A myocardial bridge (MB) is a common congenital heart anomaly in which one of the coronary arteries tunnels through the heart muscle (myocardium) itself. In most people, the coronary arteries rest on top of the heart muscle and feed blood down into smaller vessels (e.g. the septal arteries) which then carry blood to the heart muscle itself (i.e. populate throughout the myocardium). However, if a band of muscle forms around one of the coronary arteries during the fetal stage of development, then a myocardial bridge is formed – a "bridge" of heart muscle over the artery. Every time the heart squeezes to pump blood, the band of muscle exerts pressure and it very rarely constricts the artery, reducing blood flow to the heart. Even a very thin ex. <1 mm and/or short ex. 20 mm MB can cause significant symptoms, although this has not been completely proven. MBs can range in length from a few millimetres to 10 cm or more. The overall prevalence of myocardial bridge is at 19%, although the prevalence found at autopsies is much higher (42%). Myocardial bridge is usually a harmless condition, and in many cases bridges don't seem to cause any symptoms. However, some people with myocardial bridges may experience angina, or chest pain.
Symptoms and signs While many people have very tiny myocardial bridges that cause no symptoms, others have longer and/or deeper bridges causing significant symptoms, including children. For example, some patients cannot run or exercise at all, others can exercise despite symptoms such as shortness of breath or feelings of tightness in the chest, and still others find improvement of symptoms during exercise. Many competitive athletes have had severe myocardial bridges and unroofing surgery. The symptoms of myocardial bridges differ slightly from patient to patient depending on the length, depth, and location of the bridge. Common symptoms include:
Dizziness Shortness of breath Fatigue Chest pain/angina Palpitations/arrhythmia i.e. irregular heart rhythm Squeezing/tightness/pressure/pain in chest, shoulder, jaw, armpit, neck, and/or down the arm Inability to exercise, walk, do chores, have to sit/lie down Feeling like chest is “going to explode” Feeling of something “clamping down” in the chest Fainting/passing out (syncope) and/or feeling like one is about to faint
Complications Myocardial bridges can cause numerous complications – which are often as misunderstood in the medical community as the condition itself. These include:
angina pectoris (chest pain radiating from the heart) endothelial dysfunction, which causes vasospasms – both sometimes very severe arrhythmia (irregular heartbeats) tachycardia (abnormally high heart rate) plaque forming just before (proximal to) the myocardial bridge Note that studies have shown that plaque does not form inside myocardial bridges, yet there is virtually always plaque just before the myocardial bridge in adults. Some common triggers of myocardial bridge symptoms are:
exertion/exercise anything that raises the heart rate, even positive events low-oxygen environments ex. hot humid weather, high altitude sitting/lying/doing nothing for long periods Notably, high heart rate or tachycardia greatly increases ischemia (low oxygen to the heart) caused by myocardial bridges. Studies have shown that this is because the compressed artery reopens only very slowly each heartbeat and thus stays in a state of semi-compression for most or all of the diastolic period. Thus as the heart rate increases, the time the artery has to reopen (diastolic period) decreases dramatically – to the point that with very high heart rates, the artery never fully reopens and blood flow is constantly reduced.
Diagnosis There are three key tests currently used to diagnose myocardial bridges by Stanford University: CT scan, cardiac catheterization, and stress ultrasound.
CT scan – on which the myocardial bridge often appears as a compressed or squashed area of the artery in which, notably, the fatty areas surrounding the artery (shown in black on CT scans) disappear, since the artery is tunneling through muscle not fat in this area. CTs often allow an assessment of an approximate length and depth of the myocardial bridge, but compression cannot be assessed accurately from a CT scan. IVUS cardiac catheterization including dFFR measured during dobutamine challenge – from which readings of dFFR and percentage compression as well as measurements of the approximate length and depth (shown as the halo or echolucent band) of the MB are taken. It is critical to note that in order to be meaningful in diagnosing a myocardial bridge, it is critical to measure dFFR i.e. the diastolic period, not mean FFR. This is because, contrary to a common misconception, myocardial bridges cause compression of the artery during diastole as well as systole, as explained above. This has been shown in multiple studies. It is also critical that the dobutamine challenge be used, elevating the heart rate, because dFFR decreases significantly at high heart rates as shown by Yoshino et al., 2014. Stress echocardiogram (i.e. before and after running on a treadmill) – used to identify evidence of ischemia i.e. a lack of oxygen delivered to the ventricle due to the MB. This test does not visualize the MB itself but rather its effects on the ventricle. Multiple studies have shown that ischemia from MBs is indicated by the appearance "septal buckling" in the stress echocardiogram, as the septum gives out under stress., which a 2013 paper by Lin et al. describes as "a transient focal buckling in the end-systolic to early-diastolic motion of the septum with apical sparing which correlates prospectively with the presence of LAD MB." As much of the science of testing for MBs is relatively new, patients frequently go undiagnosed. Stanford's center for myocardial bridges has offered second opinion services from a distance for some ten years, including to numerous international patients. Notably, EKG is not a reliable or conclusive diagnostic tool for diagnosing MBs. Some symptomatic MB patients show normal EKG results and others abnormal. Many doctors have suggested that there is a need for more awareness of MBs among doctors and better testing, including testing of young people as the disease is congenital. According to a 2007 study:
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