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Atrial and atrioventricular septal defects


Defects of the atrial septum are common cardiac anomalies. The surface cardiac anatomy is assessed for cardiac chamber and pulmonary artery enlargement, status of the mitral and tricuspid valves, location of the pulmonary veins (especially the right superior pulmonary vein), persistent left superior vena cava, and patent ductus arteriosus.

Morphology

Figure 4-1, A There are three main types of atrial septal defects. The most common is the secundum type, in which the defect occupies the location of the foramen ovale. The ostium primum type is actually a defect of the atrioventricular septum and is located low in the septum. The defect is crescent-shaped and is associated with atrioventricular valve abnormalities. The sinus venosus type of atrial septal defect is located high in the septum, near the superior vena cava orifice, and is often associated with an anomalous connection of the right superior pulmonary vein. B Atrial septal defect, secundum type. The ostium secundum defect is an embryologic failure of atrial septation related to excessive fenestration of the septum primum during formation of the ostium secundum or inadequate coverage of the ostium secundum by the septum secundum as it descends from the roof of the atrium. This specimen shows some features of both mechanisms: there are fenestrations extending inferiorly to the limbus of the fossa ovalis, and the atrial septum is clearly thicker in the fossa above the fenestrations. There is also a patent foramen ovale due to failure of fusion after birth. Thus, there are two defects within the fossa ovalis, both some distance above the annulus of the tricuspid valve. C Atrial septal defect, ostium primum type. The term atrioventricular septal defect has been proposed to describe the group of defects variously termed endocardial cushion defects and atrioventricular canal defects . The pathognomonic feature of this group of malformations, whatever the specific type, is a defect at the site of the atrioventricular septum. The normal atrioventricular septum is composed of the fibrous extension of the central fibrous body, located at the junction of the aortic root and the atrioventricular valve rings, and a muscular portion that is the cephalic segment of the inlet ventricular septum. These defects also have a virtually identical common atrioventricular junction guarded by a basically six-leaflet common atrioventricular valve. It is the anatomy of these leaflets bridging the ventricular septum that differentiates the partial and complete forms of the defect. In the partial form of the anomaly, the bridging leaflets of the atrioventricular valve are joined by a connecting tongue of valve tissue and are usually firmly adherent to the crest of the ventricular septum. This specimen demonstrates an intact fossa ovalis with a small rim of septum that extends to the coronary sinus. The septal rim borders a low-lying atrial septal defect extending to the atrioventricular valves. This defect is caused by the embryologic failure of the septum primum to fuse with the endocardial cushions. The left atrioventricular valve shows a prominent “cleft” in its septal portion, but the valve is fused to the ventricular septum. D Atrioventricular septal defect, left ventricular outflow tract. The left ventricular outflow tract is abnormal in all patients with this defect. It is longer and narrower than normal because of the left atrioventricular valve’s attachment to the ventricular septum at the edge of the septal deficiency, which may or may not be an open defect. The extent of narrowing is determined by how far the septal deficiency extends across the septum below the aortic valve. In this case, the narrowing of the left ventricular outflow tract is moderate, extending only halfway across the diameter of the aortic orifice. E Atrioventricular septal defect, left ventricular outflow tract, severe narrowing. This specimen shows severe narrowing of the left ventricular outflow tract due to the left atrioventricular valve’s attachment to a large ventricular septal deficiency that involves most of the septum below the aortic valve. Note that there is no opening in the ventricular septum due to dense atrioventricular valve tissue occluding the septum. F Atrioventricular septal defect, left ventricular outflow tract, severe narrowing. This specimen illustrates a steeper angle than that in Figure 4-1 , E , with severe narrowing of the left ventricular outflow tract.

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