|Year : 2014 | Volume
| Issue : 1 | Page : 22-26
The relationship between myocardial bridge type and proximal stenosis in hypertensive patients
Gouse Mohimuddin1, Xindao Yin1, Hui Xu1, Guanghui Xie1, Wenzhen Zhou1, Xinying Wu1, Qingqing Xu1, Sondagur A. R. Ziyad2, Mohammed M Mohiuddin3, Jiqiang Yue4
1 Department of Radiology, Nanjing Hospital Affliated to Nanjing Medical University, Nanjing, Jiangsu, People's Republic of China
2 Department of Cardiology, Nanjing Hospital Affliated to Nanjing Medical University, Nanjing, Jiangsu, People's Republic of China
3 Mahadevappa Rampure Medical College, Gulbarga, Karnataka, India
4 Department of Statistics, Nanjing Medical University, Nanjing, Jiangsu, People's Republic of China
|Date of Web Publication||7-Apr-2014|
Department of Radiology, Nanjing Hospital Affliated to Nanjing Medical university, Nanjing, Jiangsu
People's Republic of China
Source of Support: Department of Radiology, Nanjing’s First Hospital, Nanjing, Jiangsu, P. R. China, Conflict of Interest: None
Background: Myocardial bridges (MBs) are a congenital abnormality in which a part of the coronary artery is tunneled or immersed in the myocardium, most commonly found in the left anterior descending (LAD) artery. In the artery proximal to the bridge, there is formation of atherosclerosis causing stenosis. However, the tunneled and distal part of the artery is free from atherosclerosis.
Objective: This study will investigate the relationship between MB type and proximal stenosis in hypertensive patients.
Methods: A total of 140 MB patients were selected form Nanjing Hospital database, all aged between 31 and 91 years. A total of 33 were further excluded from the study. The data of the remaining 107 patients, including the type of bridge, length, depth, blood pressure, cholesterol level, and artery involved, were all taken. The Pearson's Chi-square test was done to compare the relationship between the bridge type and degree of stenosis in the artery proximal to the bridge in hypertensive patients.
Results: Among those 107 patients, 69 (64.4%) had a superficial bridge and 38 (35.5%) had a deep bridge. The P value of the superficial bridge group was (0.993), while that of the deep bridge group was (0.580).
Conclusion: In this study, superficial bridges were the most common form of MB and the most common artery involved was the (middle segment of) LAD. There were no signs of atherosclerosis in the MB (or tunneled) arteries or the distal parts. This study found no link between MB type and proximal stenosis in hypertensive patients.
Keywords: Deep bridge, hypertension, myocardial bridge, stage 1, stage 2, stenosis, superficial bridge
|How to cite this article:|
Mohimuddin G, Yin X, Xu H, Xie G, Zhou W, Wu X, Xu Q, Ziyad SA, Mohiuddin MM, Yue J. The relationship between myocardial bridge type and proximal stenosis in hypertensive patients. Nig J Cardiol 2014;11:22-6
|How to cite this URL:|
Mohimuddin G, Yin X, Xu H, Xie G, Zhou W, Wu X, Xu Q, Ziyad SA, Mohiuddin MM, Yue J. The relationship between myocardial bridge type and proximal stenosis in hypertensive patients. Nig J Cardiol [serial online] 2014 [cited 2020 Aug 13];11:22-6. Available from: http://www.nigjcardiol.org/text.asp?2014/11/1/22/130054
| Introduction|| |
The myocardial bridge (MB) is an anatomical variant in which part of the coronary artery runs under the myocardium.  MB was first seen by Reyman during an autopsy in 1737  and in 1960 it was angiographically observed for the first time by Portmann and Iwig.  It is also called tunnel artery.  It mostly occurs in the left anterior descending (LAD) artery. The affected artery may have two bridges. In MB depth and length varies from 1-4 mm to 4-40 mm, respectively. The detection rate of MB is higher in multidetector computed tomography (MDCT) than coronary angiography (CAG). The detection rate of MB in autopsy is between 57% and 80%.
MB can be divided into following two types: Superficial and deep. In a superficial bridge, the artery is covered by the membranous myocardium, while in deep bridges the artery is in muscular myocardium showing a ''U'' shape.
During the systolic phase, the artery beneath the bridge is compressed by the surrounding myocardium, leading to transient stenosis in the artery seen particularly in deep bridges, which relaxes during the diastolic phase and causes a ''milking'' effect in the artery. The stenosis of the artery during the systolic phase is present to lesser extent in the superficial bridge. 
The predominant ethnicity presenting MBs is unknown, and high blood pressure is one of the significant factors in the formation of atherosclerosis. The purpose of this research was to compare the severity of stenosis (caused due to atherosclerosis) in the proximal part of the bridge with MB type in hypertensive patients.
| Materials and Methods|| |
A total of 140 MB patients at Nanjing Hospital were selected for this quantitative study. Among them 33 patients with normal blood pressure (BP) and stenosis were excluded. Most of the patients had come to the hospital with complaints for chest tightness/pain and underwent MDCT scan due to suspicions of coronary artery disease and they were diagnosed with MB. Among remaining 107 patients, 68 were (63.5%) males and 39 were (36.4%) females, with the average age of patients at 63.86 ± 13.01.
