Morphological features of myocardial bridging in patients who underwent CT coronary angiography in the National Heart Centre of Nepal

Myocardial bridging (MB) is a congenital anomaly in which a segment of a coronary artery is partially or totally encased by the myocardium. It is a clinical condition with several possible manifestations. Traditionally, it is considered a benign condition as it remains clinically silent in the vast majority of cases. However, the association between myocardial ischemia and myocardial bridging has increased their clinical relevance.^1,2 Clinical consequences of MB range from angina to acute coronary syndrome to even sudden cardiac death. Numerous autopsy series have reported myocardial bridging from 5%–86%. Meta-analysis showed an overall prevalence of 42% in autopsy studies, 22% in CT coronary angiography studies, and around 6% in conventional angiography. Though pathological studies are considered the gold standard for detecting this anomaly, recent advances in MDCT are considered a reliable, non-invasive modality for diagnosing myocardial bridging. Although myocardial bridges can be found in any epicardial artery, 67% to 98% occur in the left anterior descending coronary artery (LAD).³

Angiography shows a milking effect and step-down step-up phenomenon resulting from systolic compression in the tunneled artery as well as the presence of atherosclerotic plaque proximal to the tunneled artery.⁴ Our aim was to evaluate the frequency and morphological features (types, location, atherosclerotic changes) of myocardial bridging in patients who underwent ECG-gated 640 slice MDCT coronary angiography in Shahid Gangalal National Heart Centre (SGNHC).

This was a retrospective cross-selectional study conducted at Shahid Gangalal National Heart Centre (SGNHC) in Kathmandu, Nepal, who underwent MDCT coronary angiography. Ethical approval was obtained from the institutional review board (IRB) of SGNHC. Consecutive patients who underwent CT coronary angiography between 1st January 2021 to 30th June 2021 were reviewed. CT Coronary angiography was performed to evaluate patients with no known Coronary Artery Disease who present with stable typical or atypical chest pain or a possible angina equivalent. CT Coronary angiography was performed using the Toshiba Aquilion One Vision 320 detector rows 640 Slice CT Scanner. Axial, sequential imaging with prospective gating and 0.5 mm slice thickness was performed following the intravenous administration of contrast material (65ml Ultravist 370). The patients were pre-medicated with a sublingual 0.5mg nitroglycerin tablet for coronary dilatation. The depth of the tunneled artery 1-2 mm is considered as superficial Myocardial Bridging, while the depth >2 mm is considered as deep Myocardial Bridging.⁵ All images were analyzed by one of our 3 radiologists.

In descriptive statistical analysis, quantitative variables were expressed as mean±SD and categorical variables as frequencies or percentages. In addition to probability values, 95% CIs were applied. For all analyses, P<0.05 was considered statistically significant. All statistical analyses were performed with statistical software (IBM SPSS® Statistics 25 for Windows).

1068 consecutive patients underwent CT coronary angiography between 1st January 2021 to 30th June 2021. The majority (61.3%, n=655) of the patients in this study were female, with a mean age of 57 years. Of these 1068 patients, 141 (13.2%) were found to have myocardial bridging (MB) (Table 1).

Among those 141 MB patients with mean age of 53.1 years, 133(94.3%) were superficial MB, while only 8 (5.7%) were deep MB. 138 (97.9%) MB were located in LAD, and among those LAD MB, 119(86.2%) were mid-LAD MB (Table 2).

Myocardial bridging is generally considered a benign anomaly but, in rare incidences, is clinically significant when associated with regional hemodynamic alterations, and studies have shown that such instances of myocardial bridging are linked to clinical complications that include ischemia, acute coronary syndrome, coronary spasm, arrhythmia, and sudden death.^6-10 The frequency of MB in our study was 13.2%, which was lower in comparison to various autopsy series as well as other imaging modalities, which range from 23-30%.^11,12 One of the largest studies by Risse et al. involving 1056 patients found an intramyocardial coronary artery course in 26% of patients.¹³ As in previous studies, the majority of MB was located in mid-LAD in our study as well.^14,15

Many studies suggested that in around 90% of cases, the vessel segment proximal to the bridge appears to develop atherosclerosis.¹⁶ Unlike in another study, the prevalence of atherosclerosis in the proximal segment of MB is negligible in our study. The extent and spatial distribution of atherosclerosis in the segment proximal to MB is influenced by the anatomical properties of the MB.¹⁷ The reason behind low prevalence of atherosclerosis in proximal segment of MB in our study was beyond the scope of our study. Other diagnostic techniques like Stress echocardigraphy, Myocardial perfusion imaging etc have been used in few small studies but further studies will be necessary to improve and validate these techniques before they can be accepted as a method for the diagnosis of MB.

The frequency of MB in our study was 13.2% with more than 90 percent of MB was in mid LAD. As MDCT is an evolving method, it is an effective and noninvasive method for an alternative in the diagnosis of MB.