Tal Geva Boston Children’s Hospital; Harvard Medical School, Boston, MA, USA Defects that allow interatrial communication can result from openings in the atrial septum (defined as atrial septal defect (ASD)), or from defects in the sinus venosus component of the atria (defined as sinus venosus defect), or in the tissue that separates the coronary sinus from the left atrium (defined as coronary sinus defect) [1]. A patent foramen ovale is an interatrial communication that exists normally in the fetus and in most newborns, but because of its clinical importance in certain congenital heart defects and in older patients with paradoxical emboli and stroke, it is discussed in this chapter. Atrial septal defects are further classified into secundum and primum defects. Ostium primum ASD – a defect that results from abnormal formation of the embryonic endocardial cushions – is discussed in Chapter 16. Defects of the atrial septum comprise the third most common type of congenital heart disease, with an estimated incidence of 56 per 100,000 live births [2]. This estimate represents the median incidence based on an analysis of 43 published articles spanning many decades. With improved recognition of clinically silent defects by echocardiography, recent estimates are approximately 100 per 100,000 live births [3]. Most cases of secundum ASD are sporadic; however, several investigators reported familial clusters with different modes of inheritance as well as an association with conduction defects. Examples include mutations in the cardiac transcription factor gene NKX2.5 [4–6], mutations in the GATA4 gene [7,8], a mutation in the myosin heavy chain 6 gene located on chromosome 14q12 [9], as well as several other mutations [10–12]. Secundum ASD is also frequently associated with genetic syndromes such as Holt–Oram, Noonan, Down, Budd–Chiari, and Jarcho–Levine, to mention just a few [13–20]. The classification of interatrial communications is based on an understanding of atrial septation and the relationships between the pulmonary veins, the systemic veins, and the atria. Atrial septation involves the following structures: Detailed descriptions of atrial septation can be found in several publications [21–24]. Briefly, septum primum – the first septum to appear in the developing atria at approximately 28 days of development – grows as a crescent‐shaped structure toward the developing endocardial cushions (Figure 12.1a). Its leading edge is covered by a layer of mesenchymal cells (mesenchymal cap) [24]. The space between the developing septum primum and the closing endocardial cushions is termed foramen primum or the primary foramen. Septum secundum, also called the limbus of the fossa ovalis, is a crescent‐shaped muscular infolding of the atria wall that develops to the right of the septum primum (Figure 12.1b). Foramen primum closes as a result of fusion between the mesenchymal cap of the septum primum and the now fused superior and inferior endocardial cushions (Figure 12.1c). Foramina secundi (secondary foramina) form at the same time as coalescing fenestrations within the septum primum (Figure 12.1b,c). The AV canal septum is formed, at least in part, by the superior and inferior endocardial cushions and contributes to septation of the outlet portion of the atria and the inlet portion of the ventricles (Figure 12.1). Normal development of the atrial septum results in formation of the fossa ovalis, which includes two anatomic elements: (i) muscular boundaries contributed by the septum secundum; and (ii) the valve of the fossa ovalis, which attaches on the left atrial aspect of the septum – the septum primum. The atrial component of the AV canal septum lies anterior and inferior to the fossa ovalis, separating it from the tricuspid and mitral valve annuli – the AV canal septum (Figure 12.2). The tissue that separates the right pulmonary veins from the superior vena cava (SVC) and from the posterior and inferior aspects of the right atrial free wall is termed the sinus venosus [25–27]. The tissue that separates the coronary sinus from the left atrium (LA) is termed the coronary sinus septum [28]. Figure 12.1 Diagram showing development of the atrial septum. (a) At 28 days after gestation, the septum primum (Sep 1°) develops as a crescent‐shaped structure with a layer of mesenchymal cells at the leading edge (red color). The space between the developing septum primum and the developing endocardial cushions is called the foramen primum or ostium primum. The septum secundum (Sep 2°) appears shortly thereafter to the right of the septum primum. (b) At 35 days, both the septum primum and septum secundum continue to develop. The openings within the septum primum are called the foramina secundi. (c) At 60 days, the atrial septum is nearly fully formed and the foramina secundi close by way of coalescing fenestrations within the