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Atherosclerosis: Difference between revisions
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''Ronak Delewi, MD; Hayang Yang, MsC; John Kastelein, MD, PhD''<br /><br /> | ''Ronak Delewi, MD; Hayang Yang, MsC; John Kastelein, MD, PhD''<br /><br /> | ||
{{case| | {{case| | ||
A 53 years old man, without medical history or medication visits the family physician and makes an anxious impression. His friend has recently suffered from a myocardial infarction (MI) and he is worried that he might also soon face the same situation. As for family medical history, he has a father with hypertension and an uncle with diabetes mellitus. He does not seem to have any symptoms or complaints at this moment, but he has been smoking for 25 years and is overweight. Because of these characteristics he is worried that he will suffer from a MI. Upon physical examination, his BMI was 29 kg/m<sup>2</sup>, RR was 152/90 mmHg and heart rate was 75 bpm. The family physician orders a blood test for lipid profile and glucose. Both turn out to be in the normal range. <br /> | A 53 years old man, without medical history or medication visits the family physician and makes an anxious impression. His friend has recently suffered from a myocardial infarction (MI) and he is worried that he might also soon face the same situation. As for family medical history, he has a father with hypertension and an uncle with diabetes mellitus. He does not seem to have any symptoms or complaints at this moment, but he has been smoking for 25 years and is overweight. Because of these characteristics he is worried that he will suffer from a MI. Upon physical examination, his BMI was 29 kg/m<sup>2</sup>, RR was 152/90 mmHg and heart rate was 75 bpm. The family physician orders a blood test for lipid profile and glucose. Both turn out to be in the normal range. <br /> | ||
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== 1.2 Arterial vessel with atherosclerosis == | == 1.2 Arterial vessel with atherosclerosis == | ||
[[File:RCA_atherosclerosis.jpg|thumb|Atheroclerotic plaque in a coronary artery]] | |||
=== Three pathologic stages of atherogenesis === | === Three pathologic stages of atherogenesis === | ||
Atherogenesis can be divided into five key steps, which are 1) endothelial dysfunction, 2) formation of lipid layer or fatty streak within the intima, 3) migration of leukocytes and smooth muscle cells into the vessel wall, 4) foam cell formation and 5) degradation of extracellular matrix. Via these consecutive steps, an atherosclerotic plaque is formed. The formation of the plaque can also be divided into three major stages namely 1) the fatty streak, which represents the initiation 2) plaque progression, which represents adaption and 3) plaque disruption, which represents the clinical | Atherogenesis can be divided into five key steps, which are 1) endothelial dysfunction, 2) formation of lipid layer or fatty streak within the intima, 3) migration of leukocytes and smooth muscle cells into the vessel wall, 4) foam cell formation and 5) degradation of extracellular matrix. Via these consecutive steps, an atherosclerotic plaque is formed. The formation of the plaque can also be divided into three major stages namely 1) the fatty streak, which represents the initiation 2) plaque progression, which represents adaption and 3) plaque disruption, which represents the clinical complication of atherosclerosis.<br /> | ||
[[File:Figure_7_-_Fatty_streak_formation_revealing_platelet_aggregation_on_the_endothelial_surface.png|right|thumb|Figure 5. Fatty streak formation]] | [[File:Figure_7_-_Fatty_streak_formation_revealing_platelet_aggregation_on_the_endothelial_surface.png|right|thumb|Figure 5. Fatty streak formation]] | ||
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Endothelial dysfunction is a primary event in atherogenesis, which can be caused by various agents, such as physical stress and chemical irritants. Endothelial dysfunction is also observed in other pathological conditions, which are often related to atherosclerosis such as hypercholesterolemia, diabetes, hypertension, heart failure, cigarette smoking and aging.<br /> | Endothelial dysfunction is a primary event in atherogenesis, which can be caused by various agents, such as physical stress and chemical irritants. Endothelial dysfunction is also observed in other pathological conditions, which are often related to atherosclerosis such as hypercholesterolemia, diabetes, hypertension, heart failure, cigarette smoking and aging.<br /> | ||
Endothelial cells can display different reactions according to various levels of physical stress. There are two atheroprotective endothelial functions from physical stress. When endothelial cells are exposed to laminar flow, which | Endothelial cells can display different reactions according to various levels of physical stress. There are two atheroprotective endothelial functions from physical stress. When endothelial cells are exposed to laminar flow, which display minimal physical stress, they secrete NO. NO functions as an anti-atherosclerotic substance through vasodilation, inhibition of platelet aggregation and anti-inflammatory effects. The second function is executed, when exposed to laminar flow by an expression of the antioxidant enzyme superoxide dismutase. This enzyme performs anti-atherosclerotic role by acting against reactive oxygen species, which are produced by chemical irritants or transient ischemia in the vessel.<br /> | ||
