Can Myocardial Bridging Cause Heart Attack

Myocardial bridging, a congenital heart condition where a segment of a coronary artery tunnels through the heart muscle, has long been considered a benign anomaly. However, growing evidence suggests that in certain individuals, it can contribute to significant cardiac events, including heart attacks. This article explores the complexities of myocardial bridging, its potential to induce myocardial infarction, and the factors that influence this risk.

Understanding Myocardial Bridging

Myocardial bridging (MB) is an anatomical variation characterized by a coronary artery, typically the left anterior descending (LAD) artery, coursing through the myocardium (heart muscle) rather than lying on its surface. During systole (the contraction phase of the heart), the overlying muscle compresses the tunneled artery, causing temporary narrowing. This compression usually resolves during diastole (the relaxation phase), allowing normal blood flow to resume.

• Prevalence: MB is more common than previously thought. Autopsy studies reveal a prevalence ranging from 15% to 85%, although the clinical significance of these findings varies widely.
• Location: The LAD artery is most frequently affected, particularly its mid-segment.
• Types: MB can be classified as superficial or deep, depending on the depth of the tunneled segment within the myocardium. Deeper bridges are generally associated with more pronounced systolic compression.

How Myocardial Bridging Can Lead to Myocardial Infarction

The mechanisms by which MB can precipitate a heart attack (myocardial infarction) are multifactorial and not completely understood. While the systolic compression of the artery is transient, it can initiate a cascade of events that ultimately compromise myocardial perfusion.

1. Endothelial Dysfunction: The repetitive compression and decompression of the tunneled artery can damage the endothelium, the inner lining of the blood vessel. Endothelial dysfunction is a key early step in atherosclerosis (plaque buildup). Damaged endothelium becomes more permeable to lipids and inflammatory cells, promoting the formation of plaques within the artery wall.

2. Atherosclerosis: While MB was initially believed to protect the tunneled segment from atherosclerosis due to the systolic compression, studies have shown that atherosclerosis can occur proximal (upstream) to the bridge. The altered blood flow dynamics caused by the bridge, including increased shear stress, contribute to plaque formation in the segments of the artery before it enters the myocardial tunnel.

3. Vasospasm: MB can induce coronary artery spasm, a sudden constriction of the artery that reduces blood flow to the heart muscle. The exact mechanism of vasospasm in the context of MB is unclear, but it may be related to endothelial dysfunction, increased sensitivity to vasoconstrictor substances, or abnormal autonomic nervous system activity.

4. Reduced Diastolic Filling Time: In individuals with hypertrophic cardiomyopathy (HCM) or other conditions that cause increased heart rate or shortened diastole, the time available for blood flow through the bridged segment during diastole is reduced. This can lead to myocardial ischemia (inadequate blood supply) and potentially infarction.

5. Plaque Rupture: If a plaque has formed proximal to the MB, the altered hemodynamics and increased shear stress can make the plaque more vulnerable to rupture. Plaque rupture triggers the formation of a blood clot (thrombus) that can completely block the artery, leading to a heart attack.

6. Left Ventricular Hypertrophy: In some cases, chronic myocardial ischemia caused by MB can lead to left ventricular hypertrophy (LVH), an enlargement of the heart muscle. LVH increases myocardial oxygen demand and can further exacerbate ischemia, increasing the risk of infarction.

Factors Influencing the Risk of Heart Attack in Myocardial Bridging

Not everyone with myocardial bridging experiences adverse cardiac events. Several factors determine the likelihood that MB will lead to a heart attack.

• Depth and Length of the Bridge: Deeper and longer bridges cause more significant systolic compression and are more likely to induce ischemia.
• Presence of Atherosclerosis: The presence of atherosclerotic plaques, particularly proximal to the bridge, significantly increases the risk of infarction.
• Associated Cardiac Conditions: Individuals with hypertrophic cardiomyopathy (HCM), coronary artery ectasia (abnormal widening of the coronary arteries), or other cardiac conditions are at higher risk.
• Heart Rate: Increased heart rate, whether due to exercise, stress, or underlying conditions, reduces diastolic filling time and can worsen ischemia in individuals with MB.
• Medications: Certain medications, such as beta-blockers, can slow the heart rate and increase diastolic filling time, potentially reducing ischemia. Conversely, medications that increase heart rate or blood pressure could exacerbate symptoms.
• Lifestyle Factors: Smoking, high cholesterol, hypertension, and other risk factors for atherosclerosis can accelerate the progression of endothelial dysfunction and plaque formation, increasing the likelihood of myocardial infarction in individuals with MB.

Diagnosis of Myocardial Bridging

Myocardial bridging is often discovered incidentally during coronary angiography, a procedure in which a catheter is inserted into a blood vessel and guided to the heart to inject dye and visualize the coronary arteries.

• Coronary Angiography: The classic angiographic finding of MB is the "milking effect," a visible narrowing of the artery during systole that disappears during diastole. However, angiography may not detect all cases of MB, especially those with shallow bridges.

• Intravascular Ultrasound (IVUS): IVUS is an imaging technique that uses ultrasound to visualize the inside of the coronary arteries. IVUS can detect MB even when it is not apparent on angiography and can also assess the presence and severity of atherosclerosis.

