The Facts

Blood clots are a collection of sticky blood cells that form when a blood vessel is damaged. The body creates blood clots as a normal response to blood vessel damage. The main job of a blood clot is to seal the leak in a damaged blood vessel. This prevents the blood from leaking out and protects the person from bleeding.

Clots (or thrombi) that block the arteries and prevent flow of blood and oxygen to an organ can lead to areas of tissue damage (infarcts). When blood clots break away (called an embolism) from the area they're meant to protect, they can endanger other organs.

Clots that block blood flow are the main culprits in most heart attacks and strokes. They can also damage other organs:

• When a blood clot (thrombus) forms in one or more arteries that supply blood to the heart, it blocks the blood flow to a part of the heart muscle, reducing or completely cutting off the oxygen supply to the cells in that area. As a result, the part of the heart muscle that is deprived of oxygen dies, and a heart attack occurs.
• Clots that block the flow of oxygen to the brain are the primary cause of strokes.
• Clots that form in the eye may cause sudden blindness.

Presence of an obstructing blood clot (thrombus) is referred to as thrombosis. Thrombosis in a vein is almost always associated with phlebitis (inflammation of a vein). Thrombophlebitis is an inflammation of a vein in the area where a blood clot has formed.

Thrombophlebitis is classified as either superficial or deep. In other words, thrombosis can affect either superficial (surface) or deep (below the surface) veins, causing thrombophlebitis.

Superficial thrombophlebitis occurs when a blood clot affects veins near the skin surface, or superficial veins.

Deep venous thrombosis (DVT) occurs when a blood clot affects deeper, larger veins, such as those in the lower legs and thighs. DVT is more worrisome than superficial thrombophlebitis. These clots can break away (called emboli) from a blood vessel and cause a pulmonary embolism if they travel to the lung. (For more information on pulmonary embolism, see the section "Symptoms and Complications.") DVT is more common for people over 40 years of age.

Causes

Blood clots are the result of:

• Disruptions in blood flow (due to a blockage)
• Injury to the blood vessels
• Changes in blood composition (e.g., too many clotting factors in the blood)

Ischemic strokes (strokes caused by blood clots blocking an artery that supplies the blood to the brain) can be due to blood clots that have formed in the heart as a result of rhythm disorders known as atrial fibrillation and atrial flutter.

Atrial fibrillation is a type of an irregular heartbeat (arrhythmia) that triggers a rapid, quivering beat in the upper chambers of the heart (the atria). This irregular pumping may cause some blood to remain in the heart chamber and form clots, which can then travel to the brain. Emboli can also form at the site of artificial heart valves, after a heart attack, or as a result of heart valve disorder or heart failure.

Cardiogenic embolism refers to clots that form inside the heart and travel to the brain. A stroke may occur in someone who has suffered a heart attack. A heart that has been damaged by a heart attack doesn't pump blood properly, which can cause formation of a blood clot that travels to the brain. The presence of artificial replacement heart valves can also lead to blood clot formation.  People with artificial heart valves can take blood-thinning medications to help to prevent clots from forming.

Some blood clots may form in a narrowed artery as a result of atherosclerosis, commonly known as hardening of the arteries. In this case, the arterial walls slowly thicken, harden, and narrow over time until blood flow is reduced. As a result, these arteries become vulnerable to injury. If they tear, a blood clot may form, completely blocking the already narrowed artery and shutting off oxygen to part of the heart or brain.

Blood clotting problems and rare blood disorders also cause blood clots to form, but the cause of blood clots is not always known.

For some women, the use of oral contraceptives (birth control pills) may increase the risk of blood clot formation. The risk is higher for women over 35 years of age who smoke or have a history of previous blood clots.

A variety of things can cause inflammation of a superficial vein. One common cause is due to trauma or injury, for example from solutions or medications given intravenously (into a vein) in hospitals. Piercing the vein to give the solution or medication can cause irritation. Any trauma to a vein (such as an injury from a car accident) will trigger inflammation in the area, leading to pain, discomfort, redness, and swelling. During this process, there is increased blood flow to the injured area, and a blood clot often forms in the inflamed or injured area of the vein. Superficial thrombophlebitis is an uncomfortable condition but rarely causes serious problems.

Sometimes, thrombophlebitis is caused by a bacterial infection in the vein. The usual culprit is a bacteria called Staphylococcus, commonly found on the skin.

In certain cases, thrombophlebitis develops without an obvious reason. It may develop in the leg veins of pregnant women, in people with varicose veins, and in some people with cancer in the abdomen (particularly the pancreas). Women over the age of 35 years who smoke and take oral contraceptives (birth control pills) are at a higher risk of developing blood clots.

DVT occurs when blood clots form in the deep veins of the legs or pelvis, and is often caused by:

• prolonged sitting or bed rest
• surgery or trauma (especially hip surgery, gynecological surgery, heart surgery)
• medications such as estrogen, and birth control pills with higher levels of estrogen
• injury to the leg or immobilization (such as casting after a broken bone)
• hospital admission (current or within the past 3 months)
• cancer and cancer-related treatment
• infection

Certain inherited conditions can make DVT more likely to occur. Blood flow in the veins depends on contraction of surrounding muscles, and with inactivity, such as extended bed rest, the blood starts to collect and blood clots can easily form.

Symptoms and Complications

Blood clots that result in a heart attack may cause chest pain (angina) that usually starts in the centre of the chest and moves to the jaw, the back, the left arm, or occasionally the right arm. Less commonly, the pain may be felt in the abdomen.

Heart attack pain is usually severe but not always. Some people have "silent heart attacks" without any symptoms. Some people have reported feeling a sense of impending doom as a heart attack comes on. There's tightness and often a pounding in the chest. The heart may speed up and beat irregularly. Shortness of breath or difficulty breathing, nausea or vomiting, fainting, or collapse may also occur.

