The major coronary arteries, which normally are distributed over the epicardial surface of the heart, occasionally have a segmental intramyocardial course. During systole, this segment of the vessel is compressed, a condition referred to as milking or systolic "myocardial bridging" [1,2]. This phenomenon was first recognized more than 200 years ago , was first reported in depth in 1951, and was recognized angiographically in 1960.
On angiography, bridging is recognized as compression of a segment of a coronary artery during systole, resulting in narrowing that reverses during diastole. The dynamic and phasic nature of the obstruction serves to differentiate bridging from fixed coronary stenosis (image 1).
It has been thought that most instances of bridging are of little clinical significance. However, there are reports suggesting that severe bridging of the major coronary arteries can produce myocardial ischemia, coronary thrombosis and myocardial infarction, as well as predispose the patient to atherosclerosis or sudden death [4-10].
PREVALENCE OF BRIDGING
The reported prevalence of bridging varies according to the method of evaluation. Pathologic studies have found a mean frequency of myocardial bridging of 25 percent (range 5 to 86 percent), similar to that observed in noninvasive imaging studies using coronary computed tomography [1,2,11-14]. In one autopsy study, the incidence was 50 percent . Although all major epicardial coronary arteries can be affected, involvement of the LAD is the most common.
Angiographic studies have noted somewhat different findings. Among patients undergoing coronary angiography, the reported prevalence of myocardial bridging is 1.7 percent (range 0.5 to 16 percent), which is almost always confined to the LAD (image 1) [1,2,5,15-19]. A higher prevalence has been observed in patients with hypertrophic cardiomyopathy and in recipients of cardiac transplants [1,19]. (See 'Bridging in patients with hypertrophic cardiomyopathy' below.)