How does lower extremity venous thrombosis form? Can it be detected by ultrasound?

Author: He Wen, Chief Physician, Beijing Tiantan Hospital, Capital Medical University

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

Lower limb vein thrombosis can cause lower limb edema, pigmentation, ulcer formation, and limb movement disorders. The most dangerous thing is that the thrombus breaks off and flows into the lungs with the blood, causing pulmonary embolism. Once pulmonary embolism occurs, the mortality rate is very high.

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Therefore, we should pay attention to lower limb venous thrombosis, detect it early and treat it early.

1. What are the high-risk factors for lower limb venous thrombosis?

The cause of lower limb venous thrombosis is currently not very clear. It may be related to factors such as advanced age, long-term bed rest, trauma; tumors, heart disease; and pelvic surgery, limb surgery, and extremity surgery. These are all high-risk factors for thrombosis.

There are several situations that lead to lower limb venous thrombosis. The first is endothelial damage, the second is slow blood flow, and the third is hemodynamic changes. Any of these situations can lead to thrombosis.

For example, trauma and iatrogenic procedures can cause damage to the vascular endothelium; long-term bed rest can cause hemodynamic changes; high blood viscosity can cause blood to flow more slowly and easily form blood clots.

For people with these risk factors, it is recommended that they undergo routine lower extremity venous ultrasound examinations to screen for thrombosis. Of course, some people who do not have these high-risk factors but have edema in their lower extremities should also undergo examinations to find out whether they have venous thrombosis.

2. Is it sufficient to just do ultrasound examination to diagnose lower limb venous thrombosis?

Ultrasound is the preferred method for detecting lower extremity venous thrombosis and is currently the gold standard for the diagnosis of lower extremity venous thrombosis. Ultrasound is a non-invasive examination, has no radiation, and is inexpensive. It can understand changes in hemodynamics and show whether thrombosis has formed.

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Ultrasound diagnosis of deep vein thrombosis in the lower extremities generally does not require contrast agents. Under what circumstances is it necessary? For example, if the venous blood flow is very slow, the color Doppler ultrasound may not show it clearly, as if there is no blood flowing in the veins. After the ultrasound contrast agent is injected, it can be distinguished whether a thrombus has formed or a blood vessel is blocked. For patients with edema and swollen legs, conventional ultrasound display is limited, and ultrasound contrast can make up for the deficiency. For calf vein thrombosis, sometimes the sound velocity is vertical, and the blood flow velocity is very slow, so it is sometimes difficult to show. Ultrasound contrast can show whether the blood flow is unobstructed and whether there is a thrombus.

3. What are the manifestations of lower limb venous thrombosis under ultrasound?

In acute venous thrombosis, the vascular dilation is more than twice that of the artery, and the lumen is filled with low-echo emboli. The lumen cannot be flattened when the ultrasound probe is pressurized, while normal veins are easily flattened. If it is completely blocked, there will be no blood flow signal in this area. If there is stenosis, it can be seen that the flow velocity in the stenosis increases, the venous return slows down, and there are more changes in the spectrum.

Generally, acute thrombosis is formed within two weeks. When the thrombus is just formed, its structure is relatively loose, and the relationship between it and the tube wall is not very clear. It can be seen swinging back and forth and is extremely easy to fall off. Of course, the thrombus has an excellent thrombolytic effect.

In the subacute stage, that is, when the thrombus has formed for more than half a month, the thrombus begins to organize and fibrose. At this time, the relationship between the thrombus and the vessel wall becomes closer and closer, the blood vessels begin to shrink to a certain extent, the echo increases, and even some side branches are formed, and the thrombolytic effect will be greatly reduced.

In the chronic stage, the thrombus has been formed for more than half a year, and the thrombus begins to organize, fibrose, and even calcify. The thrombus and the vessel wall form adhesions, the structure is not clear, the vessel wall is thickened, and collateral circulation is often formed. Of course, the blood vessels cannot be flattened when the probe is pressurized, and there may even be a phenomenon of recanalization. The thrombolysis effect of chronic thrombus is not very good.