Myocardial Bridge

A myocardial bridge (MB) is an anatomical variant in which the myocardial muscle partially covers the epicardial coronary arteries -. Although MB can be detected in any coronary artery, most involve the left anterior descending coronary artery (LAD). This variant has historically been regarded as benign, because contraction of the bridged muscles alters blood flow within the underlying LAD during systole, whereas coronary flow in the LAD occurs predominantly during diastole. However, an MB can lead to significant clinical issues, such as arrhythmia, myocardial ischemia conduction disturbances 4, , myocardial infarction 6) and sudden death 7) in a subset of patients. II. Prevalence and diagnostic testing The prevalence of MB varies widely according to the detection methods applied. The reported MB rates among numerous necropsy series (Fig. 1) 8) range from 5% to 86% 9) and an average of ~25% of adults have MB. The reported rates of MB are higher according to pathological series including thin MB or even myocardial strands with minimal hemodynamic consequences, than those determined by coronary angiography, which typically detects systolic compression as a “milking effect” (Fig. 2) . Coronary angiography is the most popular means of diagnosing MB in the clinical setting, with detection rates ranging from 0.5% to 12% at rest and up to 40% upon provocation or after intracoronary nitroglycerin injection 2-4, -. Numerous factors have been presumed to account for the reported mismatch between the rates of “tunneled arteries” that run intramurally through the myocardium compared with angiographic findings. These include MB thickness and length, the reciprocal orientation of the coronary artery and myocardial fibers, loose connective or adipose tissue around the bridged segment, aortic outflow tract obstruction, in which the systolic tension that develops in the MB overcomes the intracoronary artery pressure, the intrinsic tone of the wall of the coronary artery, a proximal coronary fixed obstruction that causes a decrease in distal intracoronary pressure, and the status of myocardial contractility . Intravascular ultrasound (IVUS) can clearly visualize eccentric or concentric systolic compression in the tunneled segment of an artery that persists into diastole 4, 5, 8, -, accompanied by a highly specific echolucent “halfmoon” appearance throughout the cardiac cycle (Fig. 3) 5, 8, . Vessel compression can be detected by IVUS under coronary provocation even in the absence of angiographically significant milking. The prevalence of MB determined by IVUS, which is more sensitive than angiography for detecting minor compression, is 23%. Optical coherence tomography (OCT) can also detect MB with a homogeneous specific “band” appearance outside the adventitia (Fig. 4). Review Article