Watch the video Pathogenesis of Atherosclerosis
Plaque Formation Image by Jared C BYU-I F16
Arteriosclerosis is the hardening of arterial walls that can be caused by a variety of conditions. Atherosclerosis is a specific kind of arteriosclerosis. Atherosclerosis is the thickening and stiffening of artery walls because of the accumulation of a fatty material called plaque. Atherosclerosis often affects coronary arteries and can lead to a myocardial infarction (heart attack). Atherosclerosis is also common in carotid arteries where clots can form and cause strokes if they dislodge and become an embolus.
There are three stages of atherosclerosis:
- The first stage is fatty streaks and represents the first evidence of the disease. Fatty streaks consist of smooth muscle tissue filled with cholesterol and macrophages.
- The second stage, a fibrous atheromatous plaque, is a more progressed lesion that consists of lipids accumulated in macrophages and smooth muscle cells as well as scar tissue and calcification. This plaque may bulge into the lumen of a small blood vessel but is still covered by endothelial cells.
- If the endothelial cells covering the plaque are compromised, the final stage called a complicated atherosclerotic lesion ensues. The formation of a lesion greatly increases the risk for thrombus formation as blood flow is slowed further in the area of the plaque and the blood is also in contact with connective tissue components beyond the endothelial barrier that can trigger a clotting response.
The pathophysiology, or progression of atherosclerosis through these stages begins when endothelial cells become damaged due to things like hypertension, smoking, hyperglycemia, and hypercholesterolemia. Damage to endothelial cells makes it so LDLs are able to enter into the tunica intima (the inner layer of the vessel wall). These damaged endothelial cells also express adhesion molecules that recruit circulating WBCs to the area. Once in the tunica intima, WBCs produce free radicals that oxidize LDLs and make them more likely to attract and activate more WBCs. WBCs and smooth muscle cells then engulf the oxidized LDLs. WBCs laden with oxidized LDLs produce even more free radicals which further oxidize LDLs, recruit more WBCs, and further damage the tissue. We see that a positive feedback loop has begun. Macrophages saturated with LDL particles are called foam cells because the excess amount of lipid in the cell gives it a foamy appearance. Once foam cells die, they release their contents, which can then be engulfed by other macrophages to continue the cycle or calcified and cause hardening of the vessel wall. The buildup of all this lipid, damage, and dead WBCs forms a plaque with a lipid core. Endothelial cells cover the plaque and over time, calcium salts and dead cells continue to accumulate and cause hardening. This plaque in the arterial wall is atherosclerosis. Eventually, areas of the plaque may jut out into the vessel lumen and cause narrowing of the vessel. If the endothelial cells are later compromised, blood clots can form on the vessel wall and potentially occlude blood flow. Healthy endothelial cells can inhibit clotting, but damaged cells cannot. A clot that forms and attaches to the vessel wall is called a thrombus, but if the clot breaks loose and floats downstream, it is considered an embolus.
Risk factors for atherosclerosis can be broken down into three categories:
- Nonmodifiable risk factors are increasing age, male gender, genetic disorders of lipid metabolism, and a family history of premature coronary artery disease.
- Potentially modifiable risk factors include cigarette smoking, obesity, hypertension, hyperlipidemia (high LDL, low HDL), and diabetes mellitus.
- Additional nontraditional risk factors are inflammation marked by elevated C-reactive protein levels, hyperhomocysteinemia, and increased lipoprotein (a) levels. Lipoprotein (a) is not Apo A1. Lipoprotein (a) is an LDL variant that contains a specific protein variant that is actually bound to the Apo B100. This variation in the LDL appears to correlate with even higher risk of atherosclerosis development. A special test to look for lipoprotein (a) called Lp(a) may be done to assess a more accurate picture of atherosclerosis risk.