Chapter 132. Malformations of The Heart and Great Vessels - Review of Medical Embryology Book

Malformations of the heart and great vessels are common because the development of the structures is very complex. The incidence is about 0.7% of live births and 2.7% of stillbirths. Only the major conditions are described and outlined here

DEXTROCARDIA (displacement of the heart to the right): if the heart tube bends to the left instead of the right, there is transposition, in which the heart and its vessels are reversed left to right, as in a mirror image. It is uncommon, yet is the most frequent of positional abnormalities

Isolated dextrocardia: abnormal heart positioning without other visceral displacement; however, one may see other heart defects
Dextrocardia with situs inversus includes transposition of the viscera, such as the liver being on the left side and the heart on the right. Few other heart defects accompany

Extrathoracic ectopia cordis: most common; heart protrudes through a sternal defect caused by failure of the lateral folds to fuse in the thorax during week 4 of development. Other congenital defects also are seen. Survival rate is low
Incomplete ectopia cordis: the heart lies within the body but sternal and mediastinal defects cause abnormal positioning of the heart in the chest

ATRIAL SEPTAL DEFECTS are one of the most common of congenital defects. The clinically significant atrial defects are discussed here

Patent foramen ovale (secundum type): most common congenital heart defect; includes both the septum primum and septum secundum defects. Usually results from an abnormal resorption of the septum primum (foramen ovale valve) during formation of the septum secundum

If resorption occurs in an abnormal location, the septum primum is fenestrated
If excessive resorption of the septum primum occurs, there is a short septum which fails to cover the foramen ovale
If a large foramen ovale results from a defective development of the septum secundum, the normal septum primurn will not close the foramen ovale at birth
Large atrial septal defects may result from excessive resorption of the septum primum and a large foramen ovale
A patent foramen ovale without atrial septal defects may be due to abnormal intra-atrial pressure after birth

Endocardial cushion defect with primary type of atrial septal defect: relatively common. The septum primum fails to fuse with the endocardial cushions, leaving a patent foramen primum

In the complete type: seen in 20% of people with Down's syndrome, but otherwise is uncommon; fusion of the cushions does not occur, resulting in a large hole in the center of the heart (atrioventricularis communis or persistent atrioventricular canal). This condition is usually accompanied by a patent foramen primum and a ventricular septal defect

Sinus venosus type of atrial septal defect: very rare; seen high in the interatrial septum, peripheral to the fossa ovalis. It is due to incomplete absorption of the sinus venosus into the right atrium and failure or abnormal development of the septum secundum

Common or single atrium: very rare; no interatrial septum. Due to failure of the septum primum and septum secundum to develop