Myocardial bridge


 * Associate Editor-In-Chief: ; Assistant Editor-In-Chief: Meagan Doherty, B.S.

Synonyms and keywords: Myocardial bridging, MB

Overview
A myocardial bridge is defined as a segment of a major coronary epicardial coronary artery that "tunnels" or passes intramurally through the myocardium beneath the muscle bridge. Myocardial bridges are generally located in the distribution of the left anterior descending artery (LAD) and its diagonal branches.

Epidemiology and Demographics
The reported frequency varies from 0.5% to 16% in coronary angiographic series. Although myocardial bridging in children is rare, myocardial bridging in children may occur in association with hypertrophic cardiomyopathy, with an incidence of 28% among children with diagnosed hypertrophic cardiomyopathy.

Complications
Complications of myocardial bridging include the following:
 * Ischemia and acute coronary syndromes
 * 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 (MB) is associated with a worse prognosis as compared to hypertrophic cardiomyopathy alone.

Pathophysiology
There are two basic morphologic variants:
 * Deep variant
 * Superficial variant

Angiographic Findings

 * Systolic narrowing of the epicardial artery
 * Angiographic manifestation depends on
 * Thickness and length of myocardial bridge
 * Reciprocal orientation of the coronary artery and myocardial fibers
 * Presence of loose connective 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

Myocardial Bridge Example
Image courtesy of C. Michael Gibson MS. MD.

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

Medical Treatment

 * Beta Blockers
 * Calcium channel blockers

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.

Surgery
Surgery is limited to patients with symptoms that persist despite medical treatment.

Surgical Procedures

 * 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. Potential complications include right ventricular perforation and left ventricular aneurysm.  Surgical unroofing of the myocardial bridge often results in clinical improvement
 * Cardiac surgery is not indicated for hypertrophic cardiomyopathy associated myocardial bridging, and medical therapy may provide adequate antiischemic therapy