This is the most common type of ASD in the general population and accounts for 70 % of ASDs and 6–10 % of all congenital heart disease [2]. The secundum ASD is usually an isolated lesion, but it can be seen in association with other forms of congenital heart disease. The two common associations are mitral stenosis (Lutembacher syndrome) and absent radius (Holt–Oram syndrome). Secundum ASD is also associated with mitral valve prolapse.

Figures 11.2, 11.3, 11.4, 11.5, and 11.6 are representative illustrations of the spectrum of secundum ASD.

Primum atrial septal defects (Fig. 11.7), also called atrioventricular septal defects, account for 15–20 % of ASDs [2]. This ASD presumably results from failure of the endocardial cushion to close the ostium primum and occurs between the fossa ovalis and the level of the atrioventricular valves. Primum ASDs are often associated with atrioventricular valve abnormalities (cleft mitral valve) and with trisomy 21 and heterotaxy syndromes. Unrepaired primum ASDs are very rare in adults [3] since correction is usually necessary prior to adulthood.

A superior sinus venosus atrial septal defect occurs at the junction of the superior vena cava (SVC) and right atrium and accounts for 5–10 % of all ASDs [2]. It often coexists with partial anomalous pulmonary venous return, usually from the right upper lobe to the SVC near its junction with the right atrium. Sinus venosus ASDs are associated with an increased incidence of pulmonary hypertension due to increased flow to the lungs from the anomalous pulmonary venous return. An inferior sinus venosus ASD is much less common than a superior sinus defect and occurs at the junction of the inferior vena cava (IVC) and right atrium.

Treatment of sinus venosus ASDS is surgical closure and reimplantation of the anomalous pulmonary veins. Alternatively, a baffle may be created to correct return of the anomalous pulmonary venous flow back to the left atrium [2]. Rarely, the superior vena cava can enter the right atrium via an accessory vein.

Figures 11.8, 11.9, 11.10, 11.11, and 11.12 are examples of various types of sinus venosus ASD.

Unroofed coronary sinus ASD is rare, comprising less than 1 % of all atrial septal defects [2]. It is thought to result from a failure of separation of the superior wall of the coronary sinus with the left atrium, leading to a direct communication between the coronary sinus and the left atrium. It is usually associated with a persistent left superior vena cava (LSVC). Persistent LSVC occurs in 0.1–0.5 % of the general population and 8 % of anomalies drain into the left atrium [4]. An unroofed coronary sinus ASD is seen in 75 % of patients with persistent LSVC that drains into the left atrium [4]. Unroofed coronary sinus is classified into four groups. Type I is a completely unroofed coronary sinus with persistent LSVC. Type II is a completely unroofed coronary sinus without persistent LSVC. Type III is a partially unroofed midportion coronary sinus defect. Type IV is a partially unroofed terminal portion coronary sinus defect. The fenestration from the coronary sinus into the left atrium typically occurs between the left atrial appendage and the left upper pulmonary vein [4].

Figure 11.13 is an example of an unroofed coronary sinus type ASD.

Common atrium (confluence of two or more types of ASD defects) is characterized by complete absence of atrial septum and occurs in association with other complex defects, frequently heterotaxy syndrome. It is very rarely seen in the adult population [5].

Figure 11.14 depicts a common atrium.

Patent foramen ovale (PFO) is the most common form of interatrial communication. It is caused by a failure of fusion of the flap valve of the fossa ovalis. The prevalence of PFO declines progressively with age (34 % up to age 30 years, 25 % for age 30–80 years, and 20 % older than 80 years) [7]. Two types of PFO exist. The first is the incompetent valve type and results in right-to-left shunting only when right atrial pressure exceeds that of the left atrium such as during a Valsalva maneuver. The second type is called the stretched type and is due to high left atrial pressure (such as seen in left heart failure) which stretches the fossa ovalis flap valve to the point of incompetence, resulting in a left-to-right shunt. While a single defect is most common in PFO, multiple fenestrations may also be seen.

Figure 11.15 is an example of a PFO seen on CT.

Atrial septal aneurysm (ASA) is associated with a PFO in 30 % of the cases [8]. ASA is defined as an abnormal bulging of the interatrial septum with an excursion of at least 10 mm and a base span of at least 15 mm [8]. This is thought to be due to redundancy of the valve of the fossa ovalis and/or excessive mobility of the atrial septum with ballooning into the right or left atrial chamber. ASA may be associated with mitral valve prolapse and atrial arrhythmias.

Figure 11.16 demonstrates an atrial septal aneurysm.

Clinical findings of isolated atrial septal defects are usually related to left-to-right shunting. The degree of shunting is determined by the size of the defect and the relative compliance of the right and left ventricles. Secundum ASDs that have a sufficient rim of tissue around the septal defect can be closed percutaneously with septal occluder devices, most commonly the AMPLATZER septal occluder device. All other hemodynamically significant ASDs require surgical closure.

Echocardiography is the method of choice to diagnose ASD but CT can be used when echocardiography is indeterminate [8–12]. The CT diagnosis of ASD is based on identifying a defect in the septal tissue and/or the presence of increased contrast opacification in the right heart despite proper timing of the scan after contrast bolus administration. The typical appearance of an ASD on CT is that of a contrast jet from the left atrium to the right atrium. Other findings of a left-to- right shunt include right ventricular and right atrial enlargement. Secundum ASD can be differentiated from a patent foramen ovale by the appearance of the jet. A PFO has a very thin, channel-like appearance and the contrast jet is at a slight angle or parallel to the interatrial septum or atrial roof as it courses from the left atrium to the right atrium (Fig. 11.15) [13, 14]. The secundum and sinus venous defects are broader and the former flows into the central part of the left atrium, while the latter flows along the lateral wall (Figs. 11.2, 11.3, 11.4, 11.5, 11.6, 11.8, 11.9, 11.10, 11.11, and 11.12) [14, 15].

Associated findings of partial anomalous pulmonary venous return may be noted in sinus venosus-type ASDs (Figs. 11.8, 11.9, 11.10, 11.11, and 11.12). With primum ASDs, an associated cleft mitral valve or associated ventricular septal defects may be noted.

Morphological changes of pulmonary hypertension found in long-standing larger ASDs include pulmonary arterial and right heart chamber enlargement.

CT can provide accurate anatomic assessment prior to percutaneous closure of an ASD or PFO. The necessary measurements include an assessment of the long- and short-axis views of the defect and measurement of the area of the defect (Fig. 11.17) In addition, the thickness of the rim of surrounding tissue is important to note. Measurements are made in end systole.