How dangerous is a myocardial bridge?

Can be normal. A myocardial bridge is when a coronary artery the normally runs on top of the heart actually dives into the muscle and is compressed with each heart beat. This isn't as bad ad it sounds because most blood flow occurs during diastole or when the heart is relaxed. Very rarely, some bridges can be symptomatic and cause angina symptoms but this should be a diagnosis of exclusion.
Myocardial bridge. A bridge is a condition you're born with in which the coronary artery runs through the heart muscle instead of on the outer surface of the heart. It's benign. Remember, it's been there since birth!
Usually notdangers. If you mean you have an epicardial coronary that someone has told you displays myocardial bridging, then usually benign; however, if this causes symptoms, see a doctor.