Venous thrombosis is a condition where blood clots form inside your veins, blocking the normal flow of blood and potentially leading to serious complications if left untreated.

Epidemiology

Venous thrombosis is a widespread health concern affecting people across the globe. In the United States alone, as many as 600,000 venous thromboembolism events occur each year, making it a significant public health challenge that requires attention and awareness.^[1] The condition is quite common, with approximately 1 to 3 out of every 1,000 adults developing deep vein thrombosis or pulmonary embolism annually in the United States.^[4]

This condition does not discriminate by age, though certain age groups face higher risks. While acute venous thrombosis can occur at any age, it is less common in children and adolescents but becomes increasingly frequent in people over the age of 60.^[13] In fact, people over 60 are at notably higher risk of developing this condition.^[6] The prevalence increases with age, meaning that our elderly population faces a greater burden from this disease.

Venous thrombosis ranks as the third most common vascular disease in the United States, trailing only behind heart attacks and strokes.^[13] This positioning highlights how widespread the condition is and underscores the importance of understanding its risks and prevention strategies. Tragically, up to 300,000 people die each year in the United States as a result of deep vein thrombosis and pulmonary embolism.^[13] In France, approximately 100,000 cases of venous thrombosis occur every year, with 10,000 to 20,000 deaths annually from pulmonary embolism, a serious complication of the condition.^[9]

More than half of all deep vein thrombosis cases happen as a result of being in the hospital from a medical illness or following surgery.^[13] This fact is particularly important because it shows that many cases are linked to periods when patients are less mobile than usual, such as during recovery from an operation or while receiving treatment for other conditions.

Causes

Understanding what causes venous thrombosis helps explain why blood clots form where they shouldn’t. Blood clotting is actually a normal and healthy response when you have a cut or wound on the outside of your body. Your blood naturally coagulates, or partially solidifies, to prevent too much bleeding. On the inside of your body, this process produces a clot. When a wound or infection begins to heal, the clot is supposed to dissolve. However, sometimes it doesn’t, and sometimes blood clots form even when there was no wound or obvious reason.^[2]

The formation of venous thrombosis typically involves one or more underlying problems with how blood flows or clots. Blood clots can develop if the blood flowing through your veins moves too slowly or becomes sluggish. When blood doesn’t move quickly enough, it has more opportunity to clump together and form a clot.^[13] Injury to your veins is another root cause. When the inner lining of a vein is damaged, whether from surgery, trauma, or other factors, it creates conditions that favor clot formation.^[3]

Blood clotting can also be triggered by an infection, which irritates the lining of the blood vessel in a way similar to a wound.^[2] Many factors can contribute to these underlying problems. Sometimes blood clots form due to a tendency for your blood to clot more easily than it should. This can happen because of genetic conditions or other health factors that affect how your blood naturally behaves.

In some cases, blood clots happen for reasons that are never identified. These are called “unprovoked” or unexplained blood clots.^[11] Even though doctors cannot always pinpoint an exact cause, it’s important to take these clots seriously and follow treatment recommendations to prevent complications.

Risk Factors

While venous thrombosis can affect anyone, certain groups of people and specific situations significantly increase the likelihood of developing blood clots. Understanding these risk factors can help you recognize when you might be at higher risk and take appropriate preventive steps.

Age is one of the most consistent risk factors. People who are 65 or older face a substantially higher risk of developing venous thrombosis compared to younger individuals.^[6] This increased risk means that older adults need to be especially vigilant about symptoms and prevention strategies.

A history of previous blood clots is a significant risk factor. If you’ve had a deep vein thrombosis before, your chances of developing another one are higher.^[6] This means that once you’ve experienced venous thrombosis, ongoing awareness and sometimes continued preventive treatment become essential parts of your healthcare.

Prolonged immobility is a major contributor to blood clot formation. The risk increases significantly during situations that involve staying still for extended periods. This includes being confined to bed, staying in the hospital, or going on long journeys that last more than three hours by plane, car, or train.^[6] When you don’t move around much, blood flow in your legs slows down, making it easier for clots to form.^[3]

Several medical conditions increase your risk. Cancer is associated with higher rates of venous thrombosis.^[4] Heart failure also raises your likelihood of developing blood clots.^[6] Chronic inflammatory diseases, such as inflammatory bowel disease and certain rheumatologic disorders, are known to increase risk.^[5] Obesity puts extra pressure on your veins, which contributes to clot formation.^[5]

Pregnancy and the postpartum period create special circumstances that increase clot risk. If you’re pregnant or if you’ve had a baby in the previous six weeks, your risk is elevated.^[6] The increased pressure on veins in your pelvis and legs during pregnancy can increase your risk of developing blood clots.^[13]

Hormonal factors play an important role. Taking hormones, especially those found in birth control pills or hormone replacement therapy, increases the likelihood of blood clots.^[6] The increased levels of estrogen, a hormone that plays a key role in female reproductive health, are associated with higher clotting risk.^[4]

Certain inherited conditions can make your blood clot more easily. If you have a family history of thrombosis or genetic predispositions that cause the blood to clot more easily, such as thrombophilia (a condition that increases your tendency to form blood clots), your risk is higher.^[9] This genetic component can sometimes be detected with a simple blood test.

