The results of this study showed the incidence of MB to be 10.2% in patients who underwent RACA.
MB is a congenital coronary artery anomaly,which is usually detected incidentally in coronary angiography performed for the diagnosis of coronary atherosclerosis.The incidence of MB differs between autopsy studies (15–85%) and angiographic studies (0.5-2.5%) [10]. Furthermore, the incidence of MB varies according to the coronary angiography method.For example, 0.6% incidence of MB in CCA was reported by Harikishen et al, whereas Kantarci et al. determined MB incidence of 3.5% in CTCA [14,15].In another study, Lu et al. found the incidence of MB to be 6% with CCA, and 30% with CTCA in the same population [11].The main reason for this difference is the variability in the sensitivity of coronary angiography methods in the detection of MB.Other factors that may affect the incidence of MB are the size and ethnicity of the study samples.Luminal narrowing is dynamic in CCA and mild systolic compressions may be overlooked.Structural evaluation can be performed with CTCA and the coronary artery course can be clearly observed, but superficial MBs may not be seen.In the current study,the main reason for the higher MB incidence compared to CCA studies was thought to be the routine use of nitroglycerin and diltiazem during the procedure.Nitroglycerin and diltiazem reduce coronary artery resistance [16,17].As a result of the synergistic effect of nitroglycerin and diltiazem, even low doses can dramatically modify coronary artery resistance. Nitroglycerin augments vessel wall constriction in patients with MB and previously unseen MBs can appear after the administration of nitroglycerin in coronary angiography.In addition, nitroglycerin increases vascular compliance, and facilitates diastolic expansion as well as systolic compression, so MB can be easily detected [18].
The most common coronary artery affected by MB is the LAD mid segment, as indicated both in the current study and many other studies [19-22].Although a previous study from Turkey reported almost equal distribution of MBs in the LAD middle and distal segments and no MB present in the proximal segment, the current study results showed much higher involvement of the LAD mid segment than proximal and distal segments [23].Compared to previous CCA studies, not only the MB incidence but also the number of simultaneously affected coronary arteries was higher in the current study.Although some studies [2,5,14,23] have shown that more than one coronary artery was almost never involved simultaneously, the involvement of two or three coronary arteries together was 5.1% and 2.7%, respectively in the current study. The reason for this,just like the incidence difference,can be attributed to the use of nitroglycerin and diltiazem enabling the detection of previously unseen MBs.In a cadaver study, it was shown that almost all major branches of both coronary arteries were affected and 36% of the samples had more than one MB[5].
There has been considerable controversy regarding the functional significance of MB.Some studies have stated that MB may be associated with ischemia,arrhythmia, and sudden cardiac death,while others have claimed that it is benign and may even protect against atherosclerosis [24-28].In the current study,ventricular tachycardia was observed in 2 patients before coronary angiography, but in both cases was due to acute myocardial infarction.It is generally accepted that MB causes coronary atherosclerosis in the LAD segment proximal to the MB and enhances its natural progression through several different mechanisms [3,9,10,29].Similar to those studies, 36.2% of the current study patients with MB had CAD, of which more than 80% were proximal to the MB, and the majority were mild degree (luminal narrowing between 30% and 50%).
There are 3 different therapeutic approaches for patients with symptomatic MB: Medication, stent placement in the bridged segment, and surgical treatment [8].Due to reducing coronary compression,beta-blocker therapy is the first choice of treatment for symptomatic patients [30].Stent implantation therapy in the bridge segment remains controversial.Some studies [31,32] have stated that this therapy reduces direct coronary compression and improves coronary reserve flow,while others [32-34] have highlighted adverse effects of this therapy, such as coronary perforation,stent fracture, and early in-stent re-stenosis.Myotomy as a surgical treatment is a good option for patients refractory to medical therapy with good long-term results [8,35].All the patients in the current study were treated medically and none required surgical treatment.
This study had some important limitations,primarily that it was a retrospective,observational, single-centre study.Therefore,the population investigated might have been heterogeneous. Further investigation was not performed for the incidence of MB in patients who underwent femoral access coronary angiography and the incidence of MB in those who underwent RACA was compared with previous CCA studies.The clinical significance of MB and its relationship with symptoms was not investigated.Dedicated clinical studies would be required to determine the relationship of MB with clinical symptoms by proving the presence of ischemia,especially in the absence of atherosclerotic coronary artery stenosis.