CTA visualizes myocardial bridging better than catheter angiography
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.

Myocardial bridging of distal left anterior descending artery in 47-year-old man was not identified in this conventional coronary angiogram (right anterior oblique projection in diastole). A subsequent right anterior oblique projection in systole showed considerable luminal diameter narrowing that was calculated to be 52%. CT also identified the bridging. (Source: Radiological Society of North America)  
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.

Catheter angiography had difficulty identifying shorter and shallower bridged segments. The tunneled segments missed at conventional coronary angiography had a mean length (20.9 mm) and depth (2.6 mm) significantly shorter and shallower than the segments catheter angiography identified: 28.3 mm and 2.9 mm, respectively.

Conventional angiography also had trouble identifying bridging in segments with a lower percentage of systolic compression. The percentage of systolic compression at CT in the patients in whom conventional coronary angiography failed to demonstrate myocardial bridging (mean, 9.9%) was significantly lower than the systolic compression assessed with CT and conventional coronary angiography (mean, 31.3%).

“Even though demonstration of systolic compression and the milking effect is considered diagnostic, the signs are rather insensitive in shallow variants of myocardial bridging that demonstrate only minimal or no systolic compression. Similarly, the step down–step up phenomenon may be absent in superficial variants of myocardial bridging,” Leschka said.

Overall, the mean percentage of systolic compression as assessed with CT (31.3%) significantly correlated with the mean obtained at conventional coronary angiography (28.2%).

The authors concluded that visual estimation of myocardial bridging at conventional angiography is limited to segments with systolic compression of more than 20%.

“The authors need to be applauded for doing this thorough work,” said U. Joseph Schoepf, MD, director of CT research and development at the Medical University of South Carolina, Charleston. “It highlights a very simple truth: 3D volumetric imaging has an advantage over a modality that only shows the vessel lumen.”

Volume-rendered CT image in end diastole (100% of R-R interval) in right anterior oblique view depicts tunneled proximal right coronary artery segment (dotted circle in magnified view) in 51-year-old man. A conventional coronary angiogram during systole showed no compression and no milking effect in the segment; thus, the diagnosis of myocardial bridging was not made with conventional angiography. (Source: Radiological Society of North America)  
Schoepf said that in his clinical experience, myocardial bridging occurs in about 20% of the CTA studies they perform. He has never seen, however, a chest pain case where the symptoms were attributable to myocardial bridging.

The best visual technique to diagnose myocardial bridging, according to Schoepf, is curved multiplanar imaging, where the artery stretches out along its course making it easy to identify tunneled segments.

Leschka and colleagues also noted that the coronary segment proximal to myocardial bridging frequently shows a disproportionate degree of atherosclerotic plaque formation, explained by its low wall shear stress. They suggest, therefore, that early identification with CTA of myocardial bridging could become important.

Schoepf noted that the tunneled segments almost never show atherosclerosis. Recent research suggests the tunneled portion is not exposed to the cytokines released by epicardial fat, which trigger atherosclerosis.