THE EMBRYOLOGICAL BASIS FOR CARDIOVASCULAR CONGENITAL ABNORMALITIES
The development of the heart is a complex process that requires significant movement of tissues and changes in relationships of one structure to another. The cardiovascular system is also developmentally precocious so that we may not be aware of many abnormalities that may be occurring. Defects in the heart and the great vessels can lead to death not only after birth but also in utero. When considering the development of the heart is it important to keep in mind the normal anatomy. It is also important to remember the normal development of each structure. With that in mind, we will proceed with the assumption that the normal development of the heart is understood. Abnormalities will be described on the basis of how the normal development may not have occurred properly and caused these defects.
1. Heart - The Atrium
The development of the atria involves the formation of an interatrial septum that arises from four structures: 1. septum primum, 2. septum secundum, 3. the incorporated sinus venosus, and 4. the endocardial cushion tissue. Four types of defects can occur that result from the failure of one or more of these structures to develop normally.
The most common interatrial septal defect is the foramen ovale defect (ostium secundum defect). This defect occurs when ostium secundum becomes too large as the result of the loss of too much septum primum tissue. Another factor also may be too little growth of septum secundum. This leaves an opening in the region of fossa ovalis that permits blood flow between the atria.
Another interatrial septal defect that occurs is the upper interatrial septal defect (sinus venosus defect). This defect occurs when the sinus venosus is not properly incorporated into the right atrium and interatrial septum. The sinus venosus may be incompletely incorporated or else may not shift to the right enough to be fully incorporated into the septum. Another possibility is that the septum secundum may not develop or may become reabsorbed. In either case there is a defect in the interatrial septum, above the fossa ovalis.
A low interatrial septal defect (ostium primum), occurs when there is the retention of the foramen primum. This may be due to the inadequate growth of septum primum, or it can be due to a malformation of endocardial cushion tissue. In either case the persistence of a lower interatrial foramen occurs. If endocardial cushion tissue is involved then there also may be a common atrioventricular canal. With a common AV canal defect, there also can be an interventricular septal defect. Abnormal valve formation also may accompany this defect.
Finally, an uncommon interatrial defect is the common atrium or cor triloculare biventriculare. This defect is the result of the lack of septum primum and septum secundum formation. An interatrial septum is not present and the atria form one large chamber.
2. Heart - The Ventricle
The interventricular septal defects are common defects of the heart. This is understandable given the complex movements that are needed for the contribution to this structure from endocardial cushion tissue, muscular interventricular septal tissue and aorticopulmonary septal tissue. This is especially true given the shifting and spiraling of the aorticopulmonary septum during normal development of the heart.
The pars membranaceum defect of the interventricular septum occurs as the result of the abnormal contribution of endocardial cushion tissue, a lack of connective tissue from the muscular interventricular septum, or the lack of aorticopulmonary tissue. During discussions of other types of defects it is seen that this interventricular septal defect is often associated with other congenital cardiac abnormalities.
There can be defects in the muscular interventricular septum. These are called pars muscularis defects. During normal development there are numerous sinuses or canals between the two ventricular chambers. Normally they are obliterated with the final formation of the interventricular septum. However, at times the interventricular connections remain and an interventricular defect such as that seen on the right can occur.
Finally, there can be interventricular defects that occur with the complete absence of the interventricular septum. This defect is called cor triloculare biatriatum. In this defect there are two atrial chambers that both empty into a single ventricular chamber.
3. The Heart - Bulbus Cordis Defects
One type of bulbus cordis defect is the absence of part or all of the aorticopulmonary septum. The caudal part of the septum can be absent so that there is a partial persistent truncus arteriosus defect. There can also be a complete absence of the aorticopulmonary septum so that the pulmonary arteries arise from the common truncus arteriosus. In both cases, there is an accompanying IV defect which results from the lack of a contribution of the bulbar septum to the IV septum.
Another type of defect is transposition of the great vessels. This is due to the improper spiraling of the aorticopulmonary septum. The defect depicted is transposition of the outflow tract. Here the aorta arises from the right ventricle and the pulmonary artery arises from the left ventricle. In the transposition depicted here, there is the formation of the IV septum. For survival of the infant, it is necessary that there also be either an interatrial septal defect or a patent ductus arteriosus.
Here, there is complete transposition of the great vessels with an interventricular septal defect. The aorta arises from the right ventricle. The pulmonary artery arises from the left ventricle. This defect is due to the improper spiraling of the aorticopulmonary septum that normally occurs during development.
In this figure there is incomplete transposition of the great vessels with both arising from the right ventricle. In this case the transposition occurs due to the improper spiraling of the aorticopulmonary septum. The improper position of the great vessels results from the lack of a leftward shift of these vessels that normally occurs during development. Thus there is an accompanying IV septal defect that occurs with the improper alignment of the aorticopulmonary septum with the IV septum.
