CT angiography depicts a higher rate of myocardial bridging than conventional coronary angiography. In addition, the depth—not the length—of the tunneled segment significantly correlated with the percentage of systolic compression, a novel finding, according to investigators from University Hospital Zurich, Switzerland.
Although myocardial bridging is considered by many to be a normal variant, reports do exist of instances where it has led to myocardial ischemia, tachycardia-induced ischemia, conduction disturbances, myocardial infarction, and sudden death, according to the team of cardiologists and radiologists who published the study in the March issue of Radiology.
Conventional coronary angiography, the gold standard, indirectly identifies myocardial bridging by demonstrating systolic compression of the tunneled segment and a localized change in direction of the vessel course toward the ventricle.
The indirect method, with a detection rate between 0.5% and 4.5%, is thought to underestimate the occurrence of myocardial bridging. Autopsy estimates range between 15% and 85%. The truth is probably somewhere in the middle, according to lead author Sebastian Leschka, MD, from the hospital’s Institute of Diagnostic Radiology.
Leschka and colleagues evaluated the coronary arteries of 100 patients who underwent both conventional angiography and CTA. Patients were referred to catheter angiography for various indications including stable angina pectoris (83), atypical chest pain in combination with high risk for coronary artery disease (9), or recurrent symptoms after previous balloon angioplasty with (3) or without (5) stent placement.
Researchers visually analyzed each vessel for the presence of myocardial bridging on the basis of the following indirect signs:
- systolic diameter narrowing
- milking effect, defined as diameter narrowing limited to a restricted vessel segment with extraction of contrast agent not explainable by normal coronary artery flow; and
- the step down-step up phenomenon, defined as a localized change in direction of the vessel course toward the ventricle.
Image quality of CT scans—performed on a Somatom Sensation 64 scanner (Siemens)—was diagnostic in 98% of segments.
Reconstructed CT images were evaluated at an external workstation (Leonardo, Siemens). First, the reconstruction interval with the smallest degree of motion artifacts was identified for each patient. Then, the end-systolic and end-diastolic phases were defined as the last reconstruction interval with an opened and closed aortic valve, respectively, to obtain the appropriate reconstruction time points within the cardiac cycle for measurements of systolic compression.
Multiplanar and curved planar reformations were used to depict myocardial bridging in at least two planes—one parallel and one perpendicular to the course of the vessel. Researchers measured the length and depth of the segment on ribbon planar reformations by using dedicated vessel analysis software (Cardiologist IQ, Advantage Workstation 4.0; GE Healthcare).
CT bests gold standard
CTA detected myocardial bridging in 26 out of the 100 patients, while catheter angiography identified 12 patients with the anomaly. The difference was significant.
In all CT-identified cases, the intramyocardial course of the coronary segments was directly visualized. In the catheter angiography cases, the anomalies were indirectly identified based on the step down–step up phenomenon in four patients, the milking effect in six, and systolic compression in 12.
Catheter angiography had one false negative, on the basis of a supposed step down-step up phenomenon and a calculated systolic compression of 11%, while CT demonstrated an entirely epicardial course of the artery.
Researchers found no significant correlation between the percentage of systolic compression and the length of the tunneled segment as assessed with CT. In contrast, a significant correlation was found between the percentage of systolic compression and the depth of the tunneled segment.