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Atherosclerosis
From Wikipedia, the free encyclopedia
For the journal, see Atherosclerosis (journal).
Atherosclerosis Classification and external resources 230px-Endo_dysfunction_Athero.PNG
The progression of atherosclerosis (narrowing exaggerated)
ICD-10 I70 ICD-9 440, 414.0 DiseasesDB 1039 MedlinePlus 000171 MeSH D050197

Atherosclerosis (also known as arteriosclerotic vascular disease or ASVD) is a specific form of arteriosclerosis in which an artery wall thickens as a result of invasion and accumulation of white blood cells (termed "fatty streaks" early on because of appearance being similar to that of marbled steak) and containing both living active WBCs (called inflammation) and remnants of dead cells, including cholesterol and triglycerides, eventually calcium and other crystallized materials, within the outer-most and oldest plaque. These changes reduce the elasticity of the artery walls but do not affect blood flow for decades because the artery muscular wall enlarges at the locations of plaque. However, the wall stiffening may eventually increase pulse pressure; widened pulse pressure is one possible result of advanced disease within the major arteries. It is a syndrome affecting arterial blood vessels, a chronic inflammatory response, i.e. white blood cells, in the walls of arteries, largely involving the accumulation of macrophages and white blood cells and promoted by low-density lipoproteins (LDL, plasma proteins that carry cholesterol and triglycerides) without adequate removal of fats and cholesterol from the macrophages by functional high-density lipoproteins (HDL) (see apoA-1 Milano). It is commonly referred to as a "hardening" or furring of the arteries. It is caused by the formation of multiple atheromatous plaques within the arteries.[1][2] The plaque is divided into three distinct components:

  1. The atheroma ("lump of gruel", from Greek ἀθήρα (athera), meaning "gruel"), which is the nodular accumulation of a soft, flaky, yellowish material at the center of large plaques, composed of macrophages nearest the lumen of the artery
  2. Underlying areas of cholesterol crystals
  3. Calcification at the outer base of older or more advanced lesions.

The following terms are similar, yet distinct, in both spelling and meaning, and can be easily confused: arteriosclerosis, arteriolosclerosis, and atherosclerosis. Arteriosclerosis is a general term describing any hardening (and loss of elasticity) of medium or large arteries (from Greek ἀρτηρία (artēria), meaning "artery", and σκλήρωσις (sklerosis), meaning "hardening"); arteriolosclerosis is any hardening (and loss of elasticity) of arterioles (small arteries); atherosclerosis is a hardening of an artery specifically due to an atheromatous plaque. The term atherogenic is used for substances or processes that cause atherosclerosis.[citation needed]

Atherosclerosis is a chronic disease that remains asymptomatic for decades.[3] Atherosclerotic lesions, or atherosclerotic plaques, are separated into two broad categories: Stable and unstable (also called vulnerable).[4] The pathobiology of atherosclerotic lesions is very complicated but generally, stable atherosclerotic plaques, which tend to be asymptomatic, are rich in extracellular matrix and smooth muscle cells, while, unstable plaques are rich in macrophages and foam cells and the extracellular matrix separating the lesion from the arterial lumen (also known as the fibrous cap) is usually weak and prone to rupture.[5] Ruptures of the fibrous cap expose thrombogenic material, such as collagen,[6] to the circulation and eventually induce thrombus formation in the lumen. Upon formation, intraluminal thrombi can occlude arteries outright (e.g. coronary occlusion), but more often they detach, move into the circulation and eventually occluding smaller downstream branches causing thromboembolism. Apart from thromboembolism, chronically expanding atherosclerotic lesions can cause complete closure of the lumen. Chronically expanding lesions are often asymptomatic until lumen stenosis is so severe (usually over 80%) that blood supply to downstream tissue(s) is insufficient, resulting in ischemia.

