Significance of anatomical properties of myocardial bridge on atherosclerosis evolution in the left anterior descending coronary artery
Introduction
Myocardial bridge (MB) is a common anatomical situation in the left anterior descending coronary artery (LAD) [1] in which part of LAD is covered by myocardial tissue. Since a detail description of MB [2], it has been designated as intramural coronary artery, mural coronary artery, or coronary artery overbridging [3]. Frequency of MB in LAD is relatively high, sometimes over 50% by autopsy study [4]. It is widely accepted that arterial intima beneath MB is significantly spared from atherosclerotic changes [1], [4]. This atherosclerosis suppression under MB compared with the coronary segments proximal to MB has been also evident in histopathologic [5], [6], [7] and image analysis [8] studies. It has been also suggested through morphological examination of endothelial shapes that hemodynamic changes leading to high shear stress in LAD segment under MB contribute to atherosclerosis suppression in this region [6]. This hypothesis is further supported by a morphological observation using a cholesterol-fed rabbit model [9]. These studies on LAD with MB indicate that hemodynamic alterations resulting from systolic compression by MB consistently influence distribution of atherosclerotic lesions.
On the other hand, disturbed coronary flow due to systolic compression by MB may also evoke clinical symptoms of ischemic heart disease [3], [10], ventricular fibrillation [11], atrioventricular block [12], and sudden cardiac death [13]. These conditions are probably caused by coronary ischemia due to retrograde blood flow from the coronary segment under MB into the segment proximal to MB through MB compression during systole. Such blood flow subsequently impedes coronary flow and causes persistent reduction of arterial diameter during systole [14], [15]. Angiographically visible severe narrowing of LAD lumen by MB compression during systole may contribute to coronary ischemia [10]. An LAD deeply situated in the myocardium can be distorted by systolic compression of MB [16]. Considering these angiographic and morphological evidences [10], [13], [16], anatomical properties of MB, such as its location within LAD as well as its thickness and length, can be regarded as critical factors that directly influence extent of coronary stenosis and disturbed blood flow in LAD. In this study, we histomorphometrically investigated the relationship between various anatomical properties of MB and extent of atherosclerosis in human LAD.
Section snippets
Materials
MB group consisted of 200 autopsied cases with MB(s) in LAD but without any morphologically proven cardiovascular disease (Fig. 1). Control group similarly having no cardiovascular disease consisted of 100 autopsied cases without MB in LAD. They were obtained from consecutive autopsies at Ohmori Hospital of Toho University from 1998 to 2002. MB group consisted of 149 males and 51 females, with mean age 66.6 ± 13.7 years, and control group for 71 males and 29 females, with mean age 67.4 ± 14.7
Atherosclerosis ratio in MB segments
In MB group, one MB was observed in 150 cases (75%), two MBs for 45 cases (22.5%), and three MBs for 5 cases (2.5%). Mean value of atherosclerosis ratio in the segments proximal to MB was 1.93 ± 0.99, MB segments for 0.86 ± 0.51, and the segments distal to MB for 0.98 ± 0.65. Atherosclerosis ratio of MB segments was significantly (p < 0.0001) lower than that in the segments proximal to MB (Fig. 2a), and this was consistent for all age groups (Fig. 2b). Atherosclerosis ratio in all the three LAD
Discussion
This study is the first to comprehensively examine the significance of anatomical properties of MB on extent of atherosclerosis in LAD. Present results indicate that atherosclerosis suppression distinctly takes place in LAD intima under MB, irrespective of MB location, thickness and length, and moreover that these anatomical properties of MB are critical to regulate atherosclerosis evolution not only in LAD intima under MB but also LAD intima proximal to MB.
In previous studies, existence of MB
Acknowledgements
This study was partly supported by a MEXT Grant (30101893) of Japan and the Project Research Grant (16-25) of Toho University School of Medicine.
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