A vascular stenosis is a narrowing of one of the arteries. Developmentally, a stenosis can be the result of many different factors. When a stenosis occurs during development it not only results in a defect of the vessel itself, but also can produce hypertrophy or hyperplasia of the accompanying ventricle. In the case seen in this figure, there is a subvalvular aortic stenosis. This can be due to an abnormal increase in connective tissue or muscular tissue in the region. The reason for the increase in tissue may range from abnormal mitral valve formation to improper development of the annulus fibrosus in that region. In this case of aortic stenosis there could be an accompanying hypoplastic left ventricle that results in hypertrophy of the muscular left ventricular wall and a small left ventricular chamber.
Other types of stenosis include supravalvular aortic stenosis that can occur anywhere along the length of the aorta. Another type of stenosis is the valvular aortic stenosis. In this case the aortic valves can be malformed and result in the narrowing of the outflow tract. This can happen to the pulmonary outflow tract as well.
Another type of stenosis is due to the unequal partitioning of the aorta and pulmonary artery so that one vessel lumen is too small. The most common syndrome that results from this type of stenosis is Tetralogy of Fallot. Here the unequal partitioning of the aorta and pulmonary artery leaves the pulmonary artery narrowed. This then results in the classic 4 characteristics of Tetralogy of Fallot: 1. the pulmonary stenosis; 2. a failure of the aorta to shift to the left so that it comes to lie over the IV septum; 3. an IV septal defect; and 4. right ventricular hypertrophy due to the narrowing of the right ventricular outflow tract. (For ultrasound images from the University of Pennsylvania, click here).
Other defects of the heart and great vessels occur and their complexity and variety go beyond the scope of this course. Due to improved clinical approaches to congenital heart disease many of these defects are now being detected earlier and being surgically corrected with increased success.
2. Major Vessels - Arteries
The most common defect of the cardiovascular system is the patent ductus arteriosus. This occurs when the ductus arteriosus fails to close during the few weeks after birth. It is now known that the closure of the ductus arteriosus, which is functionally closed normally 15 to 20 hours after birth, is mediated by an increased partial pressure of O2 in the blood that occurs shortly after birth. In addition, bradykinins released from the lungs may also play a role in the closure of the ductus. Prior to birth the dilation of the fetal vasculature is under the control of prostaglandins. Prostaglandins relax the musculature of the wall of the ductus. In some cases of patent ductus arteriosus, administration of inhibitors of prostaglandins can result in the closure of the ductus.
Another type of defect is coarctation of the aorta. This is the narrowing of the aorta, which is often the result of a persistence of muscular tissue in the region of the ligamentus arteriosus. Most of the coarctations occur in the region opposite the ductus arteriosus and are called juxtaductal coarctation. Some can occur distal to the ductus, while others occur proximal to the ductus, and are called postductal and preductal, respectively.
At times there can be the retention of the right dorsal aorta between the right subclavian artery (which arose in part from the 7th intersegmental artery) and the left dorsal aorta. In this case there is the formation of the double aortic arch. This may be entirely compatible with life.
In another type of defect the arch of the aorta forms from the 4th aortic arch on the right and a mirror image of the normal development of the arch of the aorta occurs. In this case the descending aorta forms on the right and the brachiocephalic artery is on the left. This is depicted in this figure and as seen can occur with the aorta normally arising from the left ventricle. This is a defect in the aortic arch formation and not the bulbus cordis. It is now thought that neural crest tissue contributes to the formation of the aortic arches and this region and so these defects may be the result of abnormal neural crest migration.
There are other abnormalities that occur in the formation of the arteries. They include the formation of a right aortic arch with a retroesophageal left subclavian artery. In this case the left subclavian lies posterior to the esophagus and may actually constrict it. Another abnormality is a normal left aortic arch with an anomalous right subclavian artery which arises from the retained right dorsal aorta.
2. Major Vessels - Veins
As noted in gross anatomy there are many variations in the location of the large veins. This is the result of the fact that the veins often develop in a variety of patterns. An example is the position of the portal vein, superior mesenteric vein, inferior mesenteric vein, and the splenic vein. Therefore, many variations in the development of the veins are completely compatible with life. There are some examples that are worth mentioning for this discussion.
One example is the double superior vena cava. This is due to the complete retention of the left anterior and common cardinal veins. It may or may not be accompanied by the remnants of the thymico-thyroid anastomosis of veins with the larger vessel the remnant of the thymico-thyroid veins (called the bridging vein) the smaller vessel, the left superior vena cava. In the case, the left superior vena cava empties into the right atrium. However, in this example, the left superior vena cava arises from the left atrium.
Anomalies of the inferior vena cava and its tributaries also occur.
Finally, pulmonary vein anomalies also occur and there are many variations. In some cases the anomalous pulmonary veins can enter the right atrium, though most often not directly. Again sophisticated imaging techniques have allowed for better detection of many of these variations.
For a further illustration of congenital heart abnormalities, check out the Temple Pathophysiology courses' Congenital Heart Disease page.
For gross pictures of heart abnormalities, go to the Pediatric Cardiology Almanac. Look specifically for the multimedia educational resources and the pathology atlas.
For radiographs of various congenitial heart conditions, try the University of Minnesota's Congenital Heart Disease page.