A thrombus is a blood clot in the circulatory system. It attaches to the site at which it formed and remains there, hindering blood flow.
Doctors describe the development of a thrombus as thrombosis.
A thrombus is most likely to occur in people who are immobile and in those with a genetic predisposition to blood clotting.
A thrombus can also form after damage occurs in an artery, vein, or surrounding tissue.
In this article, we look at blood clotting and the different types of thrombi. We also look at the symptoms, diagnosis, and treatment of a thrombus.
A blood clot is usually a normal physical response to injury.
It quickly forms a plug that can reduce or prevent bleeding. However, a thrombus can cause severe health problems, as it interrupts the function of a blood vessel.
A section of a blood clot that breaks free from the thrombus and circulates in the bloodstream is called an embolus.
An embolus moves through the vascular system until it lodges in a different part of the body.
An embolus is a dangerous and potentially fatal complication of thrombosis. It is especially dangerous if it reaches the heart, lungs, or brain (embolism).
Doctors categorize thrombi based on the type of blood vessel in which they develop:
When a thrombus forms in an artery, such as in the heart or brain, it is called an arterial thrombosis.
When a thrombus occurs in a vein, it is called a venous thrombosis. When this happens in the deep veins of the leg, it is called deep vein thrombosis (DVT).
Clotting occurs due to a series of chemical reactions between blood cells known as platelets and proteins called clotting factors.
When a person is in good health, the body regulates the clotting process according to its needs.
However, a clot can form more easily when a person:
- uses tobacco
- has high cholesterol
- has obesity or is overweight
- has cancer
- has diabetes
- is stressed
- has an inactive lifestyle
Some of these factors also increase the risk of atherosclerosis, a condition wherein fatty plaque deposits line the blood vessels and clog them.
Atherosclerosis makes blood clots more likely to block the arteries and the veins.
The physical effects of a thrombus depend on its location in the body.
Both arterial and venous thrombosis can reduce or totally impede blood flow. This can lead to severe and even life threatening complications.
A thrombus does not usually cause any symptoms until it blocks or heavily restricts the flow of blood.
In the sections below, we cover the symptoms of arterial and venous thrombosis:
Symptoms of arterial thrombosis
A thrombus in an artery can result in:
- unstable angina, which is a type of chest pain
- heart attack
- ischemic stroke
- peripheral arterial limb ischemia, a condition that significantly reduces blood flow to the limbs
These conditions all require prompt medical attention.
People should seek emergency treatment if they experience any of the following symptoms:
- chest pain
- shortness of breath
- drooping on the lower half of the face
- a sudden loss of strength in one arm or leg
- a limb that has become cold, pale, and painful
Symptoms of venous thrombosis
A thrombus in a vein, usually a deep vein in the leg, may lead to
- pain, swelling, and tenderness, usually in the calf
- aching and warmth of the skin in the affected area
- red skin, particularly at the back of the leg below the knee
People who experience any of these symptoms should seek urgent medical attention.
DVT can be a life threatening condition; emboli can travel through the bloodstream before blocking arteries elsewhere in the body.
Doctors use several different methods to diagnose the presence of a thrombus. For example, they may use:
- Duplex ultrasound: This is the most common test for diagnosing DVT. A duplex ultrasound uses sound waves to create images of the blood flowing through the arteries and veins.
- A D-dimer test: This test measures the levels of a substance in the blood that results from the breakdown of blood clots. High levels of this substance may indicate the presence of DVT or another type of blood clot. However, the test is not definitive. If the result is normal and few risk factors are present, a person does not have a high risk of DVT.
- Venography: For venography, a doctor will inject a dye into a vein in the affected leg. This dye makes the vein visible on some types of X-ray, such as a fluoroscopy. If the scan shows a slower-than-usual blood flow through the vein, a thrombus may be present.
- MRI and CT scans: These scans create detailed images of organs, tissues, and blood vessels.
- A VQ scan: This is a nuclear imaging study. It uses a radioactive substance called a radiotracer to reveal, on a scan, the flow of air and blood within the lungs.
A doctor may request blood tests to check for a genetic blood clotting disorder. This may be necessary in cases of repeated unexplained blood clots.
Thrombi in the liver, kidney, or brain may develop due to an inherited clotting disorder.
The aim of treating a thrombus is to achieve the following quickly and effectively:
- gain control over the symptoms
- restore the blood flow
- reduce and remove the thrombus
Doctors typically recommend the following treatments to deal with the effects of thrombi:
Surgery for the effects of thrombosis will always be a medical emergency.
The procedure can involve directly accessing and unblocking an affected artery. In other cases, the surgeon will divert blood flow or completely bypass the blocked artery.
Inferior vena cava filters
Inferior vena cava (IVC) filters are small mesh devices that a surgeon can put in the inferior vena cava (a large vein), usually under local anesthetic.
The IVC filter traps fragments of the blood clot and prevents them from reaching the heart and lungs.
An IVC filter can be permanent, and doctors typically combine this treatment with anticoagulation medication therapy where possible. However, a surgeon may remove the IVC filter if the person’s risk of a blood clot declines.
Anticoagulants, or blood thinners, have a misleading name; they do not give the blood a thinner consistency.
Instead, they reduce the risk of a clot forming, which can reduce the size of a thrombus.
When taking anticoagulant medications, a person should visit a specialized anticoagulant management service instead of a primary care physician.
If anticoagulants are not effective, or if a person does not tolerate them well, a doctor will consider other treatment options.
Doctors may recommend that people wear compression stockings while taking anticoagulant therapy for DVT.
The stockings help prevent calf pain and swelling, as well as reduce the risk of complications.
A person should wear compression stockings for as long as their doctor recommends.
Raising the affected leg
As well as wearing compression stockings, people should try to keep the affected leg elevated above hip level during the night.
This can relieve pressure in the veins, improve blood circulation, and help prevent complications.
Once a doctor has prescribed compression stockings, they will usually recommend more frequent walking to stimulate blood circulation.
It is not always possible to prevent a thrombus. However, people can take steps to reduce their risk.
For example, a person can:
- avoid or quit tobacco smoking
- prevent excessive weight gain or lose weight to avoid obesity
- adopt a healthful diet
- exercise regularly
It is particularly important for a person to
Those with a higher risk of developing a blood clot may also require anticoagulant therapy alongside medications to reduce blood pressure and blood cholesterol levels.
Successful treatment is possible — even for potentially fatal medical emergencies due to thrombosis — if a person gets appropriate treatment at the right time.
Aftercare is particularly important. Complications can occur months or years after the thrombus initially forms, even following successful treatment.
Post-thrombotic syndrome (PTS) is one potential complication of DVT. This refers to surrounding tissue damage that occurs due to the presence of DVT.
PTS can lead to increases in pressure in the vein following a blood flow blockage, ulcerations, and pain.
This complication can lead to permanent vascular damage and — in rare circumstances — may lead to amputation of the affected limb.
The speed and success of recovery depends on the location of the clot, as well as the duration and extent of blood flow disruption. The sooner a doctor starts to treat a thrombus, the lower the risk of long term damage or complications.