Thrombosis (Blood Clot)

How thrombosis happens

Clotting normally stops bleeding when a vessel is injured. Thrombosis happens when clotting occurs inside an intact vessel. Three factors, known as Virchow's triad, make it more likely:

1. slow or stagnant blood flow: immobility, long travel, paralysis, heart failure
2. damage to the vessel wall: surgery, injury, atherosclerosis, catheters
3. blood that clots more easily: cancer, pregnancy, estrogen medicines, inherited conditions, dehydration, inflammation

Venous thrombosis

Deep vein thrombosis (DVT)

A clot in a deep vein, usually in the leg or pelvis. Deep vein thrombosis causes:

• swelling of one leg, calf or thigh
• pain or tenderness, often a cramping ache in the calf
• warmth and redness or discoloration
• visible surface veins

About half of people with DVT have few or no symptoms.

Pulmonary embolism (PE)

A clot, usually from a leg DVT, that travels to the lungs. Pulmonary embolism causes:

• sudden shortness of breath
• sharp chest pain, worse with deep breaths
• fast heart rate
• coughing up blood
• lightheadedness or fainting

PE can be fatal and needs emergency care.

Other venous clots

• cerebral venous thrombosis: clots in brain veins, causing severe headache, seizures and stroke-like symptoms
• abdominal vein clots: in the liver, portal or mesenteric veins, causing belly pain
• arm vein clots, often linked to catheters or vigorous arm activity
• superficial thrombophlebitis: a clot in a vein near the surface, causing a tender, red cord

Arterial thrombosis

Arterial clots usually form on top of fatty plaques (atherosclerosis) that rupture:

• heart attack: a clot in a coronary artery
• stroke: a clot in an artery supplying the brain, or an embolus from the heart, often due to atrial fibrillation
• acute limb ischemia: sudden loss of blood flow to a leg or arm, causing pain, pallor, coldness, numbness and weakness
• mesenteric ischemia: a clot in the gut arteries, causing severe belly pain

Risk factors

For venous clots

• recent surgery, especially orthopedic, or hospitalization
• immobility, long flights or car trips
• cancer and chemotherapy
• pregnancy and the 6 weeks after birth
• estrogen-containing birth control or hormone therapy
• obesity and smoking
• previous clot or family history
• inherited thrombophilia, such as factor V Leiden
• antiphospholipid syndrome
• inflammatory bowel disease, COVID-19 and other serious infections

For arterial clots

• high blood pressure, high cholesterol, diabetes and smoking
• atrial fibrillation
• older age and family history of heart disease

Diagnosis

• D-dimer blood test: a normal result in a low-risk person helps rule out a venous clot
• duplex ultrasound for DVT
• CT pulmonary angiography for PE
• ECG and troponin for heart attack
• CT or MRI of the brain for stroke
• CT angiography for arterial clots
• clotting disorder tests in selected people

Treatment

Venous clots

• anticoagulants (blood thinners): direct oral anticoagulants such as apixaban or rivaroxaban, heparin injections or warfarin. They stop the clot growing and prevent new clots while the body breaks down the existing clot
• duration: usually at least 3 months, longer or indefinitely for unprovoked or recurrent clots
• clot-dissolving medicines or catheter-directed clot removal for large PE with low blood pressure or severe DVT
• IVC filter in selected people who cannot take blood thinners
• compression stockings may ease leg symptoms

Arterial clots

• heart attack: aspirin, other antiplatelets, and opening the artery with a stent or clot-busting drugs
• stroke: clot-busting medicine within hours and mechanical clot removal in eligible patients
• limb ischemia: heparin and urgent surgery or catheter procedures
• long-term antiplatelets, statins and control of risk factors

Prevention

• move around and walk often after surgery or illness
• on long trips, walk every 1 to 2 hours, flex your ankles, and stay hydrated
• blood thinner injections or tablets, and compression devices, during and after high-risk surgery or hospitalization
• stop smoking, manage weight, blood pressure, cholesterol and diabetes
• discuss risks before starting estrogen if you have had a clot or have a strong family history

Frequently asked questions

What are the signs of a blood clot?

It depends on the location. A leg clot causes one-sided swelling, pain, warmth and redness. A lung clot causes sudden breathlessness and sharp chest pain. A clot in the heart or brain causes heart attack or stroke symptoms. Lung, heart and brain clots are emergencies.

What causes thrombosis?

Clots form when blood flow slows, a vessel wall is damaged or the blood clots too easily. Common triggers include surgery, immobility, long travel, cancer, pregnancy, estrogen medicines, smoking, obesity and inherited clotting disorders. Arterial clots usually form on cholesterol plaques.

How long does it take for a blood clot to dissolve?

The body gradually breaks down a clot over weeks to months. Blood thinners prevent it from growing while this happens. Treatment for a DVT or PE usually lasts at least 3 months. Leg swelling may take longer to settle.

Can a blood clot go away on its own?

Small clots sometimes dissolve without treatment, but untreated deep vein clots can grow or travel to the lungs. Anyone with symptoms of a clot should be assessed promptly rather than waiting.