septum primum. The foramen ovale remains patent throughout pregnancy. CPV, common pulmonary vein; IVC, inferior vena cava; LA, left atrium; LV, left ventricle; LVV, left venous valve; RA, right atrium; RV, right ventricle; RVV, right venous valve; SVC, superior vena cava. The patent foramen ovale (PFO) is the space between a well‐developed (valve‐competent) septum primum and a normally formed septum secundum (Figure 12.3). A PFO is a normal interatrial communication during fetal life, characterized by streaming of flow from the ductus venosus and, to a lesser extent, from the inferior vena cava, through the foramen ovale to the LA (Figure 12.4a, Video 12.1). After birth, left atrial pressure normally exceeds right atrial pressure and, as a result, the septum primum apposes the septum secundum (the superior limbic band of the fossa ovalis) and the foramen ovale narrows (Figure 12.4b, Videos 12.2 and 12.3). A PFO is seen in almost all newborns, but its frequency decreases with advancing age [29–31]. Complete anatomic closure of the foramen ovale occurs in 70–75% of adults [32]. This is a defect within the fossa ovalis, usually due to a single or multiple defects within the septum primum (Figure 12.5). The septum secundum is usually well formed. Most secundum ASDs are not confluent with the venae cavae, right pulmonary veins, coronary sinus, or the AV valves. With the exception of PFO, secundum ASD is the most common cause of an atrial level shunt. The size of secundum ASDs varies from several millimeters to 2–3 cm. Large defects are usually associated with marked deficiency, or even complete absence, of septum primum. Rarely, a secundum ASD results from deficiency of septum secundum (the muscular limb of the fossa ovalis). In such cases, the superior border of the defect can reach the SVC–RA junction (“high” secundum defect) but, in contrast to a sinus venosus defect, does not involve the right upper pulmonary vein. Primum ASD is an endocardial cushion defect with an interatrial communication located between the anterior–inferior margin of the fossa ovalis and the AV valves. It is considered a form of partial AV canal defect with two separate AV valve annuli and no ventricular septal defect of the AV canal type (see Figure 12.1 and Chapter 16). A sinus venosus defect (SVD) is a communication between one or more of the right pulmonary veins and the cardiac end of the SVC and/or the posterior–inferior wall of the RA (Figure 12.5) [25,27]. SVDs comprise approximately 4–11% of ASDs [33]. From an anatomic standpoint, a SVD is not an atrial septal defect because it does not allow direct communication between the left and right atria. Instead, the interatrial communication is through one or more systemic and pulmonary veins. The most common location of a SVD (approximately 87% [25]) is between the right upper pulmonary vein and the SVC, below the insertion of the azygos vein (SVC‐type SVD). The defect results from partial or complete absence of the sinus venosus tissue that separates the anterior aspect of the right upper pulmonary vein from the posterior wall of the SVC [25,26]. The deficiency of the sinus venosus tissue can extend peripherally to involve secondary branches of the right pulmonary veins, resulting in drainage of several pulmonary veins to the SVC. The LA orifice of the right upper pulmonary vein is usually widely patent, allowing for a communication between the LA and the cardiac end of the SVC. The combination of an interatrial shunt through the LA orifice of the right upper pulmonary vein (LA to SVC to RA) and drainage of right upper pulmonary vein blood to the RA through the SVC typically result in a large left‐to‐right shunt and marked right ventricular volume overload [34]. Figure 12.2 Diagram showing the anatomy of the atrial septum and neighboring structures. (a) Right atrial aspect. (b) Left atrial aspect. (c) Echocardiographic view, where the arrow points to the superior limbic band (septum secundum) and the arrowhead points to the septum primum. Ao, aorta; CS, coronary sinus; CT, crista terminalis; EV, Eustachian valve; FO, foramen ovale; IVC, inferior vena cava; LA, left atrium; LAA, left atrial appendage; PA, pulmonary artery (main); RA, right atrium; RAA, right atrial appendage; RLPV, right lower pulmonary vein; RPA, right pulmonary artery; RUPV, right upper pulmonary vein; SLB, superior limbic band (septum secundum); SP, septum primum; Sup PVs, superior pulmonary veins; SVC, superior vena cava; TBV, Thebesian valve. Less frequently, the defect also involves the right lower and/or middle pulmonary veins and the middle or inferior aspects of the RA (Figure 12.6). This type of SVD has been called the inferior vena cave (IVC) type although direct involvement of the IVC is extremely rare. For that reason, the term sinus venosus defect of the right atrial type is preferred [25]. In the study of