Atrial Septal Defects
ASDs are classified according to their embryogenesis, their location relative to the fossa ovalis, and their size. There are four types of ASDs, listed here in order of frequency, with ostium secundum ASDs making up approximately 80% of cases (Fig. 4–1):
The most common type of ASD, the ostium secundum ASD, generally occurs in isolation. It is located centrally in the atrial septum, superimposed on the fossa ovalis. The ostium primum ASD is the second most common type. Although ostium primum defects may occur alone, they usually occur as part of a more complex congenital cardiac anomaly, the atrioventricular septal defect. The ostium primum ASD is located low in the atrial septum immediately adjacent to the atrioventricular valves. Sinus venosus ASDs are rare and can be divided into two types:
1. Sinus venosus ASDs of the superior vena cava (SVC) type are located just inferior to the orifice of the SVC in the right atrium so that the SVC overrides the ASD, providing blood to both atria. Sinus venosus ASDs of the SVC type closely approximate the site where the right pulmonary veins enter the left atrium, allowing anomalous pulmonary venous drainage through the ASD. This type of ASD is present in partial anomalous pulmonary venous return to the right atrium.1
The rare coronary sinus ASD is located at the expected site of the ostium of the coronary sinus in the right atrium. In this anomaly, the deficient or “unroofed” coronary sinus opens, through a defect in its distal wall, directly into the left atrium. The coronary sinus may enlarge greatly, resulting from increased flow. Usually, a persistent left SVC enters the upper left aspect of the left atrium. Rarely, the coronary sinus is absent rather than simply “unroofed.” This variation is associated with cyanosis after birth because a large volume of desaturated (anomalous) left SVC blood pours into the left atrium where it mixes with and dilutes the oxygen-rich blood returning from the lungs.
Between the fourth and sixth weeks of gestation, the primitive atrium is divided into right and left halves by a complex series of events. Throughout this process, some degree of interatrial blood flow is maintained. The septum primum, a thin, mobile, crescent-shaped membrane, develops along the cephalad portion of the atrium and grows caudally toward the endocardial cushions (Fig. 4–2, A). The space between the septum primum and the endocardial cushions, termed the ostium primum, is eventually obliterated when the septum primum fuses with the endocardial cushion (Fig. 4–2, B to D). Before complete fusion, however, multiple small fenestrations develop in the septum primum, coalescing to form the ostium secundum (Fig. 4–2, B to D), maintaining free blood flow from the right to the left primitive atrium.
A second crescent-shaped membrane, the septum secundum, subsequently develops just to the right of the septum primum (Fig. 4–2, D to F). The thick muscular septum secundum is formed from invagination of the roof of the primitive common atrium. As this membrane grows toward the endocardial cushions, it covers the ostium secundum (Fig. 4–2, E and F). Its crescent-shaped lower border never fuses entirely with the endocardial cushion, leaving an opening that represents the foramen ovale (Fig. 4–2, E and F). The upper portion of the septum primum is gradually resorbed, and the remaining lower portion becomes the valve of the foramen ovale or foraminal flap (Fig. 4–2, G and H).
The most common type of ASD, the ostium secundum defect, is caused by a shortened valve of the foramen ovale (foraminal flap) resulting from excessive resorption of the septum primum or to deficient growth of the septum secundum (Fig. 4–3).
The ostium primum ASD occurs when the lower portion of the atrial septum, which is partially formed from the endocardial cushions, is deficient and fuses incompletely with the endocardial cushions (Fig. 4–4). This defect can occur in isolation but is usually part of the more complex atrioventricular septal defect, also referred to as an endocardial cushion defect or an atrioventricular canal defect, in which a single large abnormal atrioventricular valve separates the atria and ventricles in the presence of a large ASD and ventricular septal defect (VSD) (Fig. 4–5).
The sinus venosus ASD occurs during embryogenesis when the right horn of the sinus venosus, which normally encompasses the orifice of the SVC and the inferior vena cava, develops abnormally, leaving an opening in the atrial septum near one of these orifices (see Fig. 4–1).
The coronary sinus ASD occurs when improper development of the coronary sinus allows it to communicate directly with the left atrium through a defect in the wall of its distal extremity, leaving an open channel from the right atrium through the coronary sinus and into the left atrium (see Fig. 4–1). Even in the absence of structural defects, the anatomy of the normal neonatal atrial septum is complex (Fig. 4–6).
ASDs occur in 1 in 1500 live births,2,3 and they comprise approximately 6.7% of congenital heart disease in live-born infants.4 Overall, ASDs are twice as common in female infants than in male infants.5,6
ASD is the fifth most common form of congenital heart disease, and it is the most common form in adult patients. A patient’s life span without surgery can easily reach 50 years, and frequently ASDs are discovered incidentally at autopsy in asymptomatic individuals.
Ostium secundum defects account for more than 80% of all ASDs5 and occur twice as often in female patients.7 Sinus venosus defects account for 5% to 10% of all ASDs and occur with equal frequency in male and female patients.8 Coronary sinus defects are rare. Various combinations of these types of ASD can occur together.
Rarely, ostium secundum ASDs are familial and may occur through multiple generations.9–12 Perhaps the best known example of familial inheritance of ASD is the Holt-Oram syndrome.13 This disorder is characterized by an autosomal dominant pattern of inheritance with a penetrance of nearly 100%. In addition to a septum secundum ASD, patients have congenital deformities of the upper limbs (most commonly with absent or hypoplastic radii) and cardiac rhythm abnormalities such as a right bundle branch block or first-degree atrioventricular block. Approximately 40% of these cases represent new mutations. The remainder are inherited from a parent.