• Fractional Flow Reserve (FFR): FFR is a technique used during coronary angiography to measure the pressure gradient across a narrowed artery segment. An FFR value of 0.80 or less indicates that the narrowing is causing significant ischemia. FFR can be used to assess the functional significance of MB, particularly in cases where there is uncertainty about whether the bridge is causing symptoms.

• Cardiac Computed Tomography Angiography (CCTA): CCTA is a non-invasive imaging technique that uses CT scans to visualize the coronary arteries. CCTA can detect MB and assess the presence of atherosclerosis, but its ability to assess the functional significance of MB is limited.

• Stress Testing: Stress testing, such as exercise stress testing or pharmacological stress testing, can be used to evaluate whether MB is causing myocardial ischemia. During stress testing, the heart is made to work harder, either through exercise or by administering medications that increase heart rate and blood pressure. If MB is causing ischemia, the stress test may reveal abnormalities in the electrocardiogram (ECG) or imaging studies.

Management and Treatment of Myocardial Bridging

The management of myocardial bridging depends on the severity of symptoms and the presence of associated cardiac conditions.

1. Medical Management:

   • Beta-Blockers: Beta-blockers are often the first-line treatment for MB. They slow the heart rate, increase diastolic filling time, and reduce myocardial oxygen demand.
   • Calcium Channel Blockers: Calcium channel blockers can also be used to reduce heart rate and blood pressure, and they may also help to prevent coronary artery spasm.
   • Nitrates: Nitrates can dilate the coronary arteries and improve blood flow to the heart muscle. However, they may also cause hypotension (low blood pressure) and should be used with caution.
   • Antiplatelet Agents: Antiplatelet agents, such as aspirin or clopidogrel, may be prescribed to reduce the risk of blood clot formation, particularly in individuals with atherosclerosis.
   • Statins: Statins are medications that lower cholesterol levels. They are often prescribed to individuals with MB who also have high cholesterol or atherosclerosis.

2. Surgical Management:

   • Myotomy: Myotomy involves surgically dividing the muscle fibers overlying the tunneled artery. This relieves the systolic compression and improves blood flow. Myotomy is typically reserved for patients with severe symptoms that are not adequately controlled with medical therapy.
   • Coronary Artery Bypass Grafting (CABG): CABG involves bypassing the blocked or narrowed artery with a healthy blood vessel from another part of the body. CABG may be considered in patients with MB who also have significant atherosclerosis in other coronary arteries.
   • Unroofing: Unroofing is a surgical procedure that involves removing the myocardial bridge. This procedure is more extensive than myotomy and is typically reserved for cases where the bridge is very deep or long.

3. Percutaneous Coronary Intervention (PCI):

   • PCI, also known as angioplasty, involves inserting a catheter into the blocked or narrowed artery and inflating a balloon to widen the artery. A stent, a small metal mesh tube, is then placed in the artery to help keep it open. PCI is generally not recommended for the treatment of MB alone because the stent can be compressed by the overlying muscle. However, PCI may be considered in patients with MB who also have significant atherosclerosis proximal to the bridge.

Lifestyle Modifications

Lifestyle modifications play a crucial role in managing myocardial bridging and reducing the risk of heart attack.

• Healthy Diet: A heart-healthy diet low in saturated and trans fats, cholesterol, and sodium can help to prevent atherosclerosis and improve overall cardiovascular health.
• Regular Exercise: Regular exercise can help to lower blood pressure, improve cholesterol levels, and reduce the risk of heart disease. However, individuals with MB should avoid strenuous exercise that can increase heart rate and exacerbate symptoms.
• Smoking Cessation: Smoking is a major risk factor for atherosclerosis and heart disease. Quitting smoking is one of the best things you can do for your heart health.
• Weight Management: Maintaining a healthy weight can help to lower blood pressure, improve cholesterol levels, and reduce the risk of heart disease.
• Stress Management: Stress can increase heart rate and blood pressure, which can worsen symptoms of MB. Finding healthy ways to manage stress, such as yoga, meditation, or spending time in nature, can help to improve cardiovascular health.

When to Seek Medical Attention

Individuals with myocardial bridging should seek medical attention if they experience any of the following symptoms:

• Chest pain or discomfort
• Shortness of breath
• Dizziness or lightheadedness
• Fatigue
• Irregular heartbeat

These symptoms may indicate myocardial ischemia or other cardiac problems. Prompt diagnosis and treatment can help to prevent serious complications, such as heart attack.

Conclusion

Myocardial bridging, once considered a benign anatomical variant, is now recognized as a potential cause of myocardial ischemia and infarction in susceptible individuals. The complex interplay of endothelial dysfunction, atherosclerosis, vasospasm, and altered hemodynamics contributes to the risk. While not all individuals with MB will experience adverse cardiac events, careful evaluation and appropriate management are essential to mitigate the risk of heart attack. A combination of medical therapy, lifestyle modifications, and, in selected cases, surgical intervention can effectively manage symptoms and improve outcomes for individuals with myocardial bridging. If you suspect you may have myocardial bridging or are experiencing related symptoms, consulting with a cardiologist is crucial for accurate diagnosis and personalized treatment strategies.