These complications of advanced atherosclerosis are chronic, slowly progressive and cumulative. Most commonly, soft plaque suddenly ruptures (see vulnerable plaque), causing the formation of a thrombus that will rapidly slow or stop blood flow, leading to death of the tissues fed by the artery in approximately five minutes. This catastrophic event is called an infarction. One of the most common recognized scenarios is called coronary thrombosis of a coronary artery, causing myocardial infarction (a heart attack). The same process in an artery to the brain is commonly called stroke. Another common scenario in very advanced disease is claudication from insufficient blood supply to the legs. Atherosclerosis affects the entire artery tree, but mostly larger, high-pressure vessels such as the coronary, renal, femoral, cerebral, and carotid arteries. These are termed "clinically silent" because the person having the infarction does not notice the problem and does not seek medical help, or when they do, physicians do not recognize what has happened.

Signs and symptoms

Atherosclerosis is asymptomatic for decades because the arteries enlarge at all plaque locations, thus there is no effect on blood flow. Even most plaque ruptures do not produce symptoms until enough narrowing or closure of an artery, due to clots, occurs. Signs and symptoms only occur after severe narrowing or closure impedes blood flow to different organs enough to induce symptoms.[7] Most of the time, patients realize that they have the disease only when they experience other cardiovascular disorders such as stroke or heart attack. These symptoms, however, still vary depending on which artery or organ is affected.[8]

Typically, atherosclerosis begins in childhood, as a thin layer of white-yellowish streaks with the inner layers of the artery walls (an accumulation of white blood cells, mostly monocytes/macrophages) and progresses from there.

Clinically, given enlargement of the arteries for decades, symptomatic atherosclerosis is typically associated with men in their 40s and women in their 50s to 60s. Sub-clinically, the disease begins to appear in childhood, and rarely is already present at birth. Noticeable signs can begin developing at puberty. Though symptoms are rarely exhibited in children, early screening of children for cardiovascular diseases could be beneficial to both the child and his/her relatives.[9] While coronary artery disease is more prevalent in men than women, atherosclerosis of the cerebral arteries and strokes equally affect both sexes.[10]

Marked narrowing in the coronary arteries, which are responsible for bringing oxygenated blood to the heart, can produce symptoms such as the chest pain of angina and shortness of breath, sweating, nausea, dizziness or light-headedness, breathlessness or palpitations.[8] Abnormal heart rhythms called arrhythmias (the heart is either beating too slow or too fast) are another consequence of ischemia.[11]

Carotid arteries supply blood to the brain and neck.[11] Marked narrowing of the carotid arteries can present with symptoms such as a feeling of weakness, not being able to think straight, difficulty speaking, becoming dizzy and difficulty in walking or standing up straight, blurred vision, numbness of the face, arms, and legs, severe headache and losing consciousness. These symptoms are also related to stroke (death of brain cells). Stroke is caused by marked narrowing or closure of arteries going to the brain; lack of adequate of blood supply leads to the death of the cells of the affected tissue.[12]

Peripheral arteries, which supply blood to the legs, arms, and pelvis, also experience marked narrowing due to plaque rupture and clots. Symptoms for the marked narrowing are numbness within the arms or legs, as well as pain.

Another significant location for the plaque formation is the renal arteries, which supply blood to the kidneys. Plaque occurrence and accumulation leads to decreased kidney blood flow and chronic kidney disease, which, like all other ares, are typically asymptomatic until late stages.[8]

According to United States data for 2004, in about 66% of men and 47% of women, the first symptom of atherosclerotic cardiovascular disease is a heart attack or sudden cardiac death (death within one hour of onset of the symptom). Cardiac stress testing, traditionally the most commonly performed non-invasive testing method for blood flow limitations, in general, detects only lumen narrowing of ≈75% or greater, although some physicians claim that nuclear stress methods can detect as little as 50%.

Case studies have included autopsies of American soldiers killed in World War II and the Korean War. A much-cited report involved autopsies of 300 American soldiers killed in Korea. Although the average age of the men was 22.1 years, 77.3 percent had "gross evidence of coronary arteriosclerosis".[13] Other studies done of soldiers in the Second Indochina War showed similar results, although often worse than the ones from the earlier wars. Theories include high rates of tobacco use and (in the case of the Vietnam soldiers) the advent of processed foods after World War II.

