Pediatric Atrial Septal Defects
- Author: Michael R Carr, MD; Chief Editor: Steven R Neish, MD, SM more...
Background
Congenital heart defects (CHD) are common in children, with an incidence of approximately 8 cases per 1000 live births. These defects can cause an array of problems in the primary care of children. Atrial septal defects (ASDs) are a prevalent form of CHD. An understanding of human embryology is essential for diagnosing these abnormalities, understanding the pathophysiology and in planning long-term treatment.
Pathophysiology
Cardiac tissues are first detectable on the 18th or 19th day of fetal life. Cardiac development continues for the next several weeks. The atrial septum begins to form during the fourth week of gestation and is complete by the end of 5 weeks' gestation.
Classic model of cardiac development
According to the classic model of cardiac development, the process begins when a thin crescent-shaped membrane (septum primum) begins to form along the dorsal and cranial walls of the atrium. The space between the septum primum and the endocardial cushions (ostium primum) becomes progressively smaller as the septum primum grows toward the endocardial cushions. Before the ostium primum completely closes, small perforations develop in the anterosuperior wall of septum primum and ultimately coalesce to form a second interatrial communication, the ostium secundum. Meanwhile, the leading edge of the septum primum fuses with the endocardial cushions, and the ostium primum disappears.
Near the end of 5 weeks' gestation, the second phase of the process begins when a second crescent-shaped membrane (septum secundum) begins to form within the atrium to the right of the first septum. This membrane also begins to grow toward the endocardial cushions, covering the ostium secundum. However, the septum secundum remains incomplete. The foramen ovale is the opening remaining after the septum secundum completely forms.
The final phase of the process begins when the upper portion of the septum secundum proceeds to degenerate and finally disappears. The fully formed atria now have 2 overlapping but incomplete septae. The upper portion of the septum secundum covers the ostium secundum and creates a one-way valve allowing right-to-left shunting of blood in the fetus.
Van Praagh and Corsini model of cardiac development
Van Praagh and Corsini proposed another model of cardiac development.[1] According to their model, the septum primum (also known as the flap valve of the foramen ovale) grows from the portion of the left venous valve of the sinus venosus that is furthest left. As it extends from the most dorsal aspect of the atrium, the septum primum begins to meet the septum secundum, which is an invagination of the most rostral portion of the primitive atrium. The marginal edges of the septum primum eventually meet the left aspect of the septum secundum.
During embryonic and fetal life, the central portion of the septum primum billows into the left atrium due to the normal right to left shunting at the atrial level. After birth, the remainder of the septum primum adheres to the left aspect of the septum secundum.
Recent identification of an anomaly called deviated superior attachments of septum primum provides evidence in favor of the Van Praagh and Corsini model. Additional detailed morphologic analysis of murine cardiac development is needed to determine which model is correct.
Types of atrial septal defects
Four basic types of atrial septal defects are known. Patients who simultaneously have the first 3 types of atrial septal defect, as described below, are said to have common atrium.
The first type is an ostium secundum defect. The most common yet least serious type of atrial septal defect is an ostium secundum defect. This defect occurs in the area of the fossa ovalis and presumably results from excessive fenestration or resorption of septum primum, underdevelopment of septum secundum, or some combination of the 2 conditions.
In approximately one half of patients with left atrioventricular (AV) valve underdevelopment (ie, hypoplastic left heart syndrome or Shone complex), the superior attachments of the flap valve of the foramen ovale lie on the left atrial roof, well to the left of the septum secundum. Weinberg et al (1986) called this anomaly "(leftward and posterior) deviation of the superior attachments of septum primum."[2] This deviation is observed extremely rarely in patients with a normal-sized left AV valve. Of importance, the classic model does not explain its existence well. This type can be regarded as a variation of an ostium secundum defect, although it is most rigorously designated as a malalignment-type atrial septal defect.
A second variant of the ostium secundum defect is its association with an aneurysm of the atrial septum. This is thought to be due to redundancy of the valve of the fossa ovalis. It may be associated with mitral valve prolapse or atrial arrhythmias (see the images below).
Subcostal echocardiographic view of a child with a secundum atrial septal defect (ASD). Note the position of the defect in the atrial septum. RA = Right atrium; LA = Left atrium; SVC = Superior vena cava. 
Subcostal long-axis view of the same child as in the previous image with a secundum atrial septal defect (ASD). RA = Right atrium; LA = Left atrium; RUPV = Right upper pulmonary vein. 
Parasternal short axis view of a child with a secundum atrial septal defect (ASD). RA = Right atrium; LA = Left atrium; AO = Aorta. The second type is an ostium primum defect. This atrial septal defect presumably results from failure of the endocardial cushions to close the ostium primum. Because endocardial cushions also form the mitral and tricuspid valves, ostium primum defects are virtually always associated with a cleft in the anterior mitral valve leaflet (see the images below).
Apical echocardiographic view of a primum atrial septal defect (ASD). Note the position of the defect when compared with a secundum ASD. RA = Right atrium; LA = Left atrium; RV = Right ventricle; LV = Left ventricle. 
