Relation of myocardial bridges and loops on the coronary arteries to coronary occulsions.

Abstract 1. 1. The muscular overbridgings on the coronary arteries of 70 human hearts are described and classified into (a) muscular bridges under which the artery submerges during its course over the surface of the ventricles, and (b) muscular loops which attach the artery to the atrial myocardium during its course in the atrioventricular groove. These formations occur in 85.7 per cent of all hearts; the occurrence is more frequent in the region of the left coronary artery. 2. 2. The muscular bridges formed by ventricular myocardium—usually 10 to 20 mm. long, up to 5 mm. thick, and rarely mentioned in the literature—are most frequent in the proximal half of the anterior descending branch (60.0 per cent), next most frequent in the oblique branch of the left coronary artery (18.5 per cent), and rarer (2.8 to 14.2 per cent) in other branches of the left and right coronary arteries. 3. 3. The muscular loops, formed by atrial myocardium—usually 10 to 15 mm. long, 100 to 300 μ thick, and not mentioned in the literature up to now—are most frequent in the first stretch of the circumflex branch of the left coronary artery (40.0 per cent), and in the terminal stretch of the right coronary artery (27.1 per cent). 4. 4. The stretch of the artery before the bridge or loop corresponds to the sites at which occlusions most frequently occur, just as the order of frequency is the same as that of the occurrence of these formations (the origin of the anterior descending branch, the proximal stretch of the circumflex branch of the left, and the terminal stretch of the right coronary arteries). 5. 5. Microscopic examination of 36 hearts showed a very close relationship of the muscular formation to the adventitia. The muscular overbridging may have more layers (circular and longitudinal). In the muscular loops, fibrous degeneration in old persons was observed to greater or smaller degree. 6. 6. The intima of the artery under the bridge is normally thin, whereas especially before the bridge it is strongly hyperplastic, but sometimes also behind the bridge. 7. 7. The degree of hyperplasia of the intima in unbridged and bridged arteries before the bridge (most frequently the site of occlusions) is compared. As to the surface of the sections, hyperplasia of the intima is of the same degree, but there is a difference in its shape. Hyperplasia of the intima in the unbridged artery is more regular, whereas in the bridged artery it is irregular, and there is a greater tendency to the formation of sclerotic plaques. 8. 8. The stretches of the arteries before the muscular bridges and loops could become one of the morphologic conditions, a site of predilection, for a sclerotic process. In this way the coincidence between sites of the most frequent overbridgings and those of the most frequent occlusions of the coronary arteries could be explained.