Lifestyle factors also matter. Smoking damages blood vessels and increases the risk of clot formation.^[3] Being overweight or obese, which means having excess body weight that puts additional pressure on your veins, raises your risk.^[6] Having varicose veins, which are enlarged and twisted veins visible under the skin, can increase your chances of developing blood clots, particularly in the superficial veins.^[6]

Dehydration is another factor that increases risk. When you don’t drink enough water, your blood can become thicker, making it more likely to clot.^[6] This is especially important to remember during travel or when you’re not feeling well.

Symptoms

The symptoms of venous thrombosis can vary depending on where the blood clot is located and how large it is. Understanding what to look for is crucial because early recognition can lead to prompt treatment and prevent serious complications. However, it’s important to know that venous thrombosis can sometimes occur without any obvious signs, which makes the condition harder to detect.^[1]

When a blood clot forms in the deep veins of your leg, which is called deep vein thrombosis or DVT, you might experience several noticeable changes. About half of people with deep vein thrombosis have no symptoms at all, but when symptoms do appear, they can be quite distinctive.^[4] Up to 30% of people with a deep vein thrombosis don’t have symptoms, and sometimes the symptoms are very mild and may not raise immediate concern.^[13]

The most frequent symptom of venous thrombosis in the leg is pain.^[9] This pain often starts in the calf and can feel like cramping or soreness.^[3] The pain or tenderness may only happen when you’re standing or walking.^[13] Sometimes patients experience very few symptoms, such as persistent discomfort in the calf, but even these mild signs should not be ignored.^[9]

Swelling is another common symptom. The affected leg or arm may swell, and sometimes this swelling happens suddenly.^[3] You might notice that one leg looks noticeably larger than the other, and in some cases the swelling is accompanied by skin that appears “shiny” in the affected area.^[9]

Changes in how your skin looks and feels can signal a blood clot. The area of your leg or arm that’s swollen or hurts may be warmer than usual.^[13] Your skin might become red or discolored. Depending on your natural skin color, this change might appear as red, purple, or darkened skin, and these color changes may be harder to see on brown or black skin.^[6]

In the case of superficial vein thrombosis, which affects veins closer to the skin’s surface, you might feel hardening of the vein in addition to pain, swelling, and redness.^[9] Some people experience a slight fever, around 38°C or 100.4°F.^[9]

The veins near your skin’s surface may become larger than normal, appearing more prominent or bulging.^[13] When blood clots affect veins deep inside your abdomen, you might experience abdominal pain or flank pain instead of leg symptoms.^[13] If the clot affects veins in your brain or head, you might have a severe headache that usually comes on suddenly.^[13]

If a blood clot breaks loose and travels to your lungs, causing a pulmonary embolism or PE, the symptoms change dramatically and require immediate medical attention. You can have a pulmonary embolism without any symptoms of deep vein thrombosis in your legs.^[4] The warning signs of pulmonary embolism include sudden shortness of breath, which means you have difficulty breathing that comes on quickly.^[3]

Chest pain or discomfort is a serious symptom of pulmonary embolism. This chest pain usually worsens when you take a deep breath or cough.^[3] You might experience a faster than normal or irregular heartbeat.^[4] Some people cough or cough up blood when they have a pulmonary embolism.^[4]

Very low blood pressure, lightheadedness, or fainting are also signs of pulmonary embolism that require emergency care.^[4] If you develop any symptoms of a pulmonary embolism, you should seek medical help immediately because this condition can be life-threatening.^[4]

Prevention

Preventing venous thrombosis involves a combination of lifestyle changes, medical interventions, and awareness, especially during high-risk situations. Many blood clots can be prevented with proper precautions, making prevention an essential part of managing your health.

If you are preparing to go to the hospital for a procedure or have other risk factors for venous thromboembolism, it’s crucial to talk with your healthcare provider about a plan for preventing blood clots from forming.^[17] Your provider should check if there’s a higher chance you’ll develop venous thrombosis and, if so, may give you treatment to prevent it, such as medicine or compression stockings.^[6]

Movement is one of the most powerful tools for preventing blood clots. Staying active and exercising regularly helps keep your blood circulating properly. Aim for at least 30 minutes of moderate exercise daily, which significantly improves blood flow and reduces your risk.^[18] Activities like walking, cycling, and swimming are excellent options.^[18]

After surgery, your provider may give you several suggestions to help prevent venous thrombosis. Helping your blood circulate makes it harder for clots to form, so your provider may tell you to move around as soon as possible after surgery and as you heal.^[17] If you cannot get up and walk, try to flex and stretch your feet to improve blood flow in your calves.^[17]

For people with sedentary jobs or lifestyles, it’s important not to sit still for long periods of time. Try to get up and move around every hour or so.^[6] Try not to stay seated for more than two hours at a time.^[19] If you’ve had deep vein thrombosis in one of your legs, don’t cross your legs when you sit down, as that position can affect your circulation.^[19]

