Myocardial bridging
Definition:Myocardial bridge (MB) is a band of myocardial tissue overlying a segment of an epicardial coronary artery – the tunneled artery (TA). Angiographically, MB is characterized by systolic compression of the TA. MB is a congenital abnormality with an incidence of 0.5 - 16% in angiographic and 15 – 85 % in autopsy studies. This usually benign condition has been associated with stable and unstable angina, myocardial infarction, myocardial stunning, atrioventricular nodal block, ventricular tachycardia and sudden death. Factors associated with an increased risk of symptomatic MB include its length, thickness and location, severity of systolic arterial compression, presence of left ventricular hypertrophy, increased heart rate, low systolic blood pressure and concomitant coronary vasospasm. MB appears to cause myocardial ischemia by delayed relaxation leading to diastolic TA compression, systolic TA compression especially during tachycardia when systolic coronary flow becomes increasingly important due to shortened diastolic periods and possibly endothelial dysfunction and coronary artery spasm. Evaluation:Atypical chest pain may be a presenting symptom in up to 45% of patients with MB. Sensitivity of exercise stress testing and perfusion imaging studies in patients with MB is comparable with that seen in patients with single-vessel coronary disease. MB with minimal lumen diameter reduction of 70% during systole and 35% during diastole is considered to be hemodynamically significant. Coronary flow reserve (CFR) is reduced in patients with MB to less then 3. However, CFR is not site specific and both concomitant atherosclerotic lesions and microvascular disease can overestimate severity of MB. Intravascular ultrasound (IVUS) helps to delineate extend of MB. Provocation tests with intracoronary nitroglycerine or intravenous dobutamine uncover angiographically silent MB if characteristic “half-moon” phenomenon is present on IVUS examination. Medical therapy:Negative chronotropic agents - beta-blockers and calcium channel blockers improve myocardial perfusion by prolonging diastole. Rate-independent antiischemic properties of beta-blockers seem to be mediated by reduced systolic and diastolic vascular compression. Calcium channel blockers may be particularly useful in patients with MB-associated vasospasm. Nitrates may worsen symptoms of MB due to increased vessel compression and should be used with caution. Antiplatelet agents counteract possible enhanced platelet aggregation in patients with MB. Surgical therapy:Coronary artery bypass grafting in symptomatic MB is currently indicated if: 1. 1. There are other indications for cardiac surgery Minimally invasive, supra-arterial myotomy may become preferred surgical therapy if its safety is documented. Magnetic resonance imaging or intra-operative epicardial echocardiography may then provide necessary information regarding MB thickness and vessel intramyocardial course. Percutaneous therapy:Successful percutaneous therapy of symptomatic MB has been reported. Stents effectively prevent systolic compression of tunneled artery by MB. However, concerns have been raised about symptomatic restenosis in patients with MB treated with stent placement. The frequency of this complication is unknown. We performed a systematic review of the literature. We identified 30 adult patients who were treated percutaneously for symptomatic MB with stent placement into tunneled artery. Patients were characterized by male predominance (80%), age 58±12.7 years (mean±SD) and relatively low prevalence of risk factors for coronary artery disease: hypertension 33%, hypercholesterolemia 30%, smoking 26%, diabetes mellitus 3% and a family history of CAD in 16%. Patients presented with myocardial infarction (16%) or refractory angina with myocardial ischemia documented by stress testing (84%). All patients had MB in the mid-segment of the left anterior descending coronary artery. Concomitant coronary atherosclerosis was present in 3 patients. Patients received 1.85±0.8 stent (diameter 3.1±0.23 mm, length of treated arterial segment 31.6±9.4 mm). Average balloon inflation pressure was 13.4±2.2 atmospheres. Procedural success was 96%. One procedure was complicated by tamponade, requiring emergent venous patching of an arterial perforation and internal mammary artery graft placement. Stent placement in another case was complicated by dissection with a coronary fistula into the right ventricular outflow tract that resolved spontaneously within three months. Nine patients (30%) developed recurrent angina in 1.75 - 6 months after the procedure during approximately one-year follow-up. Angiographic follow-up was performed in 24 patients (80%) at 1.75 - 12 months. Compression of the stent by the MB was found in one patient. Severe stent recoil despite repeated balloon inflation was documented by intravascular ultrasound and patient was treated with coronary artery bypass grafting. Symptomatic myocardial bridging - Case for drug-eluting stent use?Binary angiographic in-stent restenosis in patients who underwent stent placement for symptomatic MB was 30% (9 out of 30 patients). Four patients were treated with balloon angioplasty and one patient with a combination of cutting balloon angioplasty and brachytherapy. Three patients had bypass surgery with an internal mammary artery graft to the left anterior descending artery. All patients treated for restenosis were asymptomatic during further follow-up (12-18 months). Using data from a large cohort, the restenosis rate in patients with atherosclerotic CAD who receive a 3.1 x 31.6 mm stent can be predicted at 17% (confidence interval 13-22%) compared to 30% in MB stenting. The restenosis rate after stenting in symptomatic MB was 76% higher then the rate of restenosis expected in patients with atherosclerotic CAD treated with similarly sized stents (82% higher after adjustment for diabetes mellitus). Whether the use of drug-eluting stents can prevent restenosis in percutaneous therapy of MB requires further study. Clinical research of rare diseases is frequently limited to case reports. Their publication becomes increasingly difficult in the era of large clinical trials. We established this Internet registry to allow physicians to report outcomes of drug-eluting stent therapy of symptomatic MB. An Internet registry provides several advantages in facilitating data accumulation. It provides easy worldwide access to reporting, supports standardization of data collection, captures single patient experiences that otherwise might not be publishable and it supports clinical decision-making by providing references to current data. Stent placement in symptomatic MB seems to be safe and effective. Treatment with bare metal stent is associated with higher then expected rate of restenosis. The role of drug-eluting stents in the treatment of symptomatic MB requires further study. We encourage reports of drug-eluting stent therapy in symptomatic MB to this registry. |
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