper left sternal border.
  • A large AVSD with substantial left-to-right shunting creates a mid-diastolic rumbling murmur, audible along the lower left sternal border. This often occurs in association with a prominent third heart sound (S₃) in that location. These sounds are attributed to an abnormally high flow across the tricuspid component of the atrioventricular valve.
  • The apical murmur of MR occurs even with a small cleft in the atrioventricular valve. This murmur has a blowing quality and must be differentiated from the murmur caused by a VSD. However, when it occurs with a fixed split S₂, this murmur is helpful in differentiating a partial AVSD from a secundum ASD.
  • Severe MR can also cause a diastolic murmur audible over the apical area, which, in association with the systolic murmur, produces a to-and-fro quality.

Causes

For CHD, experimental and epidemiologic data suggest that a single mechanism may cause a range of anatomic malformations.

Specifically, AVSDs are presumed to occur secondary to extracellular matrix abnormalities that produce faulty development of the endocardial cushions and the atrioventricular septum.

Normal development of the human heart requires an orderly coordination of transcriptional programs. One of the most important factors for the differentiation of mesodermal progenitor cells is the homeobox protein Nkx-2.5. For example, the lack of Nkx-2.5 in mice arrests heart development prior to looping, which is lethal. In humans, 28 germline Nkx-2.5 mutations have been associated with CHD. Studies have shown that mutations in the gene Nkx-2.5 are associated specifically with AVSD and VSD.¹

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