MYOCARDIAL BRIDGING

MYOCARDIAL BRIDGING

Gary Miles, FALU OTR Asia Editor AXA Hong Kong gary.miles@.hk

Description

MB is a congenital condition in which a segment of the coronary artery runs through the myocardium (muscular middle layer of the wall of the heart). In other words, the coronary artery goes through or "tunnels" into the heart muscle, rather than resting on top of it, and then remerges again on top of the myocardium. Think of MB as the train journey between London and Paris. The train departs London and travels on land (the myocardium) until it reaches Folkstone, UK. It then descends beneath the English Channel (the middle layer of the myocardium) until it remerges on land in Calais, France. The journey under the English Channel represents the coronary artery tunnelling into the myocardium. The part of the myocardium overlying the artery is called the bridge. Although MB can be found in any epicardial artery, the majority (70-98%) involve the left anterior descending artery (LAD).1

MB was first recognised in 1737 by Reyman,2 first reported in-depth in 1951, and recognised angiographically in 1960. Long considered a benign condition, it has recently been thought of in causing myocardial ischemia, myocardial infarction, exercise-induced tachycardia, paroxysmal AV blockade and sudden cardiac death.3

The prevalence of MB is not fully known, which is in some fault due to the various methods used to detect an anatomic variant. Autopsy studies performed report MB rates of 5% to 86%.4 Ferreira et al.5 studied the anatomy of MBs in 90 hearts from subjects (ages still birth to 84 years) who did not have a history of known cardiac disease or cardiac-related death. MBs were identified in 50 hearts (55.6%): 35 hearts had a single bridge affecting the LAD, 10 contained two bridges, and five contained three bridges; however, the autopsy figures differ greatly from the angiographic detection rate of MB, which varies from 0.5% to 12%. There have been several factors expressed to

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Executive Summary A myocardial bridge (MB) is a segment of an epicardial coronary artery traveling through the myocardium, creating a "bridge" across the artery. MB is a benign congenital anomaly. However, it can be associated with acute coronary symptoms, coronary spasms, ventricular septal rupture, arrhythmias (including supraventricular and ventricular tachycardia) and sudden death.

account for the difference between the rate of MBs found on autopsy compared with observations from angiography. These include, but are not limited to: the thickness and length of the myocardial bridge, the reciprocal orientation of the coronary artery and myocardial fibres, and the state of myocardial contractility. Pathologic studies have found a mean frequency of 25%, yet in one autopsy study, the incidence was 50%. Among patients undergoing coronary angiography, the reported prevalence of MB is 1.7%, which is almost always confined to the LAD.

Generally, MB is harmless. The majority of those with MB are born with it and most never know they have the condition. Serious events are rare, and it is still unclear whether MB has a causal effect to the events. It is often found as an incidental finding on a CT angiogram, although some people can develop myocardial ischemia (lack of oxygen) because of a MB. This occurs when the heart contracts during the heartbeat. The bridge of the heart muscle can "tighten down"on the artery, pinching it, thus decreasing the blood flow.6 Fortunately, most of the blood flow through the heart happens during the "rest" phase (diastole) and not during the "contraction" phase (systole).

Although MB is present at birth, symptoms, if any, usually do not develop after the third decade.7 Symptoms occur when the bridge compresses the vessel to

ON THE RISK vol.35 n.4 (2019)

obstruct it more than 60-67% on each contraction. Generally, there are no symptoms if the obstruction was less than 50%.8 Symptomatic patients with MB as their only cardiac abnormality may present with the following:9

? Myocardial ischemia. ? Acute coronary syndromes. ? Coronary spasm. ? Exercise-induced dysrhythmias such as supra-

ventricular tachycardia. ? Atrioventricular conduction block. ? Myocardial stunning. ? Sudden death.

Tarantini et al. opined that the deeper the coronary artery was into the myocardium, the greater chance for symptoms.10 The illustration below notes that until the coronary artery reaches a "thick myocardial bridge," most are asymptomatic.

There are many diagnostic modalities used to investigate MB. However, with a lack of any gold standard for diagnosing MB, the reported diagnostic accuracies are variable.

Non-invasive diagnostic techniques: ? Multiple-slice computed tomography (MSCT). ? Stress single-photon emission computed tomography. ? Stress echocardiography.

Invasive diagnostic techniques: ? Coronary artery angiography (the most common technique for diagnosing MB). ? Intravascular ultrasound. ? Intracoronary doppler. ? Functional flow reserve.

In most patients, a myocardial bridge is not treated if asymptomatic. In symptomatic patients, three treatment strategies have been explored:11

ON THE RISK vol.35 n.4 (2019)

1. Medical treatment such as beta blockers and calcium channel blockers are usually the first line of treatment.

2. Coronary stenting. 3. Surgical myotomy and/or coronary artery bypass

grafting (CABG). Currently, surgical treatment is preferred over stent replacement due to the high risk of complications associated with stent replacement.12

Underwriting MB The underwriting assessment should be based upon whether the applicant is symptomatic or non-symptomatic (no angina, chest pain or other non-specific symptoms). Usually MB found incidentally with no symptoms can be considered without a rating for both Life and Critical Illness (CI).

Caution should be used if the applicant has any other cardiac risk factors or with any signs, abnormal findings on an ECG or stress ECG, or with a medical consulting history. If this is the case, it is best to underwrite the risk similar to single vessel coronary artery disease (CAD), at the very least, for Life coverage. For CI, you could follow the Life rating or consider excluding CAD-related benefits.

Conclusion Myocardial bridging is a congenital and generally benign condition. It is often found as an incidental finding on CT angiograms. It is characterized by the epicardial coronary artery taking an intramyocardial course. MB presents more on autopsy and CT but far less on angiographic detection. The bridge portion itself is "spared" from atherosclerosis. Symptoms are rare but can range from angina to acute coronary syndrome, to sudden cardiac death. For asymptomatic applicants, the general underwriting approach is to consider without any adverse action, in the absence of other coronary risk factors.

Notes

1. Giuseppe Tarantini et al. "Left Anterior Descending Artery Myocardial Bridging" Journal of the American College of Cardiology, Vol. 68, No. 25, 2016.

2. Jorge R. Alegria et al. "Myocardial bridging" European Heart Journal, 26, 1159-1168, 2005.

3. Li Wan and Qingyu Wu, "Myocardial bridge, surgery or stenting?" Interactive Cardiovascular and Thoracic Surgery, Vol. 4, Issue 6, December 2005.

4. Tarantini, Vol. 68. 5. Alegria, 26. 6. Myocardial Bridge, Texas Heart Institute, 2 November 2019. 7. Alegria, 26. 8. Anas Wess, A Message from the Heart to the Heart, Internal Medicine

and Cardiology Rotation, Toronto, Canada. 9. Michael Corban et al. Myocardial Bridging, Journal of the American

College of Cardiology, Vol. 63, No. 22, 2014. 10. Tarantini, Vol 68. 11. Anas Wess. 12. Ibid.

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