Apical echocardiographic view of a primum atrial septal defect (ASD). Note that the atrioventricular valves are at the same level (instead of mild apical displacement of the tricuspid valve), which is seen in the spectrum of atrioventricular canal defects. RA = Right atrium; LA = Left atrium; RV = Right ventricle; LV = Left ventricle. 
Apical color Doppler echocardiographic view of a primum atrial septal defect (ASD). Note the flow across the defect from the left atrium to the right atrium (RA), and note the mitral regurgitation (MR) through a cleft in the anterior leaflet of the mitral valve. MV = Mitral valve; LV = Left ventricle. The third type is a sinus venosus defect. This atrial septal defect is found in the posterior aspect of the septum near the superior vena cava (where it may coexist with partial anomalous pulmonary venous connection of the right upper pulmonary vein) or the inferior vena cava (where it may coexist with partial anomalous pulmonary venous defect of the right lower pulmonary vein). See the image below.
Subcostal short-axis view of a child with a sinus venosus atrial septal defect (ASD). Note the position of the defect compared with that of a secundum or primum ASD. Also note the anomalous position of the right upper pulmonary vein (RUPV). RA = Right atrium; LA = Left atrium. The fourth type is a coronary sinus septal defect. This least common type of atrial septal defect is called an unroofed coronary sinus or coronary sinus septal defect. A portion of the roof of the coronary sinus is missing; therefore, blood can be shunted from the left atrium into the coronary sinus and subsequently into the right atrium. This type is often associated with a left superior vena cava.
Left-to-right shunting
Clinical effects of isolated atrial septal defects are usually related to left-to-right shunting. The magnitude of shunt is related to the size of the defect in the septum, to the relative compliance of the left-sided and right-sided cardiac chambers, and indirectly related to the resistance of the pulmonary and systemic circulations. At birth, the right and left ventricles are of equal thickness and similar compliance. In the first few days to weeks after birth, the pulmonary vascular resistance (PVR) remains mildly elevated and has not reached its nadir.
As impedance to pulmonary blood flow decreases and the right ventricle becomes more compliant, blood is able to flow to the pulmonary vascular bed more easily, and the atrial level left-to-right shunt increases.
On occasion, the septal defect is small, with little left-to-right shunting. However, most defects that cause murmurs or symptoms are moderately large to large, and the size of the defect does little to limit left-to-right shunting. Approximately 15% of ostium secundum atrial septal defects spontaneously close by age 4 years, and others may decrease in size as to not be hemodynamically significant.
Epidemiology
Frequency
United States
Research indicates that congenital heart disease is diagnosed in 0.8% of children in the first year of life. Atrial septal defect is the second most common congenital heart defect in children and adults and occurs in anywhere from 0.67-2.1 per 1000 live births. Secundum atrial septal defects comprise just over 90% of all atrial septal defects, whereas sinus venosus and primum atrial septal defects comprise between 3-4% each. About 15-30% of healthy adults have an unfused foramen ovale in which the valve functions normally but has failed to fuse. In these individuals, a cardiac catheter passed into the right atrium can pass into the left atrium through the foramen ovale (ie, probe-patent foramen ovale).
Mortality/Morbidity
In developed countries, mortality rate of atrial septal defect is low (< 1%). Morbidity secondary to atrial septal defect is unusual and typically limited to 3 groups of patients.
Approximately 1% of infants with moderate or large (ie, nonrestrictive) atrial septal defects, but no other left to right shunting lesion (eg, patent ductus arteriosus, ventricular septal defect), have tachypnea and failure to thrive. In these individuals, the pulmonary artery pressure, when measured during catheterization or Doppler echocardiography, is at or near systemic level. Attempts to exclude mitral or left ventricular diastolic abnormalities as a cause of these hemodynamics must be undertaken; however, these attempts frequently yield equivocal data.
Patients in whom atrial septal defects go unrecognized until late childhood may develop arrhythmias (eg, atrial fibrillation, atrial tachycardia) or pulmonary hypertension. Atrial septal defects that initially appear in middle-aged or elderly adults can indicate congestive heart failure (CHF).
Patients with atrial septal defects may have an embolic stroke as the initial presentation.
Sex
The female-to-male ratio is approximately 2:1.
Age
Atrial septal defect , a congenital abnormality, is present at birth. However, in most cases, a murmur is not audible until the child is a few months old. Symptoms usually do not occur in individuals with atrial septal defect until late childhood, adolescence, or adulthood.
Secundum type (ie, ostium secundum), sinus venosus, and unroofed coronary sinus defects are sometimes not diagnosed until the third decade of life.
Ostium primum atrial septal defects are usually diagnosed in the first few years of life because of mitral regurgitation murmur or an abnormal ECG.
A common atrium (ie, a combination of sinus venosus, ostium secundum, and ostium primum defects) is usually diagnosed in the first few years of life because systemic venous blood and pulmonary venous blood often partially mix before entering each ventricle; this condition manifests as cyanosis. In addition, a common atrium may be associated with complex CHD, and patients may present relatively early because of other intracardiac abnormalities.
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