An ASD is defined as a persistent opening in the interatrial septum after birth, allowing communication between the left and the right atrium [5, 6]. When present, oxygenated blood shunts from the left atrium to the right atrium, the shunted volume depending on the size of the defect.
The reported birth prevalence of ASDs is 1.64 per 1000 live births, accounting for 13 % of all CHDs [1]. They account for 25–30 % of CHD cases diagnosed in adulthood [10]. Remarkably, most ASDs, even those with a large left-to-right shunt, go undiagnosed through most of one’s childhood and may only become symptomatic in the fifth decade or later. The exact age at which symptoms begin is highly variable and not exclusively related to the size of the shunt. With advancing age, the risk of developing exercise intolerance, atrial fibrillation or even decompensated right heart failure and pulmonary hypertension increases [11].
ASDs are detectable on both contrast-enhanced CT and MR examinations. While they can be appreciated on standard axial images, their presence and further characterization is often better appreciated on oblique, reformatted images perpendicular to the atrial septal plane.
Patent foramen ovale
During fetal life, a normal interatrial communication through the oval foramen exists, providing oxygenated blood from the inferior vena cava into the left atrium. As such, this communication is not a true ASD but a necessary passage tunnelled during embryologic development. It is commonly encountered in newborn babies. With advancing age, left atrial pressure increases, functionally closing the foramen ovale by pushing the septum primum flap against the septum secundum. In a vast majority of cases, normal development evolves to complete septal fusion. However, a failed fusion with persistent interatrial communication is called a patent foramen ovale (PFO), a common finding found in up to 25 % of adults [12].
Reflecting the embryologic purpose of arteriovenous communication, a PFO has a vertical cranio-caudal orientation in the direction of the inferior vena cava, best seen on axial images or oblique images perpendicular to the atrial septum. Both the OP and secundum flap can then be identified, surrounded by dense intravenous contrast, as demonstrated in Fig. 8. A variable amount of dense contrast can often be seen flowing into the right atrium. Incomplete fusion does not necessary imply that an interatrial shunt exists, as the septum can be functionally closed with still identifiable septum flaps. This anatomic variation is called a probe PFO (Fig. 9).
PFO’s are mostly clinically silent, frequently seen during cardiac CT examination performed for unrelated reasons [8]. When large, they can also be detected on non-ECG gated CT examinations [13]. In asymptomatic patients with an incidental PFO, no medical or interventional therapy is advised. Furthermore, a potential benefit of having a PFO is facilitation of transseptal catheter passage in radiofrequency ablation procedures. The criteria for PFO closure are not standardized. Whether transcatheter PFO closure is considered superior to medical therapy in reducing the risk of recurrent stroke is currently the subject of intense debate and research [14, 15].
Ostium primum defect
An OP defect is a less common type of ASD, accounting for 15 % of ASDs. They occur in the lower portion of the atrial septum near the AV valves. The embryological basis of this defect is shown in Fig. 10. In the presence of an OP defect, one should therefore always look for an associated valve defect, most frequently a cleft in the anterior leaflet of the mitral valve.
Since OP defects are often large they can easily be depicted on CT, revealing a large shunt flow, typically perpendicular to the interatrial septum. Oblique CT images allow detailed evaluation of the anatomy, such as the relationship with the AV valves and the absence of a flap, as illustrated in Fig. 11. As in all types of ASDs, surgical closure of the defect is indicated in those patients with a hemodynamically significant shunt that causes enlargement of the right heart structures. Suspicion of paradoxal embolism in the absence of other causes is also a valid indication for defect closure [16, 17].
Ostium secundum defect
Ostium secundum defects are the most frequent type of ASD, accounting for 75 % of all cases. They are found in the region of the fossa ovalis. The embryological basis of this ASD is illustrated in Fig. 12. Contrary to a PFO, an overlapping septal flap is lacking. As in OP defects, a shunt flow perpendicular to the axis of the interatrial septum is seen, in contrast to the more oblique cranio-caudal flow of a PFO, as demonstrated in Fig. 13.
Small defects are often asymptomatic and usually do not require treatment. Large defects, however, can initially cause volume overload of the right heart, eventually leading to pressure overload with dilatation of the right atrium and ventricle. As a consequence, right heart failure, pulmonary hypertension and right-to-left shunting may render this type of ASD to become symptomatic, usually in the fifth decade. This can be prevented by shunt closure, either surgically or endovasculary depending on the characteristics of the defect. Large defects and defects close to the edge of the interatrial wall are less suited for the endovascular approach since most closing devices cannot be anchored properly [16, 17].
Sinus venosus defect
Sinus venosus defects represent 4–11 % of atrial septum defects [18]. The name is derived from an abnormal absorption of the embryologic sinus venosus into the right atrium. In this defect, communication exists between one or more of the right pulmonary veins and the junction of the superior vena cava and the right atrium, called the superior vena cava type, as demonstrated in Fig. 14. Sometimes, only the superior vena cava is involved (Fig. 15). In fewer cases, this occurs just above the junction of the inferior vena cava and the right atrium (inferior sinus venosus defect) [19]. The presence of a sinus venosus defect may lead to volume and pressure overload of the right heart. Multiplanar CT reconstructions are very useful in the assessment of the complex anatomy of this type of ASD. Sinus venosus defects with significant hemodynamic impact need surgical correction to prevent further right heart failure and pulmonary hypertension [16, 17].