Atrial septal defect (ASD) refers to a congenital malformation in the development of the interatrial septum. In embryonic life, the interatrial septum begins as a septum primum, growing from the atrial walls to the endocardial cushions, which contains a temporary opening known as the foramen primum (Romero et al., 1988). Subsequently, a septum secundum grows on the right side of the septum primum and contains a normal opening known as the foramen ovale. The lower part of the septum primum forms a flap valve for this foramen ovale. Soon after birth, increased left atrial pressure closes this flap, normally resulting in complete closure of the foramen ovale within 2 years of life (Kerut et al., 2006).

Three different types of ASDs are possible, depending on the location within the heart (Romero et al., 1988). An inlet ASD is located near the entrance of the superior vena cava; a secundum ASD occurs in the body of the interatrial septum; a primum or outlet ASD is located near the atrioventricular junction and usually behaves similarly to an AV canal defect (see Chapter 45). Secundum ASDs typically occur because of absence of the foramen ovale flap valve. ASDs usually cause no hemodynamic effects in utero because of the normal right-to-left shunting of blood across the patent foramen ovale. If an ASD persists after birth it may allow for a left-to-right shunt, which can result in congestive heart failure or pulmonary hypertension.

Because of the difficulties in differentiating ASDs from asymptomatic patency of the foramen ovale, the precise incidence of ASD in neonatal life is difficult to establish. A patent foramen ovale may be present in up to 30% of normal adults (Kerut et al., 2006). ASDs comprise 7.5% of all congenital cardiac malformations (Ferencz et al., 1987). In one population series, the incidence of ASD was 6 in 10,000 total births (Mitchell et al., 1971).

Prenatal sonographic diagnosis of ASD is problematic because the differentiation between a normal patent foramen ovale and true ASD may be difficult. The foramen ovale normally increases in size linearly with advancing gestational age (Phillipos et al., 1994). Large defects of the septum secundum are visualized as dropout of echoes at the level of the interatrial septum (Romero et al., 1988). If an apical four-chamber view is used it may be difficult to confirm an intact interatrial septum. For adequate visualization, the ultrasound probe should be perpendicular to the septum, which can be achieved through the use of a subcostal approach to the four-chamber view (Romero et al., 1988).

The accuracy of sonographic screening for prenatal detection of ASD is poor. In a series of 7459 fetuses who were screened at 18 weeks of gestation, there were 10 ASDs, none of which were identified prenatally (Tegnander et al., 1995). In another series, only 5% of the 761 ASDs that were present in a single large population were detected prenatally (Montana et al., 1996).

The main differential diagnosis to consider following the prenatal diagnosis of an ASD is the possibility of a normal patent foramen ovale.

In the majority of cases, an isolated ASD will not cause any significant hemodynamic compromise in utero because of the normal right-to-left shunting of blood across the patent foramen ovale. It is also possible for ASDs to close spontaneously in utero, although the exact frequency of this occurrence is unclear because the accuracy of prenatal diagnosis of isolated ASD is so poor.