{| class="wikitable" border="1" style='float: right' | {| class="wikitable" border="1" style='float: right' | ||
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==== ''Smooth muscle cell migration'' ==== | ==== ''Smooth muscle cell migration'' ==== | ||
Smooth muscle cells play a central role at the phase of transition from fatty streak to plaque formation. During this phase, smooth muscle cells migrate from the media to the intima. After migration, smooth muscle cells proliferate within the intima and secrete extracellular matrix macromolecules. Additionally, foam cells, activated platelets and endothelium stimulate substances that induce the migration and accumulation of smooth muscle cells. For example, foam cells release platelet derived growth factor (PDGF), cytokines and growth factors that directly contribute to the migration and proliferation process, and they also activate smooth muscle cells and leukocytes to reinforce inflammation in the atherosclerotic lesion. Although plaque progression is traditionally known as a gradual and continuous process, recent evidence claims that this process can be strongly accentuated by bursts of smooth muscle replication. The observation of small ruptures within the plaque occurring without any clinical symptoms or signs supports this suggestion. These small ruptures expose tissue factor secreted by foam cells that stimulates coagulation and microthrombus formation in the lesion. Such | Smooth muscle cells play a central role at the phase of transition from fatty streak to plaque formation. During this phase, smooth muscle cells migrate from the media to the intima. After migration, smooth muscle cells proliferate within the intima and secrete extracellular matrix macromolecules. Additionally, foam cells, activated platelets and endothelium stimulate substances that induce the migration and accumulation of smooth muscle cells. For example, foam cells release platelet derived growth factor (PDGF), cytokines and growth factors that directly contribute to the migration and proliferation process, and they also activate smooth muscle cells and leukocytes to reinforce inflammation in the atherosclerotic lesion. Although plaque progression is traditionally known as a gradual and continuous process, recent evidence claims that this process can be strongly accentuated by bursts of smooth muscle replication. The observation of small ruptures within the plaque occurring without any clinical symptoms or signs supports this suggestion. These small ruptures expose tissue factor secreted by foam cells that stimulates coagulation and microthrombus formation in the lesion. Such microthrombi contain activated platelets that release additional factors such as PDGF and heparinase that can further stimulate local smooth muscle cell migration and proliferation. Heparinase stimulates smooth muscle cell migration and proliferation by degrading heparan sulfate, which normally counteracts this process.<br /> | ||
==== ''Extracellular matrix metabolism'' ==== | ==== ''Extracellular matrix metabolism'' ==== | ||
Metabolic processes in extracellular matrix | Metabolic processes in extracellular matrix play a central role in bridging the plaque progression to plaque rupture. Ultimately, this process weakens the fibrous cap, predisposing it to rupture. This process is influenced by the balance of matrix deposition synthesis by smooth muscle cells and degradation by matrix metalloproteinases (MMP), a class of proteolytic enzymes. For example, PDGF and TGF-β stimulate interstitial collagen production, while inflammatory cytokines such as IFN-γ inhibits collagen synthesis. TGF-β also induces formation of fibronectin and proteoglycans. It is an important regulator since it enhances the expression of protease inhibitors, leading to the inhibition of proteolytic enzymes that promote matrix degradation. On the other hand, inflammatory cytokines weaken the fibrous cap by stimulating local foam cells to secrete MMP that degrades collagen and elastin of the fibrous cap. Furthermore, the deeper parts of the thickened intima undergo necrosis due to poor nourishment.<br /> | ||
=== Plaque rupture === | === Plaque rupture === | ||
