Myocardial bridging
Synonyms and keywords: myocardial bridge, coronary bridge, coronary artery bridge, coronary bridging, coronary artery bridging

# Overview
A myocardial bridge is defined as a segment of a major coronary epicardial artery that "tunnels" or passes intramurally through the myocardium beneath the muscle bridge during systole. Myocardial bridges are generally located in the distribution of the left anterior descending artery (LAD) and its diagonal branches.
Myocardial bridging should be distinguished from intramyocardial course, which is an inborn coronary anomaly defined as a segment of a major epicardial coronary artery running and ending in the myocardium.

# Historical Perspective
Angiographically, myocardial bridging was first recognized in the 1960's.

# Pathophysiology

### Variants
There are two basic morphologic variants:
- Deep variant
- Superficial variant

### Extent
Majority of the cases, systolic bridging is usually not severe or extensive and completely reverses during diastole; except in patients with HOCM, wherein severe systolic bridging of the septal perforators may be a predisposing factor to the development of myocardial ischemia.

# Epidemiology and Demographics
The reported frequency varies with different modalities used to identify myocardial bridges. Generally, with coronary angiography, the reported frequent ranges from 0.5-16%  and with pathological or coronary CT, the frequency is between 5-86%.
Irrespective of the modality used, the left anterior descending coronary artery is most commonly involved and a higher incidence is observed in patients with hypertrophic coronary artery and cardiac transplants. Among the pediatric age group, the occurrence of myocardial bridging is rare but an increased incidence may be observed in association with hypertrophic cardiomyopathy.

# Natural History, Complications and Prognosis

## Complications
Complications of myocardial bridging include the following:
- Angina and ischemia
- Acute coronary syndromes if there is plaque rupture in or adjacent to the myocardial bridge
- Coronary artery spasm
- Ventricular septal rupture
- Arrythmias
- Exercise-induced atrioventricular conduction block
- Myocardial stunning
- Transient ventricular dysfunction
- Early death after cardiac transplantation
- Sudden cardiac death

## Prognosis
The combined presence of hypertrophic cardiomyopathy and myocardial bridging is associated with a worse prognosis as compared to hypertrophic cardiomyopathy alone.

# Diagnosis

## Coronary Angiography
- Systolic narrowing of the epicardial artery with complete reversal during diastole.
- Angiographic manifestation depends on:
Thickness and length of myocardial bridge
Reciprocal orientation of the coronary artery and myocardial fibers
Presence of loose connective tissue or adipose tissue around the bridged segment
Presence of an aortic outflow tract obstruction
Intrinsic tone of the wall of the coronary artery
Presence of a proximal coronary fixed obstruction
State of myocardial contractility
- Thickness and length of myocardial bridge
- Reciprocal orientation of the coronary artery and myocardial fibers
- Presence of loose connective tissue or adipose tissue around the bridged segment
- Presence of an aortic outflow tract obstruction
- Intrinsic tone of the wall of the coronary artery
- Presence of a proximal coronary fixed obstruction
- State of myocardial contractility
- Nitroglycerine provocation to demonstrate systolic bridging may be employed in patients with exertional angina
Shown below is an animated image depicting myocardial bridge.
Shown below are two static images of the same case depicting myocardial bridge.  Outlined with yellow in the image on the left is the part of the coronary artery during diastole.  This same part of the coronary artery is outlined in yellow in the image on the right during systole; note that this outlined part is narrowed compared to its shape during diastole.
Shown below are an animated image and a static image depicting myocardial bridge.

## Intracoronary ultrasound
- Half moon phenomenon: caused by adipose tissue that is surrounding the bridged segment of the artery 
- Ultrasound images have shown that in patients with myocardial bridging there is delayed relaxation of the artery in early diastole.. This may reduce coronary vasodilator reserve.
- An impaired coronary flow reserve has also been noted in patients with myocardial bridging. This can be corrected with intracoronary stent implantation.
- Intracoronary ultrasound demonstrates that although many times the bridged section of the artery was free of disease, plaques were found next to the bridged fragment of the artery.
- Echolucent half moon phenomenon over the bridge segment.
- Systolic compression of the bridge segment of the coronary artery.
- Retrograde flow in the proximal segment (systolic flow reversal).
- Accelerated flow velocity at early diastole (Finger-tip phenomenon).
- No or reduced systolic antegrade flow.
- Decreased diastolic/systolic velocity ratio.

# Treatment

## Medical Treatment
- Calcium channel blockers (preferably non-dihydropyridines)
- Beta Blockers may reduce contractility, but they may also increase coronary spasm by blocking beta mediated vascular relaxation
- Nitrates are contra-indicated because reduction in preload may cause reflex sympathetic stimulation and tachcardia.

## PCI/Stenting
- Stenting can abolish hemodynamic abnormalities associated with myocardial bridging and may improve clinical symptoms in patients with myocardial bridges.
- Care must be exercised in stent placement to assure that the bridge is not just moved upstream or downstream of the stent.
- Of note, there have been reports of coronary artery perforation and target vessel revascularization in patients with myocardial bridging during/following stent placement, respectively.
- Stents placed in this position may be placed under torsional strain, extrinsic compression, and repeated flexion which may theoretically increase the risk of stent strut fracture.

## Surgery
Surgery is limited to patients with symptoms that persist despite medical treatment.
Minimally invasive CABG with placement of a left internal mammary artery to the distal left anterior descending artery is one option.
Myectomy can be performed on the myocardium surrounding the coronary artery. The procedure involves surgical division (unroofing) of the coronary artery. Surgical unroofing of the myocardial bridge often results in clinical improvement. Potential complications include right ventricular perforation and left ventricular aneurysm which can be avoided with the use of echocardiography to follow the course of the vessel.
Cardiac surgery is not indicated for hypertrophic cardiomyopathy associated myocardial bridging, and medical therapy may provide adequate anti-ischemic therapy.