Women may experience slightly different heart attack symptoms than men. Women are more likely than men to have nausea, and less likely to have sweating as a symptom of a heart attack. Women may not feel the chest pain as distinctly as men. The most common places for women's chest pain to spread to are the neck, jaw, or back.

Blood clots that result in a stroke usually cause symptoms on the opposite side of the body from where they are causing the blockage in the brain. This may result in loss of feeling on one side of the face, arm or leg, or blindness. If the left side of the brain is affected, speech problems can occur. Affected people may be unable to speak or to understand spoken words. Other symptoms of stroke include confusion, blurred vision, a severe headache, or sudden loss of coordination or balance.

Signs of stroke should not be ignored, however brief or seemingly insignificant. Even if symptoms only last a few minutes and then vanish completely, it is important to seek medical attention right away.

Inflammation in superficial or surface veins (such as those used to insert intravenous [IV] lines in the arm or to draw blood) produces pain and discomfort but it usually isn't considered serious. Blood clots that form in superficial veins rarely break loose and travel in the blood to cause blockage (thromboembolism) and complications in organs such as the lungs.

In the case of DVT, a blood clot in the leg may cause pain, swelling, redness, and increased warmth. The leg may ache when standing. This usually occurs in one leg only. While many people with DVT have no signs or symptoms, the classic symptoms are:

• firm swelling
• pain or tenderness over a vein
• sharp pain when the foot is flexed upwards
• redness
• warm sensation over the affected area
• dull, aching tightness in the calf, especially with walking
• dilation (widening) of the surface veins of the leg

DVT can lead to serious complications. A blood clot that formed in deeper and larger veins, such as those of the legs, abdomen, and pelvis, can break away and become a travelling blood clot, or embolus. The embolus can travel and lodge in the lung, a condition called pulmonary embolism.

Because a clot in a deep vein may not cause symptoms early on, the first sign may occur when the clot has broken loose and travelled to the lung. Symptoms of a pulmonary (lung) embolism are breathlessness, chest pain, and bloody sputum. If you have any of these symptoms, get emergency medical care right away.

Making the Diagnosis

Tests to check for presence of blood clots may include:

• computed tomography (CT or CAT scan; a special technique that uses a computer to combine many X-ray images into a detailed image of an area body that is 100 times more clear than a regular X-ray)
• magnetic resonance imaging (MRI)
• ultrasound studies of leg veins or the arteries of the head and neck
• angiograms or venograms
• an ultrasound of the heart (echocardiogram)
• electrocardiograms (ECGs)

Certain specialized blood tests may indicate if someone has had a recent heart attack. You may need to be screened for thrombophilia (a disorder in which blood abnormally coagulates, leading to an increased risk of blood clots) if you have a family history of thrombophilia or are under 40 years of age and experiencing recurrent blood clots.

Superficial thrombophlebitis is usually diagnosed according to your symptoms. A doctor will take your medical history by asking about your symptoms and conducting a physical examination.

In the case of thrombophlebitis, an ultrasound of the suspected veins may be done to confirm the diagnosis. Since the leg pain associated with DVT is very similar to muscle pain, your doctor might look for signs of swelling and enlargement of the calf due to swollen leg veins.

A diagnosis of DVT is usually confirmed with a compression ultrasound. Compression ultrasound detects differences in echoes or sounds made by flowing blood, and can easily detect the presence of blood clots in deep veins.

Treatment and Prevention

Medications are usually used to stop progression of DVT and prevent the blood clot from worsening, breaking away, and moving to the lungs. If you think you may have DVT, seek medical attention right away.

Blood-thinning medications such as warfarin*, heparin (either unfractionated heparin or low molecular weight heparins, like enoxaparin, dalteparin, or tinzaparin), fondaparinux, or oral anticoagulants (e.g., dabigatran, rivaroxaban, apixaban, edoxaban) are usually recommended. The choice of blood thinner is highly individualized; it is based on medical history, preference, other health issues, cost, and convenience. These medications may be continued for several months after a blood clot has been diagnosed. The length of treatment usually depends on whether it was a first episode or a recurrent event, whether the event was brought on by a specific issue or not, and whether there are other health issues (e.g., active cancer).

Most people do not require admission to a hospital to treat DVT, and those with DVT can usually return to normal activities within 2 to 3 weeks.

For some people, long-term treatment with warfarin (an anticoagulant) or oral anticoagulants (e.g., dabigatran, rivaroxaban, apixaban) may be necessary to prevent new blood clots from forming. Your doctor may also recommend that you wear an elastic compression stocking on your leg to prevent DVT. Painkillers may be used to reduce the pain.

To relieve mild inflammation and discomfort, the affected area should be elevated and warm, moist packs applied for 15 to 20 minutes at a time throughout the day. For people with superficial thrombophlebitis, activities such as walking are recommended. If the inflammation and symptoms last longer than a day or two, or if symptoms become worse, see a doctor as soon as possible.

In cases where the thrombophlebitis is due to an infection, treatment with antibiotics often takes care of the problem. In rare cases, when the antibiotics aren't enough to control the infection, surgical removal of the inflamed portion of the vein may be required.

To help prevent DVT, avoid long periods of immobility such as those during long car trips or airplane flights. Try to walk around and stretch for a few minutes every hour or so. Elevate your legs above your heart level if possible, and if you have a history of blood clots, wear support stockings or socks.

Prevention of blood clots is the best way to deal with the problems of heart disease and stroke. It is important to reduce or quit smoking and to control high blood pressure. High cholesterol levels also present a risk for blood clots and may be checked by your doctor. A healthy diet and regular exercise also help to reduce the risk of heart attack and stroke.
For more information, see our articles on heart attack and stroke.