When traveling for three hours or more by plane, train, or car, there are specific precautions you can take during the journey to lower your chances of getting venous thrombosis.^[6] These include wearing loose clothing, drinking plenty of water, avoiding alcohol, and walking around when possible.^[6] If you’re in a car, take breaks often to stretch your legs, and deep knee bends can help get your circulation going.^[23] On a plane, try to move around the cabin once an hour, and while you’re in your seat, flex your ankles to keep blood flowing.^[23]

Compression therapy can help prevent blood clots by applying gentle pressure that keeps blood from pooling and clotting. Your provider may talk to you about applying pressure, for example by wearing a sleeve or boot that periodically fills with air, or by wearing compression stockings, which are special tight-fitting socks that apply more pressure around the ankles and feet to help with blood flow.^[17]

Maintaining a healthy weight is important for prevention. Being overweight increases pressure on your veins, which can contribute to the formation of blood clots.^[18] Adopting a balanced diet and maintaining a healthy weight can reduce your risk. Focus on eating plenty of fruits, vegetables, whole grains, and lean proteins.^[18]

Staying properly hydrated helps prevent blood from thickening, which reduces clot formation.^[3] Drink plenty of fluids to avoid dehydration, as venous thrombosis is more likely if you’re dehydrated.^[6]

If you smoke, quitting is one of the most important steps you can take. Smoking damages blood vessels, increasing the risk of clot formation.^[3] Similarly, it’s wise not to drink lots of alcohol, as excessive alcohol can lead to dehydration and increase clot risk.^[18]

In some cases, your healthcare provider may prescribe blood thinner medicines, also called anticoagulants, to prevent blood clots from forming. Anticoagulants such as heparin (which your provider can give as a shot), direct oral anticoagulants, and warfarin (which you take by mouth) can be used both to treat and prevent venous thrombosis.^[17] Sometimes this preventive therapy starts before surgery, or your provider may prescribe a blood thinner to take during your recovery period.^[17]

If you have risk factors like a family history of venous thrombosis, it’s important to talk to your doctor. They can help you develop a prevention strategy that might include blood thinners and other approaches tailored to your specific situation.^[18]

Pathophysiology

To understand venous thrombosis, it helps to know how normal blood flow and clotting work, and what changes occur when this condition develops. The pathophysiology describes the abnormal physical, biochemical, and mechanical changes that happen in your body when venous thrombosis occurs.

Blood normally flows through your veins back to your heart in a smooth, continuous stream. Veins are blood vessels that carry blood from the extremities of your body back to your heart.^[2] When a vein is blocked by a blood clot, blood pools behind the blockage, causing inflammation. Meanwhile, cells ahead of the blockage can’t get the oxygen and nutrients they need.^[2] This can cause serious damage to the veins, tissues, and organs that depend on that blood supply.

Blood clotting is a complex process involving many factors that cause your blood to partially solidify or coagulate. Under normal circumstances, this process helps stop bleeding when you’re injured. However, in venous thrombosis, clotting occurs inside blood vessels when it shouldn’t, creating a solid or semi-solid mass called a thrombus.^[2]

The clot can partially or completely block blood flow through your vein.^[13] When blood flow is restricted, several problems develop. The area behind the clot experiences increased pressure as blood continues to arrive but cannot pass through. This elevated pressure causes fluid to leak out of the vein and into surrounding tissues, leading to swelling. The trapped blood and increased pressure also trigger inflammatory responses in your body, causing pain, warmth, and redness in the affected area.

A particularly dangerous aspect of venous thrombosis is that blood clots can break free from where they formed. When this happens, the clot becomes an embolus, which is a circulating particle that travels through your bloodstream.^[2] If a piece of thrombus breaks loose and travels in the blood, this is called a thromboembolism.^[7]

Because veins return blood to the heart, if a piece of a blood clot breaks off from a vein, it can be transported to the right side of the heart and from there into the lungs.^[7] When an embolus lodges in the blood vessels of your lungs, it causes a pulmonary embolism. This restricts oxygen and blood flow to your lungs while increasing blood pressure in your arteries.^[2] This causes pulmonary hypertension, which is high blood pressure in the arteries of your lungs, and this in turn can cause heart failure and death.^[2]

Long-term complications can occur even after treatment. As many as half of those who get a deep vein thrombosis in their legs develop symptoms of intermittent leg pain and swelling that may last months to years. These symptoms are called post-thrombotic syndrome and can happen because of damage to the valves and inner lining of your veins, leading to blood “pooling” more than it should.^[13] This increases the pressure inside your veins and causes ongoing pain and swelling.

One-third to one-half of people who have a deep vein thrombosis will develop post-thrombotic syndrome, which is caused by the damage the clot does to the valves in the vein.^[4] People with post-thrombotic syndrome have symptoms such as swelling, pain, discoloration, and in severe cases, scaling or ulcers in the affected area.^[4] The risk of developing post-thrombotic syndrome is reduced if you begin treatment quickly and receive appropriate follow-up care.^[9]