Myocardial bridging (MB) is the most common congenital coronary variation, and is the most commonly located in the left anterior descending artery (LAD) middle segment. A 27% incidence of LAD MB has been reported in unselected necropsy series. In vivo, intravascular ultrasound (IVUS) and coronary computed tomography angiography (CCTA) are accurate imaging modalities to detect and characterize MB, whereas coronary angiography alone detects only a small proportion of MB.Association between MB and adverse cardiac events, particularly myocardial infarction and sudden cardiac death has been repeatedly described, but given the high incidence of MB in the general population, a causal relationship is still uncertain.Percutaneous coronary interventions had also been reported to carry a higher risk of complications (restenosis, perforations) when stents are implanted in a MB segment The purpose of our study is to evaluate the incidence and characteristics of LAD MB in the setting of acute coronary syndrome when the LAD is the presumed culprit artery.

We retrospectively analyzed IVUS studies of consecutive patients admitted to the cardiology department of the Universitary Hospital of Bejaia (Algeria) for acute coronary syndrome and a presumed culprit lesion located in the LAD, from January to August 2022.Intracoronary injection of 0.5 mg Isosorbide Dinitrate prior to IVUS acquisition was mandated by institutional protocol.All IVUS studies were obtained with a 60 MHz Opticross HD IVUS catheter (Boston Scientific, USA).We evaluated IVUS recordings for the presence of a MB.When a MB was identified, we measured its length, depth (Figure 1) and systolic compression index defined as (1 -systolic diameter / diastolic diameter) at the maximal compression site.We also measured minimal tunneled segment diameter (TSD) and compared it to the distal reference diameter (DRD). Diameters were measured in diastole from external elastic lamina to external elastic lamina (EEL-to-EEL). The difference between DRD and TSD was calculated.
We estimated maximum plaque burden (PB) in the tunneled segment. A PB of more than 50% defined significant tunneled segment disease.
Measurements are expressed as means ± standard deviation.A Spearman correlation analysis with calculation of the correlation coefficient "r" was performed to test whether there was a correlation between MB depth and systolic compression index and between MB depth and (DRD - TSD).A point-biserial correlation was run to determine the relationship between MB characteristics and significant tunneled segment disease.A p value < 0.05 was considered statistically significant.

Seventy-four patients were included in the study (78% males, 22% females). Mean age was 62 ± 13 years (60 ± 14 years for males, and 70 ± 7 years for females).LAD MB was identified in 37.8% of patients (28 of 74).
MB length was 14.6 ± 8.97 mm (minimum: 1.5 mm ; maximum: 33 mm; 95% confidence interval of mean: [11.15; 18.04] mm).
MB depth was 0.83 ± 0.54 mm (minimum: 0.25 mm ; maximum: 2.6 mm; 95% confidence interval of mean: [0.62; 1.04]mm).Systolic compression index was 0.11 ± 0.08 (minimum: 0 ; maximum:0.3; 95 % confidence interval of mean: [0.08;0.15]).
Significant tunneled segment disease was found in 21% of MB (6 of 28), but extended beyond the first 3 mm of tunneled segment extremities in only 2 patients (7 % of MB).TSD was smaller than the DRD by 0.41 ± 0.3mm (minimum: 0 mm ; maximum: 1.13 mm ; 95% confidence interval of mean: [0.29; 0.53]) (Figure 2).Spearman correlation analysis showed that there was no significant association between MB depth and Systolic compression index, r = 0.29, p = 0.146).However, it did show a high positive correlation between MB depth and the difference (DRD - TSD) (r = 0.69, p = < 0.001).
A Point-biserial correlation did not show a significant correlation between MB depth, length or systolic compression index and significant tunneled segment disease (respectively rpb = -0.1, p = 0.622; rpb = 0.01, p= 0.964; rpb = 0.09, p = .666).

Incidence of LAD MB in our study population was 37.8%, which is higher than the reported incidence in unselected population and chronic coronary syndrome population, suggesting a causal relationship of MB in the occurrence of acute coronary syndromes. However MB varied widely in length and depth and larger longitudinal studies are necessary to define "pathologic" MB.
Presence of significant atherosclerosis in the tunneled segment was not rare (21% of MB). Thus, if a percutaneous coronary intervention (PCI) with stent implantation was planned, landing the stent in the MB had to be considered.
Interestingly, tunneled segment diameter in diastole was found to be smaller than the distal reference diameter, which might explain the reported increased risk of coronary perforation during tunneled segment PCI.
MB depth correlated better than systolic compression index with the difference between distal reference diameter and tunneled segment diameter.