Thrombosis (Venous Thrombosis, DVT): Symptoms, Causes, and Prevention

In thrombosis, a blood clot (medically: thrombus) blocks a blood vessel. This interrupts normal blood flow. Veins are most commonly affected, particularly the deep veins of the legs and pelvis—this is referred to as deep vein thrombosis (DVT). Arterial thrombosis, or blockages in arteries, can also occur but is less common.

An adult has about 5 liters of blood in their body—roughly 8 percent of their body weight. This blood circulates continuously through arteries and veins. If a thrombus forms, blood circulation is disrupted. This can lead to serious consequences such as a pulmonary embolism—a medical emergency in which the clot travels into the pulmonary artery.

In industrialized countries, the condition occurs relatively frequently: About two out of every 1,000 people suffer from thrombosis each year.

Thrombosis can affect both veins and arteries. Venous thrombosis is the most common, particularly in the deep veins of the legs. It occurs when blood flow slows down or a blood clot forms on the vessel wall. Typical symptoms include swelling, a feeling of pressure, and pain in the affected leg. If the clot breaks loose, it can travel with the bloodstream to the lungs and cause a pulmonary embolism there.

Arterial thrombosis is much less common. It usually results from atherosclerosis—that is, a narrowing or damage to the vessel wall. This can lead to acute vascular occlusion—for example, in the heart (heart attack), in the brain (stroke), or in the legs.

Superficial thrombosis (thrombophlebitis), on the other hand, affects superficial veins, often in connection with varicose veins or minor injuries. It is usually painful but generally less dangerous. Nevertheless, it should be monitored by a doctor to prevent it from spreading to deeper veins.

Thrombosis in the Deep Veins of the Legs

The most common form is deep vein thrombosis in the legs. In this condition, a blood clot blocks the venous valves. Blood backs up, causing the affected leg to swell and become painful. Without treatment, there is a risk that the clot will break loose and travel to the lungs.

Deep vein thrombosis in the legs: A blood clot (thrombus) blocks the venous valves and causes blood to pool @ Анна Богатырева /AdobeStock

Thrombosis does not always present with noticeable symptoms right away. However, there are typical warning signs—especially with venous thrombosis:

• Swelling on one side of the leg, often in the lower leg or foot
• A feeling of tightness or pressure pain in the calf
• Pain when stepping or walking
• Bluish or reddish discoloration of the skin
• Warmth or a glossy appearance of the affected skin area

These symptoms are warning signs. If you suspect a thrombosis, you should seek medical help immediately. An untreated thrombosis can be life-threatening.

Thrombosis occurs when several factors come together. The so-called Virchow triad describes three main causes:

1. Slowed blood flow—e.g., due to prolonged bed rest or sitting for long periods (e.g., on an airplane)
2. Changes in the blood vessel wall—e.g., due to inflammation, injury, or atherosclerosis (hardening of the arteries)
3. Increased blood clotting tendency—e.g., due to dehydration, medications, or genetic predisposition

Overview of common risk factors:

• Prolonged bed rest (following surgery or hospitalization)
• Lack of physical activity (e.g., sitting for long periods while traveling)
• Dehydration
• Pregnancy and the postpartum period
• Birth control pills (hormonal contraception)
• Smoking
• Overweight
• Varicose veins
• Atherosclerosis
• Cancer
• Hereditary coagulation disorders
• Previous thrombosis

Thrombosis is often visible from the outside @ hriana /AdobeStock

First, the doctor assesses the symptoms and performs a physical examination. If there is reasonable suspicion, further diagnostic tests are conducted:

• D-dimer test: A simple blood test that shows whether blood clots are forming or breaking down at an increased rate in the body.
• Ultrasound (compression sonography): The most important test when venous thrombosis is suspected.
• Phlebography: An X-ray examination of the leg veins using a contrast agent—rarely used, e.g., when findings are unclear.
• Wells Score: A scoring system that assesses the likelihood of thrombosis based on clinical criteria.