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The concept of ‘vulnerable plaque’ has developed into a new concept of ‘vulnerable patient’ as the concept of pathogenesis of atherosclerosis was linked to a person’s susceptibility to coagulation and thus vascular events, which can be influenced by many personal factors such as genetics (e.g. procoagulant prothombin gene mutation), coexisting condition (e.g. diabetes), and lifestyle factors (e.g. smoking, obesity).<br /> | The concept of ‘vulnerable plaque’ has developed into a new concept of ‘vulnerable patient’ as the concept of pathogenesis of atherosclerosis was linked to a person’s susceptibility to coagulation and thus vascular events, which can be influenced by many personal factors such as genetics (e.g. procoagulant prothombin gene mutation), coexisting condition (e.g. diabetes), and lifestyle factors (e.g. smoking, obesity).<br /> | ||
{| | |||
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| rowspan="2" | [[File:plaque_rupture_A.svg|100px]] | |||
| rowspan="2" | [[File:plaque_rupture_B.svg|100px]] | |||
| rowspan="2" | [[File:split_arrow.svg|50px]] | |||
| [[File:plaque_rupture_C.svg|100px]] || [[File:plaque_rupture_clot.svg|100px]] | |||
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| [[File:plaque_rupture_D.svg|100px]] | |||
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| colspan="4" width="450px" | Progression of coronary atherosclerosis can be gradual (bottom) or can lead to plaque rupture with acute occlusion of a coronary vessel due to clot formation | |||
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== Complications of atherosclerosis == | == Complications of atherosclerosis == | ||
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The clinical complications of atherosclerosis are highly dependent on the location and size of affected vessels, the duration of the chronic process, and the type of plaque, since the severity of impairment of atherosclerosis differs throughout the vasculature. For example, ‘stable plaque’ can easily result into angina pectoris due to its thick fibrous cap that directly affects the diameter of the relatively small coronary vessels. On the other hand, ‘vulnerable plaque’ is non-stenotic, but can easily cause acute thrombosis and therefore myocardial infarction due to its fragility towards rupture when located at physically stressed areas such as bifurcations. Often with ‘vulnerable plaques’ there are relatively few symptoms, however they are more numerous and dispersed throughout the arteries compared to ‘stable plaque’. Thus, you can either have an occlusion due to the growing plaque or due to the embolization of the ruptured fragments of the original plaque. Due to the difficult detection of ‘vulnerable plaques’ while they are widely dispersed, it is highly important to tackle the risk factors prior to plaque rupture. Thus in the following paragraph, we will highlight the clinical risk factors associated with atherosclerosis. The four major clinical consequences of atherosclerosis are listed and explained below.<br /> | The clinical complications of atherosclerosis are highly dependent on the location and size of affected vessels, the duration of the chronic process, and the type of plaque, since the severity of impairment of atherosclerosis differs throughout the vasculature. For example, ‘stable plaque’ can easily result into angina pectoris due to its thick fibrous cap that directly affects the diameter of the relatively small coronary vessels. On the other hand, ‘vulnerable plaque’ is non-stenotic, but can easily cause acute thrombosis and therefore myocardial infarction due to its fragility towards rupture when located at physically stressed areas such as bifurcations. Often with ‘vulnerable plaques’ there are relatively few symptoms, however they are more numerous and dispersed throughout the arteries compared to ‘stable plaque’. Thus, you can either have an occlusion due to the growing plaque or due to the embolization of the ruptured fragments of the original plaque. Due to the difficult detection of ‘vulnerable plaques’ while they are widely dispersed, it is highly important to tackle the risk factors prior to plaque rupture. Thus in the following paragraph, we will highlight the clinical risk factors associated with atherosclerosis. The four major clinical consequences of atherosclerosis are listed and explained below.<br /> | ||
# Acute narrowing of the vessel lumen: When the plaque ruptures, it will release its pro-coagulants in the bloodstream and that will lead to the formation of thrombus at the rupture site. The rupture often occurs at sites of erosion and fissuring on the fibrous cap surface. This thrombus may cause a complete occlusion of a particular vessel and result in ischemic necrosis (infarction) of the tissue that this particular vessel is supplying to. Clinically this is manifested as stroke, MI, gangrene of several possible organs such as intestine, spleen or lower extremities. These | # Acute narrowing of the vessel lumen: When the plaque ruptures, it will release its pro-coagulants in the bloodstream and that will lead to the formation of thrombus at the rupture site. The rupture often occurs at sites of erosion and fissuring on the fibrous cap surface. This thrombus may cause a complete occlusion of a particular vessel and result in ischemic necrosis (infarction) of the tissue that this particular vessel is