Data of each individual patient's age, sex, BP, blood cholesterol level, diabetes status, bridge length, bridge type, artery involved, and stenosis were all recorded. Patients consent was taken prior to the MDCT scan and oral consent from the ethical committee was taken to carry out this research.
Procedure for the computed tomography examination/image processing and analysis
Coronary angiographies were done using a Siemens CT somatom definition dual-source scanner. The image was taken from the bifurcation of the trachea through to the end of diaphragm. Contrast iopadimal was used for the scan (100 mL) and 35-50 mL of normal saline was flushed through after the contrast was infused. Two radiologists visualized the images to evaluate the coronary arteries. Multiple planar reconstruction (MPR) and curved planar reconstruction (CPR) were mostly used so as to accurately diagnose of the type, length, depth, and location of the atherosclerosis.
Diabetes, cholesterol and smoking
If the fasting plasma blood sugar was ≥7.0 or random plasma blood sugar was more than 11.1 mmol/L, the patient was said to be diabetic. If total blood cholesterol was more than 5.5 mmol/L, hypercholestolemia was identified. In regards to smoking, occasional smoking was not considered significant to this study, but a daily smoking habit was considered positive.
Detection of MB
The detection of MB was based on the coronary artery being covered by membranous and muscular myocardium. The length of the bridge was calculated from the point where the coronary artery covered by the membranous or muscular myocardium began and until the covering stopped. The depth of the bridge was calculated only in cases of deep bridge, as the artery becomes embedded in muscular myocardium and this covering can be calculated by measuring the thickness of the myocardium covering the artery. In superficial bridges, the covering is very thin and therefore cannot be calculated.
Detection of stenosis
By using MPR and CPR, the affected artery in the MB was visualized and then the artery's stenosis calculated. The classification of the stenosis was based on the American Heart Association's standards, called Nobile classification. The stenosis was divided into three grades 1. <50% stenosis 2. 50%-75% stenosis 3. >75% stenosis. This study is focused on the stenosis in the artery proximal to the bridge.
Category of blood pressure
Patients' BP was divided into four categories in accordance with the American Heart Association's standards (1) Group N (normal) 90/60-119/79, (2) Group P (prehypertension) 120/80-140/90, (3) Group 1 (stage 1) 140/90-160/100, and (4) Group 2 (stage 2) ≥160/100 mm Hg. Patients with normal and prehypertension were excluded from the study while patients whose blood pressure falls in stage 1 and stage 2 were accepted (hypertensive) for this research.
Statistical analysis was done using SSPS software 11.0. If P value of less than 0.05 was considered statistically significant [Table 1].
| Results|| |
In this study, there were more hypertensive males (63.5%) than females (36.4%) shown in [Table 2]. The mean age of stage 2 hypertensive patients was marginally higher than that of stage 1 patients (69.33 ± 11.96), and the average age of all patients included in this study is 63.86 ± 13.01. Hypercholesterolemia (19.2%) and smoking (30.7%) was more common in stage 1 hypertension, while the stage 2 hypertension group showed a higher rate of diabetes (27.2%) as compared with stage 1 hypertension diabetics (17.3%).
Among the 107 patients, superficial bridges were the most common type of bridge (69 patients or 64.4% of those studied) [Figure 1]. The mean average length of the bridge was 10.26 mm and among these patients, 65 (94.2%) of superficial bridges were in the LAD artery. The most common location of the bridge in the LAD was in the middle segment (> 80%) of the artery, while the second most common location was the proximal part of the artery. Among these 69 superficial bridges, 2 (2.8%) were located in RCA and 2 (2.8%) in the diagonal arteries.
|Figure 1: Superficial bridge with grade 1 stenosis in the artery proximal to the bridge|
Click here to view
There were 38 (35.5%) patients (out of 107) suffering from myocardial deep bridge [Figure 2], and the mean length and depth of these bridges were 7.14 and 4.29 mm, respectively. More than 90% of the bridge was found in the middle segment, while the remainder was in the proximal segment. Of 38 deep bridge patients, 34 (89.4%) patients had bridges in the LAD artery, 1 (3%) patient had a bridge in the RCA, 1 (3.0%) in the diagonal arteries, and 2 (6.0%) had deep bridges in the posterior descending artery.
|Figure 2: Deep bridge with grade 2 stenosis in the artery proximal to bridge|
Click here to view
Stenosis and blood pressure
Among 107 patients, 52 (48.5%) had stage 1 hypertension and 55 (51.4%) patients had stage 2 hypertension. Of those myocardial superficial bridged patients in the stage 1 hypertension group [Table 1], patients were evenly spread across grades 1, 2, and 3 stenosis (10 to 11 patients each). In the stage 2 hypertension group, grade 2 type stenosis was most prevalent, affecting 14 of the patients.
Among the myocardial deep-bridged patients [Table 1] in the stage 1 hypertension group, patients were again evenly spread between the three grades of stenosis (9-11 patients each). In the stage 2 hypertension group, stenosis was greatly reduced in comparison to all other groupings.
