Discussion
In this study, the prevalence of MB in patients with HCM was 24.7%. The main findings are as follows. First, in patients with HOCM, the surgical treatment of MB can significantly reduce the incidence of nonfatal MI and combined endpoints (including all-cause death and nonfatal MI). Second, considering the patency rate of the graft vessels, myocardial unroofing is the recommended treatment for eligible patients, and every effort to perform unroofing when technically possible may be preferable for long-term outcomes.
The prognostic implications of MB in patients with HCM are controversial. It has been suggested that compared with non-MB patients, patients with MB have more abnormalities during exercise testing and a greater incidence of chest pain, cardiac arrest, and ventricular tachycardia.9 In one study involving pediatric patients with HCM, the presence of MB may be an additional risk factor for sudden cardiac death and myocardial ischemia.4 In contrast, another study found that MB does not result in myocardial ischemia and may not cause arrhythmias or sudden death in children with HCM.5 In addition, a previous study reported that MB is associated with a higher prevalence of chest pain. However, the chest pain assessment in patients with HOCM is complicated because chest pain may be related to an underlying cardiomyopathy, associated fixed coronary artery disease, or MB and its sequelae.10 In our study, all patients underwent coronary arteriography, excluding patients with coronary heart disease, and we found that the MB group had a higher prevalence of chest pain before the surgery than the non-MB group.
In this study, we found that the incidences of nonfatal MI and combined endpoints were significantly higher in patients with untreated MB after surgery, but there was no difference between the non-MB and the treated MB groups after surgery. Some case reports have shown that surgical MB treatment can significantly improve symptoms and decrease the risk of adverse cardiovascular events in patients with HOCM.11-14 Therefore, we believe that the surgical treatment of a myocardial bridge may be beneficial and can be performed safely during septal myectomy.
Presently, there are two main surgical procedures for the treatment of MB: myocardial unroofing and CABG. However, it has not been established which of these two methods is better. A previous study compared the results of CABG and myocardial unroofing in isolated MB and suggested that patients who are refractory to medication should actively undergo surgical procedures, such as myocardial unroofing or CABG, while myocardial unroofing should be recommended as the first option because of its safety and satisfactory results.15,16 In this study, we found that myocardial unroofing was better than CABG in terms of chest pain relief and a higher occlusion rate in the CABG group. Our results are consistent with those of previous studies suggesting that surgical unroofing in carefully selected patients with MB can be performed safely as an independent procedure with significant postoperative improvement in symptoms.17-19 In addition, we found that the myocardial unroofing group had a higher degree of arterial compression and the length of MB was longer in the CABG group. In fact, longer and deeper MB might be associated with a higher risk of ventricular rupture, bleeding, and aneurysm formation as a result of unroofing.20 In this study, we found that during the relatively long follow-up period, SVG had a higher primary patency than LIMA. Our results are consistent with the results of a previous study that demonstrated that LIMA patency in an isolated MB of the left anterior descending artery was low, and that SVGs should be considered in cases of CABG for MB.21 Multiple studies have reported graft dysfunction and occlusion in cases of competitive flow. 21,22Low-grade narrowing of the LAD that results in higher competitive flow, low-grade stenosis of a bypassed coronary artery, the muscular layer of LIMA, and the potential for competitive flow of MB contributed to the occlusion in those patients who underwent CABG using LIMA. In addition, it is known that during diastole there is almost normal coronary blood flow with a high probability of competitive blood flow through the graft. This situation, together with the high sensitivity of the LIMA to competitive coronary flow, might explain the remarkably low patency of LIMA grafts.
From our experience, in clinical practice, myocardial unroofing is the recommended treatment for eligible patients with HOCM complicated with MB, and every effort to perform unroofing when technically possible may be preferable for long-term outcomes. Owing to the higher risk of ventricular rupture, bleeding, and aneurysm formation as a result of unroofing, septal myectomy and myocardial unroofing should be performed by expert cardiac surgeons who are experienced in both operations.
There were some limitations to our study. First, this was a retrospective study, and at different instances, there were differences in the treatment of HOCM complicated with MB. Because of our understanding of the disease and the growing maturity of our surgical techniques, we used different methods for treating MB at different times. In addition, Second, few patients underwent coronary artery computed tomography or coronary angiography after surgery. Hence, we could not accurately evaluate the results of unroofing and the primary patency of the bridge after surgery. Third, short period was considered for observing cardiac mortality and morbidity. In the future, a long-term follow-up for these patients is needed to obtain a better understanding of the results of the different treatment methods for MB in patients with septal myectomy. Fourth, it is an inherent limitation of this observational study that the comparison is uncontrolled for selection bias, and the decision on the intervention might be affected by the baseline characteristics. However, it should be noted that the general differences among the four groups were very mild and the differences in outcomes were significant. In addition, the small number of events and uncontrolled nature were also major limitations of our study. Finally, patients with HOCM who underwent septal myectomy are known to have a better prognosis, which is close to that of an age- and sex-matched general population. Therefore, we had to admit that the number of events is small in our study.