CONGENITAL HEART DISEASE
Year : 2002 | Volume
: 3 | Issue : 2 | Page : 5-
Congenital heart disease - A: Isolated atrial septal defect embryology, clinical presentation, and diagnosis
Pediatric Cardiology Section, Department of Cardiology and Cardiovascular Surgery, Hamad Medical Corporation, Doha, Qatar
Department of Cardiology and Cardiovascular Surgery, Hamad Medical Corporation, P.O. Box 3050, Doha;Consultant Pediatric Cardiologist, Hamad Medical Corporation, Doha
This review summarizes the embryological development of the atrial septum and the abnormality resulting from improper partitioning of the atrium leading to a communication between the right and left atria. The pathophysiology and clinical presentation of isolated atrial septal defect are discussed. Confirmatory diagnosis is based on echocardiography, mainly two-dimensional and color Doppler echocardiography.
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Numan M. Congenital heart disease - A: Isolated atrial septal defect embryology, clinical presentation, and diagnosis.Heart Views 2002;3:5-5
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Numan M. Congenital heart disease - A: Isolated atrial septal defect embryology, clinical presentation, and diagnosis. Heart Views [serial online] 2002 [cited 2021 Oct 20 ];3:5-5
Available from: https://www.heartviews.org/text.asp?2002/3/2/5/64469
Congenital defects of the atrial septum are common and they may be located in different anatomic portions of the atrial septum. Atrial septal defects maybe solitary cardiac anomalies or they may be associated with other congenital cardiac anomalies. The size may vary greatly. The functional consequences are related to the anatomic location of the defect, the size of the defect, and the presence or absence of other cardiac anomalies. The incidence of atrial communication between the left and right sides vary according to the age of the patient. In the fetus, the incidence of atrial communication is 100% since the communication is necessary to keep them alive. Patent foramen ovale is also common in humans and remains throughout life in 20%. The true defects of the atrial septal wall constitute 6-10 % of all congenital heart defects (i.e. 1 in 1200 live births). Females have higher susceptibility for these defects with female to male ratio of 2:1.
Embryology and development of the atrial septum
To understand the classification of atrial septal defects (ASD), it is necessary to review the development of the atrial septum. [Figure 1] shows the stages of the development of the atrial septation.
During the first 23 days of the embryo's life, the atrial chamber is a common one and connected to the sinus venosus to drain the blood coming from right and left vens horns. After completion of cardiac looping by the third week, atrial septation begins by developing the septum primum partly from the endocardial cushion tissue. It is important to note that there is always atrial communication between the left and right sides throughout all developmental stages to maintain adequate right to left shunt and oxygenate the developing brain tissue. Even before the completion of the septum primum, fenestrations start to take place in this septum to allow blood to flow freely through it. By the early fifth week of fetal life, the septum secondum starts to appear from the superior-posterior aspect of the mid-atrial wall and extends inferiorly and anteriorly, leaving a flap of the septum primum in the mid-portion to be uncovered. This flap is pushed by the blood flow coming from the umbilical vein and remains open till the end of pregnancy [Figure 2] and [Figure 3].
Anatomy of atrial septal defects
Pathophysiology of ASD
Normally the left atrial pressure is slightly higher than right atrial pressure by 2- 5 mmHg due to less compliant left atrial wall compared to the right. As a result the blood flows from left to right across the ASD with consequent right ventricular volume overload. The pulmonary flow will increase more than systemic and takes more time for the right ventricle (RV) to empty through the pulmonary valve, resulting in delayed pulmonary component of the second heart sound (wide split 2nd heart sound). Because of increased pulmonary flow across the valve, a flow murmur (low frequency ejection systolic murmur) will be heard over the left 3rd intercostal space, which is accentuated with inspiration. The pulmonary vascular beds initially will accommodate the extra flow with no increase in pulmonary resistance. After long standing increase in pulmonary flow, certain patients (more in females) will have hyperplasia of the pulmonary arterioles and smooth muscles resulting in increased pulmonary artery pressure (pulmonary hypertension) and right ventricular hypertrophy. Rarely there will be paradoxical reversal of the shunt across the atrial defect (i.e. right to left shunt) in some patients during valsava maneuvers or deep inspiration. If there is a small clot in the systemic veins - which will normally be filtered by the lungs - the danger of systemic embolization to the brain and other vital organs will be higher especially in presence of increased right ventricular pressure. [Figure 8] summarizes these changes.