For the statistic result the Pearson test was used to determine correlations. In superficial bridge patients the P value was 0.993, which implies that there is no significant relation between superficial bridges and proximal stenosis type in hypertensive patients. The P value was lower in the deep bridge group (0.580), again indicating that there is no significant link between deep bridges and proximal stenosis in hypertensive patients.
| Discussion|| |
MB is an anatomical variation in which the artery is covered by either membranous or muscular myocardium. The golden standard method for diagnosing MB is coronary angiography; the detection rate using CAG is between 0.5% and 29.4%.  If using CAG the detection depends on the thickness of the bridge, position of the artery, and adipose tissue covering the bridge.  However with the advancement in computed tomography, a noninvasive procedure, the detection rate is 3.5%-58%, which is a broader and has higher detection rate than CAG.  The detection rate of MB is higher, if the number of detectors in MDCT is increased because smaller MB can be detected.  The increased spatial and contrast would make the visibility more clear and would ease in the diagnosis of MB.
In this research, it was found that around 69 (64.4%) and 38 (35.5%) of the patients were suffering from myocardial superficial and deep bridges, respectively, and it has been found in previous research on superficial bridges that these are the most common bridge type, which was collaborated by this study's results, also indicating that superficial bridges are most common.  LAD was the most common location of MB and artery involved, with MBs occurring mostly in the middle segment and less so in the proximal part. , No artery was found to have two bridges. The mean length of the superficial and deep bridges was found to be 10.26 and 7.14 mm, respectively, whereas the mean depth of the deep bridge was found to be 4.29 mm.
Atherosclerosis was present in the artery proximal to the bridge and resulted in stenosis thereby decreasing the blood flow to the myocardium. There was no sign of atherosclerosis in the tunneled artery beneath the MB and in the distal part of the artery.  However, there is high shear stress in the tunneled artery and reduced vasoactive secretion, so that the artery is free from atherosclerosis.  The low shear stress at the proximal part of the artery may cause lipid transfer across the arterial membrane which could lead to atherosclerosis formation. 
Endothelium is a thin layer lining the interior of blood vessels. It is an important source of mediators like nitric oxide (eNOS), endothelin-1 (ET-1), throboxane H2, and prostoglandin A2. The normal function of these vasoactives (increase or decreases the blood pressure) is to contribute to the relaxation and contraction of the artery. In the case of damage to the endothelium due to high blood pressure, this could lead to narrowing and atherosclerosis initiation due to the improper secretion of these mediators by the damaged endothelium.  In the early stages of a dysfunctional endothelium, it causes macrophage attraction and adhesion. There is also accumulation of oxidized lipid leading to fatty streaks.
In previous research, it has been found that in MB, there is impaired vasoactive secretion of mediators such as eNOS, angiotensin converting enzyme, ET-1 in the artery proximal to the bridge.  The intimal thickness of the tunneled artery and permeability is reduced, whereas in contrast permeability in the artery proximal to the bridge is found to be increased.
An increase in blood pressure and turbulent blood flow may result in increased pressure on the intima of the artery proximal to bridge resulting in injury to the intima. This causes platelet aggression and vasospasm of the coronary artery.  The injured intima forms into scars, causing cholesterol and other substances to become lodged onto the scar, and results in narrowing and plaque formation.
It has been documented by using intravascular ultrasound that there is high arterial pressure in the artery proximal to the bridge as compared with that of the aorta. It has been said in the previous studies that increases in pressure, blood flow disturbances and high wall stress in the artery proximal to bridge are the main cause for the development of atherosclerosis resulting in stenosis.  Impaired vasoactive mediator secretion, resulting in a narrowing of the artery. Both these mechanisms can result in stenosis of the affected artery, thereby reducing the blood flow.
This study is based on the hypothesis that damage to the intima of the artery proximal to the MB is due to increased blood pressure (in hypertensive patients) which causes tears in the intima. This results in scar formation, atherosclerosis formation, and worsening of atherosclerosis in patients with preexisting atherosclerosis. These will eventually lead to stenosis.
However, according to the statistical results of this study, the superficial bridge P value (0.993) and deep bridge P value (0.580) did not show a significant relation between stenosis and MBs in hypertensive patients. Previous research indicated that atherosclerosis forms in the artery due to high blood pressure, leading on to stenosis. However, this study's results did not show a significant correlation between MBs and stenosis in hypertensive patients.
One limiting factor in this study's results may be due to smaller number of patients. It would be beneficial to continue this study with a greater number of patients. If a relationship is established between MBs and stenosis, it would then be prudent to further compare the severity of stenosis in superficial and deep bridges.
| Conclusion|| |
In this study, there was strong evidence of atherosclerosis in the proximal part of the artery, while the bridged and therefore tunneled artery showed no atherosclerosis. The results of this research, therefore, indicate that there is no significant relationship between the MB and stenosis in hypertensive patients.
| Acknowledgments|| |
The authors wish to thank Dr. Sumedh Bele for his valuable help regarding the manuscript preparation.
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[Figure 1], [Figure 2]
[Table 1], [Table 2]