Banka et al. [27], RA‐type SVD accounted for 1.2% of cases with an isolated interatrial communication. Anatomic characteristics of RA‐type SVD include: (i) the posterior and/or inferior defect confluent with the posterior wall of the atria with no posterior and/or inferior rim; (ii) the confluence of the defect with the right pulmonary veins(s) or with the IVC–RA junction; and (iii) the presence of a well‐developed septum primum covering the fossa ovale (Figure 12.6) [27]. Rarely, both SVC‐ and RA‐types coexist, resulting in all right pulmonary veins draining to the right heart. This should be distinguished from leftward malposition of the septum primum resulting in anomalous drainage of the right pulmonary veins to the RA. It is important to recognize that although the right pulmonary veins in SVD almost always drain anomalously, their anatomic connections with the LA are usually normal, through their native orifices. In the rare circumstance when the LA orifice of the right upper pulmonary vein is atretic, there is no interatrial communication and the anatomic appearance is that of partially anomalous pulmonary venous connection of the right upper pulmonary vein to the SVC. Figure 12.3 Diagram of atrial septal components showing a patent foramen ovale (arrow). AVS, atrioventricular septum; FO, foramen ovale; ILB, inferior limbic band; LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle; Sept. 1°, septum primum; SLB, superior limbic band (septum secundum). Figure 12.4 Patent foramen ovale (PFO). (a) Fetal echocardiogram at 18 weeks’ gestation showing right‐to‐left flow from the ductus venosus to the left atrium (LA) through a PFO. (b) PFO with left‐to‐right flow (arrow) in a newborn. LA, left atrium; RA, right atrium; RV, right ventricle; RVOT, right ventricular outflow tract. Figure 12.5 Diagram showing types of interatrial communications. ASD 1°, primum atrial septal defect; ASD 2°, secundum atrial septal defect. Figure 12.6 Echocardiographic 2D and color Doppler images in a subxiphoid long‐axis plane (top) showing the typical appearance of a sinus venosus defect of the right atrial type (asterisk). Imaging in the subxiphoid short‐axis plane (bottom) demonstrates an intact fossa ovalis and septum primum (arrow) in the same patient. CS, coronary sinus; LA, left atrium; RA, right atrium; RLPV, right lower pulmonary vein. This is an uncommon anomaly that results from partial or complete unroofing of the tissue separating the coronary sinus from the LA, allowing the right and left atria to communicate through the defect and the coronary sinus orifice (Figure 12.7) [1]. The orifice of the coronary sinus in this anomaly is usually large as a result of the left‐to‐right shunt, resulting in a sizeable defect in the inferior aspect of the atrial septum near the entry of the IVC. The association of a coronary sinus septal defect and persistent left SVC is termed Raghib syndrome [35]. When the coronary sinus is completely unroofed, the left SVC enters the left superior corner of the LA, anterior to the orifice of the left upper pulmonary vein and posterior to the LA appendage. A common atrium is present when the septum primum, septum secundum, and the AV canal septum are absent (usually in patients with heterotaxy syndrome). Remnants of atrial septal tissue can sometime be recognized in these patients, such as a fibromuscular strand that crosses the inferior aspect of the common atrium (inferior limbic band of the fossa ovalis) or remnants of venous valve tissue. Regardless of the specific anatomic type and assuming no AV valve stenosis or hypoplasia, the amount of shunting through an interatrial communication is determined by the defect size and relative compliance of the right and left ventricles. Over the first few months of life, right ventricular compliance typically rises leading to an increasing left‐to‐right shunt. In adults, left ventricular compliance normally decreases, further augmenting the left‐to‐right flow. Shunt flow through the right heart and lungs leads to dilation of the RA, right ventricle, pulmonary arteries, and pulmonary veins. Most young children tolerate this increased pulmonary blood flow well and are asymptomatic; a few develop dyspnea or growth failure [36,37]. Defects in this age group are typically detected after auscultation of a heart murmur or incidentally when a chest radiogram, an electrocardiogram, or an echocardiogram is obtained for other indications. The risk of pulmonary arterial hypertension increases with age and up to 5–10% of patients with significant left‐to‐right shunts may develop pulmonary vascular disease by adulthood [38]. Adults with unrepaired ASDs are also at risk for exercise intolerance, atrial arrhythmias, and paradoxical emboli [39,40]. Small secundum ASDs (≤5 mm) may close spontaneously or become smaller in the first few years of life; thereafter, defects