Myocardial Bridging

Myocardial bridging is a congenital anomaly in which a segment of an epicardial coronary artery runs inside the myocardium. The overlying muscle is the bridge, and it compresses the tunnelled artery in systole.

The mid left anterior descending artery is involved in about 70 to 98% of cases. Prevalence is about 0.5 to 2.5% on angiography, 3 to 58% on CT depending on technique, and up to 40% or more at autopsy.

Most bridges are benign. Deeper bridges (over 2 mm), longer bridges (over 25 mm), and bridges in hypertrophic cardiomyopathy are more likely to cause symptoms.

Pathophysiology. Compression in systole is followed by delayed relaxation, so narrowing persists into early diastole, when most coronary flow occurs. Tachycardia shortens diastole and raises compression, and so exertion provokes ischaemia.

The bridged segment is protected from atherosclerosis. The proximal segment, exposed to low shear stress and high wall tension, often develops plaque. Endothelial dysfunction and spasm add to ischaemia.

• Most patients are asymptomatic.
• Exertional or atypical angina, dyspnoea, palpitations, and syncope.
• Acute coronary syndrome, ventricular arrhythmia, takotsubo like events, and rarely sudden death.
• Resting ECG usually normal. Ischaemic change may appear with exercise, dobutamine or stimulants.

Red flags.

• Syncope, ventricular arrhythmia or resuscitated arrest
• Infarction in a young patient with a bridge
• Coexisting hypertrophic cardiomyopathy
• Inducible ischaemia in the territory of the bridge on stress imaging

• Invasive angiography. Systolic narrowing with persistent diastolic narrowing (the milking effect), with a step down and step up appearance. Systolic narrowing over 70% is severe. Intracoronary nitroglycerin or dobutamine accentuates it.
• Coronary CT angiography. Bridge length, depth and the thickness of overlying muscle, and proximal plaque.
• Intravascular ultrasound. A half moon echolucent halo around the bridged segment.
• Physiology. Pressure wire assessment during dobutamine challenge. A diastolic fractional flow reserve of 0.76 or less is reported as significant.
• Stress imaging and ambulatory ECG for ischaemia and arrhythmia. Acetylcholine provocation when spasm is suspected.
• Echocardiography or cardiac MRI to exclude hypertrophic cardiomyopathy.

Differential diagnosis.

• Atherosclerotic stenosis of the left anterior descending artery
• Coronary vasospasm
• Spontaneous coronary artery dissection
• Hypertrophic cardiomyopathy
• Microvascular angina
• Coronary artery fistula
• Anomalous coronary origin
• Takotsubo syndrome

• Asymptomatic bridge with no ischaemia: no treatment and no restriction of activity, unless hypertrophic cardiomyopathy or arrhythmia is present.
• Avoid isosorbide, glyceryl trinitrate (except for acute relief when it clearly helps), amlodipine and nifedipine, which can increase systolic compression and heart rate and worsen symptoms. Avoid stimulants.
• Beta blocker first line. Bisoprolol 2.5 mg orally once daily, increased to 5 to 10 mg once daily, maximum 20 mg daily (do not exceed 10 mg daily at creatinine clearance under 20 mL/min or in severe liver impairment), aiming for a resting heart rate of 50 to 60/min. Contraindicated in severe asthma, sinus node disease and high grade atrioventricular block without a pacemaker. Monitor heart rate and blood pressure.
• If beta blockers are not tolerated: diltiazem 60 mg orally every 8 hours (maximum 360 mg daily), or verapamil 40 to 120 mg orally every 8 hours (usual 80 mg), maximum 480 mg daily. Avoid with reduced ejection fraction or heart block, and do not combine with a beta blocker without monitoring. Ivabradine (off label) 5 mg orally twice daily (2.5 mg twice daily at age 75 years or over), maximum 7.5 mg twice daily, when heart rate stays over 60/min. Contraindicated with heart rate under 60/min before starting and with strong CYP3A4 inhibitors (ketoconazole, clarithromycin, ritonavir). It can cause visual phosphenes and atrial fibrillation.
• Proximal plaque: aspirin 75 to 100 mg orally once daily and atorvastatin 40 to 80 mg once daily with liver enzymes at baseline and 3 months.
• Vasospasm: add diltiazem 60 mg orally every 8 hours (maximum 360 mg daily), avoiding it with reduced ejection fraction or heart block.
• Do not stent the bridged segment. Risks are perforation, stent fracture, restenosis and thrombosis.
• Surgery for disabling symptoms with proven ischaemia despite 3 months of maximal therapy: supra-arterial myotomy (unroofing), or bypass grafting when the bridge is deep or long, or when myotomy has failed. Risks are right ventricular perforation, bleeding and recurrence.
• Ventricular arrhythmia or resuscitated arrest: an implantable defibrillator by the usual criteria, with revascularisation of the bridge territory where indicated.
• Exercise: competitive sport is restricted with ischaemia, arrhythmia, syncope or hypertrophic cardiomyopathy. An asymptomatic patient with normal stress testing and ambulatory ECG may participate after full assessment.

Special groups. Pregnancy: use metoprolol tartrate 25 to 100 mg orally twice daily, maximum 200 mg daily in this setting. Hypertrophic cardiomyopathy: treat the cardiomyopathy and the bridge together.

Monitoring. Review yearly, or sooner with new symptoms. Stress imaging when symptoms change. Heart rate and blood pressure at each visit. Ambulatory ECG yearly with palpitations or syncope.

Referral and escalation. Refer to cardiology for any symptomatic bridge. Urgent for syncope, ventricular arrhythmia, acute coronary syndrome or ischaemia despite maximal therapy.