In the present study, MB was identified in 42 out of 116 patients (36%). Maximum plaque burden was not different between patients with and without MB in the proximal and distal segments, whereas significantly smaller in the MB segment compared with the corresponding mid segment in patients without MB. LCBI and maxLCBI4mm were significantly lower in both the proximal and MB segments in MB patients compared with the corresponding segments in non-MB patients. Multivariable analysis demonstrated MB and maximum plaque burden as independent predictors of LCBI and maxLCBI4mm in both the proximal and MB segments, except for the tendency that MB predicted maxLCBI4mm in the proximal segment.
MB was previously reported to accelerate atherosclerosis in the segment proximal to MB but not in the segment beneath MB, and to be associated with plaque rupture leading to myocardial infarction, especially in the younger population. These phenomena were supported by autopsy and clinical observational studies [2,12,13]. However, several studies using multi-slice computed tomography have demonstrated that MB has no association with the progression of atherosclerotic plaque [3,4]. In the present study, plaque progression in the proximal segment was not different between patients with and without MB, although it was significantly suppressed in the MB segment compared with the corresponding mid segment in non-MB patients. While MB was defined as any echo-lucent band detected by grayscale IVUS in the present study, it was unclear whether all of MB had a significant effect on coronary blood flow, leading to increased atherosclerosis. In addition, patients in the present study had relatively more comorbidities, such as diabetes (DM) and dyslipidemia (DLP), than those of previous studies [14]. Moreover, patients without MB tended to be younger than those with MB in the present study. These potential factors could make the effect of MB on atherosclerotic plaque formation in the segment proximal to MB equivocal between patients with and without MB.
In the proximal segment of LAD, a smaller amount of lipid content was observed in MB patients compared with non-MB patients whereas plaque burden was not different between the 2 groups. Wall shear stress (WSS) distribution is one of the considerable factors for plaque formation and subsequent vulnerability. In patients with MB, it has been reported that WSS at the segment proximal to MB is lower than that in the segment beneath MB [15,16]. A previous study has demonstrated the relationship between high WSS and progression of lipid-rich plaque assessed by NIRS-IVUS imaging [17]. Thus, it is possible that low WSS in the segment proximal to MB might be associated with less lipid content observed in MB patients. However, since lower WSS was reported to have an impact on more plaque progression [18], the similar amount of plaque burden between patients with and without MB observed in the present study could not be explained from this point of view. Further examination is required to clarify this issue.
In the segment beneath MB, WSS should be higher than in other segments [15,16], and higher WSS possibly results in higher LCBI [17]. However, from the results of multivariable analysis in the present study, MB was associated with lower LCBI in the segment beneath MB. Multivariable analysis also demonstrated maximum plaque burden as a significant predictor for LCBI and maxLCBI4mm in the present study. Plaque burden was reported to be positively correlated with LCBI in previous studies [19]. Therefore, less plaque burden in the MB segment might cause lower LCBI, despite possibly high WSS in the MB segment. In addition, a previous study with optical coherence tomography showed the absence of adventitial vasa vasorum formation in the segment beneath MB [20], which is involved in atherosclerotic plaque formation. This histological feature of MB may be associated with less plaque burden as well as lipid content in the MB segment.
Since patients with established CAD were included in the present study, it is conceivable that confounding risk factors, such as age, hypertension, DLP, and DM, could affect the lipid content of atherosclerotic plaque. Nevertheless, multivariable analysis demonstrated only maximum plaque burden (with a positive attitude) and MB (with a negative one) as independent predictors for lipid content in the proximal and mid segments of LAD. Although it is still uncertain whether MB has a significant effect on atherosclerotic plaque and its lipid content formation, patients with MB would not always have worse prognosis because of MB on atherosclerotic plaque vulnerability. However, further study is warranted to investigate what type of features of MB are associated with plaque stability or vulnerability, since previous reports suggest that MB can cause adverse events, such as arrhythmia, myocardial infarction, and sudden death in some patients with MB [21-23].
There are some limitations of the present study. This was a single-center study, and the number of subjects was small. The present study was a retrospective observational study and patient information was obtained from medical records only. Patients who have severe stenosis or coronary stent in the segment proximal to MB were excluded, and all remaining patients were Japanese with 80% being male. Thus, caution is warranted in applying the present study results to different populations.
In conclusion, lipid content of atherosclerotic plaque assessed by NIRS-IVUS imaging was significantly smaller in patients with MB than those without. MB could be considered as a predictor negatively associated with plaque vulnerability in the proximal and mid LAD.