tend to become larger with time [41,42]. By contrast, primum ASDs, sinus venosus septal defects, and coronary sinus septal defects almost never become smaller with time. Figure 12.7 Diagram of coronary sinus septal defects. (a) Small coronary sinus septal defect associated with left superior vena cava (SVC) to coronary sinus. (b) Unroofed coronary sinus associated with left AVC (Raghib syndrome). Note the large interatrial communication through the coronary sinus ostium. IVC, inferior vena cava; LV, left ventricle; RV, right ventricle. The atrial septum is imaged from multiple acoustic windows, including the subxiphoid, apical, left parasternal, and high right parasternal. Two‐ and three‐dimensional imaging as well as color and spectral Doppler techniques are used in concert for comprehensive evaluation of the fossa ovalis, sinus venosus, and coronary sinus. If a defect is seen from one acoustic window, its presence must be confirmed from other windows. Transesophageal echocardiography (TEE) – often used during surgical and transcatheter closure of ASDs (see later) – is also performed as a diagnostic procedure in patients with suboptimal transthoracic windows and inconclusive diagnosis. In such cases, cardiac magnetic resonance imaging (MRI) can be considered as a noninvasive alternative [34,43,44]. Contrast echocardiography is another helpful technique in certain patients (e.g., poor acoustic windows, diagnosis of left SVC associated with coronary sinus septal defect (Raghib syndrome)) [45]. The subxiphoid acoustic window is ideally suited for evaluation of the fossa ovalis because the normally oriented septum is relatively echo‐reflective from that position (Figure 12.8). This minimizes the likelihood of false dropout of acoustic signals, which can lead to erroneous diagnosis of an ASD. Imaging is performed in the long‐ and short‐axis views and the use of “in‐between” transducer angle is encouraged. A secundum ASD is seen as a defect within the fossa ovalis, confirmed by evidence of transseptal flow by color Doppler (Figure 12.8a–c, Video 12.4). In the subxiphoid long‐axis view, a secundum ASD is not contiguous with the posterior RA free wall or with the right pulmonary veins. Such findings are suggestive of a SVD of the RA type. In the subxiphoid short‐axis view, the superior limbic band of the fossa ovalis separates a secundum ASD from the SVC and the right upper pulmonary vein. The subxiphoid short‐axis view is also optimal for assessment of right ventricular volume load (right ventricular enlargement and diastolic septal flattening) and right ventricular hypertension (systolic septal flattening). The apical window is not ideally suited for evaluation of the fossa ovalis due to risk of false dropout of acoustic signal as a result of the parallel orientation of the ultrasound beam relative to the atrial septum. However, this view is helpful for measurement of the tricuspid regurgitation jet velocity for estimation of right ventricular systolic pressure. The apical window is also helpful for detection of acoustic signals during a contrast echocardiogram (Figure 12.9). The imager is looking for the appearance of acoustic reflective signals (“bubbles”) in the LA and left ventricle (representing right‐to‐left flow) or a negative jet effect in the RA representing left‐to‐right flow. Figure 12.8 Imaging of secundum atrial septal defect (ASD) from the subxiphoid window: (a) Long‐axis view showing 2D and color Doppler flow mapping. The arrow points to the secundum ASD. (b) Short‐axis view. The asterisk indicates secundum ASD; the arrow denotes the superior limbic band (septum secundum). (c) Real‐time 3D imaging (right atrial view) showing a large secundum ASD associated with deficient septum primum (asterisk). LA, left atrium; RA, right atrium; RPA, right pulmonary artery; SLB, superior limbic band; SVC, superior vena cava. The parasternal window is helpful, especially in the short‐axis plane, for imaging of the atrial septum. A low left parasternal short‐axis view can often provide adequate imaging of the atrial septum, even in patients with suboptimal subxiphoid windows (Figure 12.10). This view is helpful for measurement of the anteroposterior diameter of the defect. The presence or absence of right ventricular volume and pressure loads is also evaluated from the parasternal short‐axis view. Figure 12.9 Contrast echocardiogram in a patient with secundum atrial septal defect. During a Valsalva maneuver, contrast crosses the atrial septum and appears in the left heart.
CHAPTER 12
Anomalies of the Atrial Septum
Definition
Incidence
Etiology
Morphology and classification
Developmental considerations
Anatomy
Patent foramen ovale
Secundum ASD
Primum ASD
Sinus venosus defect
Coronary sinus defect
Common atrium
Pathophysiology
Imaging
Imaging of secundum ASDs
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