Causes
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Atherosclerosis and lipoproteins
See also: Lipoprotein

The atherosclerotic process is not fully understood. Atherosclerosis is initiated by inflammatory processes in the endothelial cells of the vessel wall in response to retained low-density lipoprotein (LDL) particles.[14]

Lipoproteins in the blood vary in size. Some data suggests that small dense LDL (sdLDL) particles are more prone to pass between the endothelial cells, going behind the cellular monolayer of endothelium. LDL particles and their content are susceptible to oxidation by free radicals,[15] and the risk is higher while the particles are in the wall than while in the bloodstream. However, LDL particles have a half-life of only a couple of days, and their content (LDL particles typically carry 3,000 to 6,000 fat molecules, including: cholesterol, phospholipids, cholesteryl esters, tryglycerides & all other fats in the water outside cells, to the tissues of the body) changes with time.

Once inside the vessel wall, LDL particles can become more prone to oxidation. Endothelial cells respond by attracting monocyte white blood cells, causing them to leave the blood stream, penetrate into the arterial walls and transform into macrophages. The macrophages' ingestion of oxidized LDL particles triggers a cascade of immune responses which over time can produce an atheroma if HDL removal of fats from the macrophages does not keep up. The immune system's specialized white blood cells (macrophages and T-lymphocytes) absorb the oxidized LDL, forming specialized foam cells. If these foam cells are not able to process the oxidized LDL and recruit HDL particles to remove the fats, they grow and eventually rupture, leaving behind cellular membrane remnants, oxidized materials, and fats (including cholesterol) in the artery wall. This attracts more white blood cells, resulting in a snowballing progression that continues the cycle, inflaming the artery. The presence of the plaque induces the muscle cells of the blood vessel to stretch, compensating for the additional bulk, and the endothelial lining thickens, increasing the separation between the plaque and lumen. This somewhat offsets the narrowing caused by the growth of the plaque, but it causes the wall to stiffen and become less compliant to stretching with each heart beat.

Some researchers believe that atherosclerosis may be caused by an infection of the vascular smooth muscle cells. Chickens, for example, develop atherosclerosis when infected with the Marek's disease herpesvirus.[16]Herpesvirus infection of arterial smooth muscle cells has been shown to cause cholesteryl ester (CE) accumulation, which is associated with atherosclerosis.[17]Cytomegalovirus (CMV) infection is also associated with cardiovascular diseases.[18]

Risk factors

Various anatomic and physiological risk factors for atherosclerosis are known.[19] These can be divided into various categories: congenital vs acquired, modifiable or not, classical or non-classical. The points labelled '+' in the following list form the core components of metabolic syndrome.

Risks multiply, with two factors increasing the risk of atherosclerosis fourfold.[20] Hyperlipidemia, hypertension and cigarette smoking together increases the risk seven times.[20]

Modifiable Nonmodifiable Lesser or uncertain

The following factors are of relatively lesser importance, are uncertain or unquantified:

Dietary

The relation between dietary fat and atherosclerosis is a contentious field. The USDA, in its food pyramid, promotes a low-fat diet, based largely on its view that fat in the diet is atherogenic. The American Heart Association, the American Diabetes Association and the National Cholesterol Education Program make similar recommendations. In contrast, Prof Walter Willett (Harvard School of Public Health, PI of the second Nurses' Health Study) recommends much higher levels, especially of monounsaturated and polyunsaturated fat.[39] Writing in Science, Gary Taubes detailed that political considerations played into the recommendations of government bodies.[40] These differing views reach a consensus, though, against consumption of trans fats.

The role of dietary oxidized fats/lipid peroxidation (rancid fats) in humans is not clear. Laboratory animals fed rancid fats develop atherosclerosis. Rats fed DHA-containing oils experienced marked disruptions to their antioxidant systems, and accumulated significant amounts of phospholipid hydroperoxide in their blood, livers and kidneys.[41] In another study, rabbits fed atherogenic diets containing various oils were found to undergo the greatest amount of oxidative susceptibility of LDL via polyunsaturated oils.[42] In a study involving rabbits fed heated soybean oil, "grossly induced atherosclerosis and marked liver damage were histologically and clinically demonstrated."[43] However, Kummerow, a prominent researcher, claims that it is not dietary cholesterol, but oxysterols, or oxidized cholesterols, from fried foods and smoking, that are the culprit.[44]

