How are carotid artery plaques formed? Can ultrasound determine the hardness of plaques?

Author: Chief Physician, Huayang Capital Medical University Xuanwu Hospital

Reviewer: Shen Chenyang, Chief Physician, Beijing Tiantan Hospital, Capital Medical University

The discovery of plaques during carotid artery ultrasound examination indicates that the carotid artery has already undergone pathological changes of atherosclerosis. Generally, atherosclerosis is divided into several stages.

1. What are the stages of atherosclerosis?

Atherosclerosis is divided into different pathological changes such as intimal thickening, plaque formation, arterial stenosis, and arterial occlusion.

The wall of the carotid artery has three layers: the intima, the media, and the adventitia. The intima of a normal person's carotid artery is very smooth and does not have any plaques. However, under the influence of various cardiovascular risk factors, the intima will be damaged, and the intima-media will fuse and thicken. When the intima-media thickness is ≥1.0mm, it is the early stage of atherosclerosis; when the intima-media thickness is ≥1.5mm, it is the plaque formation stage. The formation and gradual enlargement of plaques will lead to vascular stenosis and even occlusion.

Figure 1 Original copyright image, no permission to reprint

The formation of plaques is a complex pathological mechanism. First, under the influence of various cardiovascular risk factors such as hypertension, diabetes, and smoking, the intima layer (composed of endothelial cells) of the arterial wall is damaged and the surface is not smooth, and the intima-media fusion and thickening occur. When the thickness of the intima-media is 1.0mm≤1.5mm, it is the early stage of atherosclerosis, the first stage.

After injury, the gaps between endothelial cells widen, and inflammatory cells and fat in the blood are deposited in the blood vessel wall along the widened gaps and continuously trigger inflammatory reactions. A type of white blood cell (macrophage) involved in the inflammatory reaction can "swallow" fat in the blood, become "foam" cells, and then necrotize and disintegrate, releasing "fat particles" that are deposited in the blood vessel wall. When the thickness of the intima-media membrane is ≥1.5mm, "plaques" are gradually formed, which is the second stage of atherosclerosis.

If risk factors such as blood pressure, blood lipids, and blood sugar are not actively controlled, the plaque will grow larger and larger, slowly leading to the narrowing of the arteries and eventually occlusion. Clinically, the degree of stenosis of atherosclerotic lesions is classified as mild (less than 50% stenosis), moderate (50%-69% stenosis), and severe (70%-99% stenosis). Severe cases can lead to vascular occlusion.

Simply put, there are four different stages of the development of atherosclerosis, from intimal damage to plaque formation, to vascular stenosis, and then to arterial occlusion. However, for each individual, a carotid artery examination may reveal different arteries and different degrees of atherosclerotic lesions, and screening is performed based on ultrasound examination.

Once plaques are found during carotid artery ultrasound examination, it means that the disease has entered the stage of atherosclerosis. It is necessary to actively control risk factors, stabilize plaques, and avoid rapid progression of plaques.

2. Can carotid artery ultrasound distinguish between soft plaques and hard plaques?

Ultrasound can determine the basic properties of plaques. However, the doctor needs to conduct a comprehensive assessment of the morphology and acoustic characteristics of the plaque, the tissue structure and the acoustic imaging characteristics of ultrasound reflection, in order to determine whether the texture of the plaque is a "hard" plaque (calcified plaque) or a soft plaque (lipid).

The components of plaques include inflammatory cells and lipids, as well as necrotic and calcified tissues. If there is more calcification, the plaque will appear relatively "harder".

The following description is often seen in ultrasound reports: plaques that are mainly low-echo or isoechoic, with spots, flakes or cords visible on the surface, or spots, flakes or mostly strong echoes inside, which means that the plaques have structural characteristics of varying degrees of calcification.

If the plaque has strong echoes formed by calcification, low echoes with more lipids, or different structural characteristics such as fibrosis, we call this type of plaque detected by ultrasound a "mixed plaque". Due to the different stresses generated by blood flow on the surface of the plaque, under the influence of risk factors, especially in patients with poorly controlled risk factors such as hypertension, the original relatively "stable" plaque may also rupture and form an unstable plaque. More attention should be paid to the treatment and control of cardiovascular risk factors.

Simple medium echo plaques are relatively stable in texture, neither soft nor hard, and contain more fibrous tissue. Plaques often have a fibrous cap on the surface of the plaque. Plaques with relatively complete fibrous caps are relatively stable. However, if the fibrous cap on the surface of the plaque is incomplete and uneven in thickness, the plaque is relatively unstable. When the fibrous cap is broken, blood will flow into the plaque to "perfuse" and form ulcers. The presence of ulcers will increase the risk of plaque instability.

The surface of the plaque is irregular and "bumpy", and the red blood cells in the blood will deposit and form blood clots. The detachment of blood clots may cause stroke. If the risk factors are well controlled, blood pressure and blood sugar are controlled at normal levels, blood lipids are also up to standard, and everything is normal, it is possible that stroke will not occur within a certain period of time, or the risk of stroke will be relatively reduced.

Therefore, the stability of plaques is directly related to many risk factors. For example, fluctuations in blood pressure can easily lead to unstable plaques. Increased blood lipids can cause plaque growth and instability. Increased blood sugar can also accelerate the progression of plaques or increase and enlarge the plaques, etc.

Therefore, we should make a comprehensive consideration and cannot simply assume that soft plaques are unstable and hard plaques are stable. The stability of plaques is affected by many factors, both subjective and objective.

3. Do plaques in the carotid artery need to be removed surgically?

Whether to surgically remove the plaque depends on the clinical symptoms.

Severe stenosis, decreased blood perfusion, and repeated cerebral ischemia symptoms should be treated. If the patient has many underlying diseases and surgical treatment is risky, medical treatment can be used temporarily to stabilize the plaque.

If stroke occurs repeatedly, plaque should be removed. Carotid endarterectomy can remove the plaque.

Figure 2 Original copyright image, no permission to reprint

Another method is to place a stent, but it does not remove the plaque. The stent is used to support the plaque so that it sticks to the blood vessel wall, solving the problem of vascular stenosis. However, it is impossible to completely restore the lumen to normal, and there is a certain amount of residual stenosis.

Figure 3 Original copyright image, no permission to reprint

Surgical removal of plaques is not a permanent solution. If risk factors such as high blood lipids, high blood pressure, and high blood sugar are not controlled, plaques may grow again after surgery. Therefore, the most fundamental thing is to control the risk factors.