supplying to. Clinically this is manifested as stroke, MI, gangrene of several possible organs such as intestine, spleen or lower extremities. These occlusions may also dissolve spontaneously due to pro-fibrinolytic enzymes such as streptokinase and tissue plasminogen activator (TPA). | ||
# Chronic occlusion: When the occlusion is gradual and incomplete, it may chronically disturb the blood supply to tissues in the distribution of the affected vessel. This can result in chronic ischemia of those tissues that can additionally lead to complaints of angina pectoris or intermittent claudication or to organ atrophy (e.g. atrophy of kidney, intestines and skin due to impairment of blood flow in renal artery, mesenteric artery, peripheral vasculature among diabetics.). | # Chronic occlusion: When the occlusion is gradual and incomplete, it may chronically disturb the blood supply to tissues in the distribution of the affected vessel. This can result in chronic ischemia of those tissues that can additionally lead to complaints of angina pectoris or intermittent claudication or to organ atrophy (e.g. atrophy of kidney, intestines and skin due to impairment of blood flow in renal artery, mesenteric artery, peripheral vasculature among diabetics.). | ||
# Embolism: Embolization is the transfer of the fragments of disrupted atheroma to distal vascular sites, which results into occlusion of those sites. For example, fragments of thrombi in abdominal aorta may transfer to the popliteal artery subsequently resulting in gangrene of the leg. Ulceration of atheroma may also produce ‘cholesterol crystal emboli’. This type of emboli is visualized as needle-shaped areas in affected tissues, mostly detected in the kidney. | # Embolism: Embolization is the transfer of the fragments of disrupted atheroma to distal vascular sites, which results into occlusion of those sites. For example, fragments of thrombi in abdominal aorta may transfer to the popliteal artery subsequently resulting in gangrene of the leg. Ulceration of atheroma may also produce ‘cholesterol crystal emboli’. This type of emboli is visualized as needle-shaped areas in affected tissues, mostly detected in the kidney. | ||
# Aneurysm: After a chronic period, atherosclerotic lesion may extend into the medial layer, resulting into atrophy and loss of elastic tissue. This can subsequently cause dilatation and weakness of the artery, forming aneurysm. Over time, | # Aneurysm: After a chronic period, atherosclerotic lesion may extend into the medial layer, resulting into atrophy and loss of elastic tissue. This can subsequently cause dilatation and weakness of the artery, forming aneurysm. Over time, aneurysms may suddenly rupture and result in a life-threatening situation for the patients. | ||
== Risk factors of atherosclerosis == | == Risk factors of atherosclerosis == | ||
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==== ''Tobacco smoking'' ==== | ==== ''Tobacco smoking'' ==== | ||
Tobacco use is known to increase the risk of atherosclerosis and ischemic heart disease based on numerous studies. For example, INTERHEART study shows that smoking is responsible for 36% of the population-attributable risk of a first MI. Other studies showed that smoking is an independent major risk factor for coronary heart disease, cerebrovascular disease and total atherosclerotic cardiovascular disease. The Atherosclerosis Risk in Communities Study measured the direct effect of smoking on the development of atherosclerosis. They measured intima-medial thickness of the carotid artery of 10,914 patients for three years with ultrasound. Their result showed that current smokers had a 50% increased progression of atherosclerosis in comparison to nonsmokers during the study period. Also patients with environmental tobacco smoke exposure (passive smokers) had 20% higher rate of atherosclerotic progress versus patients without environmental smoke exposure.<br /> | Tobacco use, including environmental smoking exposure, is known to increase the risk of atherosclerosis and ischemic heart disease based on numerous studies. For example, INTERHEART study shows that smoking is responsible for 36% of the population-attributable risk of a first MI. Other studies showed that smoking is an independent major risk factor for coronary heart disease, cerebrovascular disease and total atherosclerotic cardiovascular disease. The Atherosclerosis Risk in Communities Study measured the direct effect of smoking on the development of atherosclerosis. They measured intima-medial thickness of the carotid artery of 10,914 patients for three years with ultrasound. Their result showed that current smokers had a 50% increased progression of atherosclerosis in comparison to nonsmokers during the study period. Also patients with environmental tobacco smoke exposure (passive smokers) had 20% higher rate of atherosclerotic progress versus patients without environmental smoke exposure.<br /> | ||
