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.