Clinical signs and symptoms
Usually atrial septal defects (ASD) are discovered incidentally during routine physical examination. The wide split second heart sound and pulmonary flow murmur are usually noted by most primary care physicians and cardiac referral and evaluation will reveal the defect. A mid-diastolic murmur at the left lower sternal border indicates a large size ASD. Isolated ASDs of moderate and large size do not cause major symptoms in most cases during infancy and early childhood. Failure to thrive on the basis of the ASD alone is rare. Occasionally some patients may present with easy fatigue or palpitations. Others may remain asymptomatic for several decades. Left-to-right shunting tends to increase with age in many patients. The large shunts present in many older patients cause stretching of the atria, which predisposes them to atrial arrhythmias such as atrial fibrillation, atrial flutter and tachycardia. These arrhythmias increase in incidence with age and are a major cause of morbidity and mortality. Pulmonary hypertension is not uncommon in large and moderate defects. Usually this complication occurs in the second decade of life and more in females. Pulmonary hypertension rarely occurs in infants with large ASD.
The chest X-ray in patients with atrial septal defect and sizable left-to-right shunts generally shows cardiac enlargement and increased pulmonary vascularity with a dilated pulmonary trunk and central branches [Figure 9].
The electrocardiogram (ECG) may be normal with an uncomplicated ASD and small shunt. However, incomplete right bundle (with a rSR' or rsR' pattern in the right precordial leads), right axis deviation, and right ventricular hypertrophy may be present, and atrial arrhythmias, particularly atrial fibrillation, are commonly seen. A 12- Lead ECG with significant ASD will usually show right axis deviation, right atrial enlargement and right ventricular hypertrophy [Figure 10].
Confirmatory diagnosis usually will be obtained by transthoracic echocardiogram and occasionally by transesophageal echo. Rarely angiograms will be needed to confirm the diagnosis. [Figure 11],[Figure 12],[Figure 13],[Figure 14] show these diagnostic modalities. The M-mode echocardiogram in patients with classic secundum ASD usually shows right ventricular enlargement, and paradoxical motion of the interventricular septum is present in many cases. Transthoracic two-dimensional echocardiography (2DE) can provide direct noninvasive visualization of all types of atrial septal defect. The reliability of 2DE in demonstrating the characteristic dropout in the atrial septum is best when the axis of the echo beam is perpendicular to the atrial septum. For most defects the subcostal approach provides such a perpendicular angle. In addition to direct visualization of the atrial septum, 2DE often demonstrates enlargement of the right atrium, right ventricle, and pulmonary arteries. Color Doppler also provides useful diagnostic information in these patients. It allows for the direct visualization of the flow across the ASD. If additional confirmation of diagnosis is desired, peripheral contrast echocardiography using agitated saline is effective in demonstrating right to left shunting across the ASD. A right-to-left shunt can be detected by direct visualization of microcavitation bubbles in the left atrium and left ventricle [Figure 14]. A left-to-right shunt can be detected as a negative contrast washout effect in the right atrium if good opacification of the atrium is achieved. Evaluation of the atrial septum with transthoracic echocardiography may be suboptimal in older patients with poor echocardiographic windows. In such patients, transesophageal echocardiography can provide excellent definition of the atrial septum. In addition, it is helpful in the sizing of defects, the diagnosis of sinus venosus defects, and the assessment of associated congenital anomalies or other abnormalities such mitral valve prolapse or partial anomalous pulmonary venous drainage. Transesophageal echocardiography can help to guide proper device placement during catheter atrial septal defect occlusion procedures. Transesophageal echocardiography can be successfully performed in most adults with light sedation. However, infants, children, and adolescents do not tolerate the procedure without heavy sedation or general anesthesia.
Cardiac catheterization is not essential for diagnosis when noninvasive tests clearly demonstrate the presence of an ASD. However, when pulmonary vascular disease is suspected, determination of pulmonary vascular resistance by catheterization is indicated.
Most children and adolescents with an ASD are asymptomatic even in the presence of large shunts. While the defect is often asymptomatic until adulthood, complications of an undetected lesion include irreversible pulmonary hypertension, right ventricular failure, atrial arrhythmias and paradoxical embolism. Transthoracic imaging and an M-mode echocardiogram frequently provide the first confirmation of the diagnosis of ASD. The transesophageal approach is superior to transthoracic echocardiography in its ability to image the interatrial septum and is extremely accurate in the diagnosis of all three types of septal defects.[Figure 4],[Figure 5],[Figure 6],[Figure 7]