Prompt treatment is crucial to prevent the progression of thrombosis and a pulmonary embolism. The most important treatment steps are:

Blood-thinning medications (anticoagulants):

• Treatment often begins with heparin injections
• Follow-up treatment usually involves oral medications such as rivaroxaban, apixaban, dabigatran, or warfarin
  

Compression therapy:

• Initially, wrap bandages
• Followed by medical compression stockings to relieve pressure on the veins and reduce swelling

Where the thrombosis causes congestion, you must wear the compression stocking @ tibanna79 /AdobeStock

Early mobilization:

• Patients should move around to promote blood flow

Surgical procedures:

• For large clots, e.g., in the pelvic vein, a thrombectomy (surgical removal of the clot) may be necessary

Bypass surgery:

• Only for arterial thrombosis—not for venous thrombosis

If the thrombosis is treated in a timely manner, the prognosis is good. Treatment usually lasts three to six months—or longer, or even for life, in high-risk cases.

Possible long-term consequences:

• Postthrombotic syndrome (PTS): Chronic symptoms such as leg pain, swelling, and skin changes
• Recurrent thrombosis: Especially in cases of a hereditary predisposition or inadequate follow-up care

Important: Regular medical checkups, consistent medication use, and wearing compression stockings can reduce long-term complications.

Thrombosis is a serious vascular disease that can be life-threatening—especially due to potential complications such as pulmonary embolism. Knowing the risk factors and recognizing symptoms early can help you protect yourself effectively. With modern diagnostic methods and timely treatment, the chances of recovery are generally very good.

How can you recognize a thrombosis?
The symptoms of a thrombosis usually appear in a leg, less commonly in an arm. Typical signs of a thrombosis include swelling, a feeling of tightness, and pain, which may worsen when walking or when pressure is applied to the veins. The skin is often reddened or bluish in color and feels warmer than the surrounding area. If thrombosis is suspected, medical attention should be sought promptly, as the blood clot can break loose and cause an embolism.

What causes and risk factors contribute to thrombosis?
Risk factors for thrombosis include prolonged sitting, surgery, smoking, hormonal contraception, obesity, and a congenital tendency toward increased blood clotting. Diseases of the blood vessels or inflammation of the vessel walls can also contribute to its development. Some people are at increased risk due to a family history of the condition or recurrent venous thrombosis.

How exactly does thrombosis develop?
In thrombosis, a blood clot forms and attaches to the wall of a blood vessel, obstructing blood flow. Blood pools and can further dilate or block the vessels. If a piece of the blood clot breaks off, it can travel with the bloodstream to the lungs and trigger a life-threatening embolism. This complication is one of the most serious consequences of untreated thrombosis.

How is thrombosis diagnosed and treated?
Diagnosis and treatment are typically carried out by a specialist in vascular medicine. Thrombosis can be reliably detected using imaging techniques such as ultrasound or special blood tests (D-dimers). Treatment involves the use of blood-thinning medications such as heparin or other anticoagulants to dissolve the blood clot or prevent it from growing larger. Compression therapy is also used to improve blood flow in the affected blood vessels.

Can thrombosis be completely cured?
In many cases, thrombosis heals without any lasting effects with consistent treatment. However, if it is detected too late, it can cause permanent damage to the veins. One possible long-term complication is what is known as an “open leg” or post-thrombotic syndrome. To avoid serious consequences, early diagnosis and consistent follow-up care are crucial.

How can thrombosis be prevented?
Effective prevention of venous thromboembolism involves exercise, adequate fluid intake, and wearing compression stockings—especially during prolonged sitting or after surgery. In certain cases, a doctor may recommend medication-based thrombosis prevention with heparin. Anyone with risk factors for thrombosis should seek medical advice and undergo regular checkups.

When is the risk of thrombosis particularly high?
The risk of thrombosis increases during prolonged immobility, during and after surgery, and during pregnancy and the postpartum period. Special caution is also warranted in cases of cancer or an increased tendency for blood to clot. Targeted prophylaxis helps protect the venous blood vessels and prevent the formation of new blood clots.

Glossary: Technical Terms Explained Simply

• Thrombus: A blood clot that can block a blood vessel
• Venous thrombosis: Blockage of a vein by a blood clot
• Arterial thrombosis: Blockage of an artery by a clot
• Compression stockings: Special medical stockings that improve venous return
• Anticoagulants: Medications used to thin the blood
• Pulmonary embolism: Blockage of a pulmonary artery by a blood clot
• Virchow’s triad: Three main causes that, when combined, can lead to thrombosis
• Wells score: A scoring system for assessing the risk of thrombosis