Rancid fats and oils taste very bad even in small amounts, so people avoid eating them.[45] It is very difficult to measure or estimate the actual human consumption of these substances.[46]

Highly unsaturated omega-3 rich oils such as fish oil are being sold in pill form so that the taste of oxidized or rancid fat is not apparent. The health food industry's dietary supplements are self regulated by the manufacture and outside of FDA regulations.[47] To properly protect unsaturated fats from oxidation, it is best to keep them cool and in oxygen free environments. Long term exposure to inorganic arsenic can cause atherosclerosis.[48]

Pathophysiology

Atherogenesis is the developmental process of atheromatous plaques. It is characterized by a remodeling of arteries leading to subendothelial accumulation of fatty substances called plaques. The buildup of an atheromatous plaque is a slow process, developed over a period of several years through a complex series of cellular events occurring within the arterial wall, and in response to a variety of local vascular circulating factors. One recent hypothesis suggests that, for unknown reasons, leukocytes, such as monocytes or basophils, begin to attack the endothelium of the artery lumen in cardiac muscle. The ensuing inflammation leads to formation of atheromatous plaques in the arterial tunica intima, a region of the vessel wall located between the endothelium and the tunica media. The bulk of these lesions is made of excess fat, collagen, and elastin. At first, as the plaques grow, only wall thickening occurs without any narrowing. Stenosis is a late event, which may never occur and is often the result of repeated plaque rupture and healing responses, not just the atherosclerotic process by itself.

Cellular
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Micrograph of an artery that supplies the heart showing significant atherosclerosis and marked luminal narrowing. Tissue has been stained using Masson's trichrome.

Early atherogenesis is characterized by the adherence of blood circulating monocytes (a type of white blood cell) to the vascular bed lining, the endothelium, then by their migration to the sub-endothelial space, and further activation into monocyte-derived macrophages.[49] The primary documented driver of this process is oxidized lipoprotein particles within the wall, beneath the endothelial cells, though upper normal or elevated concentrations of blood glucose also plays a major role and not all factors are fully understood. Fatty streaks may appear and disappear.

Low-density lipoprotein (LDL) particles in blood plasma invade the endothelium and become oxidized, creating risk of cardiovascular disease. A complex set of biochemical reactions regulates the oxidation of LDL, involving enzymes (such as Lp-LpA2) and free radicals in the endothelium.

Initial damage to the endothelium results in an inflammatory response. Monocytes enter the artery wall from the bloodstream, with platelets adhering to the area of insult. This may be promoted by redox signaling induction of factors such as VCAM-1, which recruit circulating monocytes, and M-CSF, which is selectively required for the differentiation of monocytes to macrophages. The monocytes differentiate into macrophages, which ingest oxidized LDL, slowly turning into large "foam cells" – so-called because of their changed appearance resulting from the numerous internal cytoplasmic vesicles and resulting high lipid content. Under the microscope, the lesion now appears as a fatty streak. Foam cells eventually die, and further propagate the inflammatory process. There is also smooth muscle proliferation and migration from the tunica media into the intima responding to cytokines secreted by damaged endothelial cells. This causes the formation of a fibrous capsule covering the fatty streak. Intact endothelium could prevent the proliferation by releasing nitric oxide.

Calcification and lipids

Calcification[2] forms among vascular smooth muscle cells of the surrounding muscular layer, specifically in the muscle cells adjacent to atheromas and on the surface of atheroma plaques and tissue.[2][3] In time, as cells die, this leads to extracellular calcium deposits between the muscular wall and outer portion of the atheromatous plaques. With the atheromatous plaque interfering with the regulation of the calcium deposition, it accumulates and crystallizes. A similar form of an intramural calcification, presenting the picture of an early phase of arteriosclerosis, appears to be induced by a number of drugs that have an antiproliferative mechanism of action (Rainer Liedtke 2008).

Cholesterol is delivered into the vessel wall by cholesterol-containing low-density lipoprotein (LDL) particles. To attract and stimulate macrophages, the cholesterol must be released from the LDL particles and oxidized, a key step in the ongoing inflammatory process. The process is worsened if there is insufficient high-density lipoprotein (HDL), the lipoprotein particle that removes cholesterol from tissues and carries it back to the liver.

