Atrioventricular Septal Defect, Partial and Intermediate 

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 the image below). 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 image depicted below).

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.

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.

Miller et al reviewed the long-term survival of infants with all types of atrioventricular septal defects with Down syndrome (n = 177) and without Down syndrome (n = 161). In this cohort, born from 1979-2003, overall survival probability through 2004 was 70% in those with Down syndrome and 69% in those without. Mortality was higher in children with a complex atrioventricular septal defect and in those with 2 or more major noncardiac malformations, but was lower in children born in 1992-2003.^[1]