Tobacco smoking can lead to many mechanisms that contribute to atherosclerosis. Smoking also leads to increased LDL levels, decreased HDL levels in blood and elevated insulin resistance. In addition it enhances oxidative modification of LDL by releasing free radicals and reduces generation of nitric oxide. This can promote endothelial dysfunction and thus lead to impairment of vasodilatation of coronary arteries and reduction of coronary flow reserve even in passive smokers. Tobacco smoking inappropriately stimulates sympathetic nervous system, increasing heart rate, blood pressure and perhaps coronary vasoconstriction. Smoking promotes a prothrombotic environment through inhibition of endothelial release of tissue plasminogen activator, elevation of fibrinogen concentration in blood, enhancement of platelet activity (possibility related to sympathetic activation) and enhanced expression of tissue factor. Smoking can even damage the vessel wall and ultimately cause a decrease in the elasticity | Tobacco smoking can lead to many mechanisms that contribute to atherosclerosis. Smoking also leads to increased LDL levels, decreased HDL levels in blood and elevated insulin resistance. In addition it enhances oxidative modification of LDL by releasing free radicals and reduces generation of nitric oxide. This can promote endothelial dysfunction and thus lead to impairment of vasodilatation of coronary arteries and reduction of coronary flow reserve even in passive smokers. Tobacco smoking inappropriately stimulates sympathetic nervous system, increasing heart rate, blood pressure and perhaps coronary vasoconstriction. Smoking promotes a prothrombotic environment through inhibition of endothelial release of tissue plasminogen activator, elevation of fibrinogen concentration in blood, enhancement of platelet activity (possibility related to sympathetic activation) and enhanced expression of tissue factor. Smoking can even damage the vessel wall and ultimately cause a decrease in the elasticity of the artery, enhancing the stiffness of vessel wall. Smoking has been associated with increased C-reactive protein and fibrinogen, suggesting a correlation with inflammatory response, which is an important part of atherogenesis. There have also been findings that show higher expression of leukocyte adhesion molecules among smokers than nonsmokers. Smoking may additionally induce tissue hypoxia through displacement of oxygen with carbon monoxide in hemoglobin. <br /> | ||
To stop smoking is known as one of the most effective preventive measures of CVD and their complications. Soon after cessation, cardiac risks due to smoking decreases in a short period, and continues to diminish when cessation is permanently preserved. The risk for cardiovascular disease among patients with coronary heart disease decreases 7-47%. Not only does cessation of smoking reduce risk of CVD, but also substantially reduce the risk of all-cause mortality.<br /> | To stop smoking is known as one of the most effective preventive measures of CVD and their complications. Soon after cessation, cardiac risks due to smoking decreases in a short period, and continues to diminish when cessation is permanently preserved. The risk for cardiovascular disease among patients with coronary heart disease decreases 7-47%. Not only does cessation of smoking reduce risk of CVD, but also substantially reduce the risk of all-cause mortality.<br /> | ||
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==== ''Diet'' ==== | ==== ''Diet'' ==== | ||
A healthy diet reduces CVD risk. In general, when following the rules for a healthy diet, no dietary supplements are needed. N-3 polyunsaturated fatty acid (PUFA) consumption mainly from oily fish, is potentially associated with beneficial effects on cardiac risk factors, notably reduction in triglycerides but not all randomized, controlled trials have shown reductions in CV events Thus current recommendations are to increase PUFA intake through fish consumption, rather than from supplements. Recently, the largest study ever conducted with a so-called ‘Mediterranean’ diet, supplemented with extra-virgin olive oil or nuts, reduced the incidence of major cardiovascular events in patients at high risk of CV events but without prior CV disease.<cite>Estruch</cite> | |||
==== ''Alcohol consumption'' ==== | ==== ''Alcohol consumption'' ==== | ||
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#Theorell Theorell, T., Lind, E., Floderus, B. “The relationship of disturbing life-changes and emotions to the development of myocardial infarction and other serious diseases.” Int J Epidemiol 1975; 4:281. | #Theorell Theorell, T., Lind, E., Floderus, B. “The relationship of disturbing life-changes and emotions to the development of myocardial infarction and other serious diseases.” Int J Epidemiol 1975; 4:281. | ||
#Vita Vita J.A., Keaney J.F. Jr. “Endothelial function: a barometer for cardiovascular risk? Circulation” 2002; 106:640. | #Vita Vita J.A., Keaney J.F. Jr. “Endothelial function: a barometer for cardiovascular risk? Circulation” 2002; 106:640. | ||
#Estruch pmid=23944307 | |||
</biblio> | </biblio> | ||