The foam cells and platelets encourage the migration and proliferation of smooth muscle cells, which in turn ingest lipids, become replaced by collagen and transform into foam cells themselves. A protective fibrous cap normally forms between the fatty deposits and the artery lining (the intima).

These capped fatty deposits (now called 'atheromas') produce enzymes that cause the artery to enlarge over time. As long as the artery enlarges sufficiently to compensate for the extra thickness of the atheroma, then no narrowing ("stenosis") of the opening ("lumen") occurs. The artery becomes expanded with an egg-shaped cross-section, still with a circular opening. If the enlargement is beyond proportion to the atheroma thickness, then an aneurysm is created.[50]

Visible features
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Severe atherosclerosis of the aorta. Autopsy specimen.

Although arteries are not typically studied microscopically, two plaque types can be distinguished:[51]

  1. The fibro-lipid (fibro-fatty) plaque is characterized by an accumulation of lipid-laden cells underneath the intima of the arteries, typically without narrowing the lumen due to compensatory expansion of the bounding muscular layer of the artery wall. Beneath the endothelium there is a "fibrous cap" covering the atheromatous "core" of the plaque. The core consists of lipid-laden cells (macrophages and smooth muscle cells) with elevated tissue cholesterol and cholesterol ester content, fibrin, proteoglycans, collagen, elastin, and cellular debris. In advanced plaques, the central core of the plaque usually contains extracellular cholesterol deposits (released from dead cells), which form areas of cholesterol crystals with empty, needle-like clefts. At the periphery of the plaque are younger "foamy" cells and capillaries. These plaques usually produce the most damage to the individual when they rupture.
  2. The fibrous plaque is also localized under the intima, within the wall of the artery resulting in thickening and expansion of the wall and, sometimes, spotty localized narrowing of the lumen with some atrophy of the muscular layer. The fibrous plaque contains collagen fibers (eosinophilic), precipitates of calcium (hematoxylinophilic) and, rarely, lipid-laden cells.

In effect, the muscular portion of the artery wall forms small aneurysms just large enough to hold the atheroma that are present. The muscular portion of artery walls usually remain strong, even after they have remodeled to compensate for the atheromatous plaques.

However, atheromas within the vessel wall are soft and fragile with little elasticity. Arteries constantly expand and contract with each heartbeat, i.e., the pulse. In addition, the calcification deposits between the outer portion of the atheroma and the muscular wall, as they progress, lead to a loss of elasticity and stiffening of the artery as a whole.

The calcification deposits,[2] after they have become sufficiently advanced, are partially visible on coronary artery computed tomography or electron beam tomography (EBT) as rings of increased radiographic density, forming halos around the outer edges of the atheromatous plaques, within the artery wall. On CT, >130 units on the Hounsfield scale (some argue for 90 units) has been the radiographic density usually accepted as clearly representing tissue calcification within arteries. These deposits demonstrate unequivocal evidence of the disease, relatively advanced, even though the lumen of the artery is often still normal by angiographic or intravascular ultrasound.

Rupture and stenosis

Although the disease process tends to be slowly progressive over decades, it usually remains asymptomatic until an atheroma ulcerates, which leads to immediate blood clotting at the site of atheroma ulcer. This triggers a cascade of events that leads to clot enlargement, which may quickly obstruct the flow of blood. A complete blockage leads to ischemia of the myocardial (heart) muscle and damage. This process is the myocardial infarction or "heart attack".

If the heart attack is not fatal, fibrous organization of the clot within the lumen ensues, covering the rupture but also producing stenosis or closure of the lumen, or over time and after repeated ruptures, resulting in a persistent, usually localized stenosis or blockage of the artery lumen. Stenoses can be slowly progressive, whereas plaque ulceration is a sudden event that occurs specifically in atheromas with thinner/weaker fibrous caps that have become "unstable".

Repeated plaque ruptures, ones not resulting in total lumen closure, combined with the clot patch over the rupture and healing response to stabilize the clot, is the process that produces most stenoses over time. The stenotic areas tend to become more stable, despite increased flow velocities at these narrowings. Most major blood-flow-stopping events occur at large plaques, which, prior to their rupture, produced very little if any stenosis.

From clinical trials, 20% is the av

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