What Is Deep Vein Thrombosis?
Deep vein thrombosis, often shortened to DVT, occurs when a blood clot forms inside one of the veins located deep within your body. These are not the veins you can see just under your skin, but rather the larger veins buried within your muscles. The clot is called a thrombus, which is simply the medical term for a blood clot that develops inside a blood vessel.[1]
Most commonly, these clots develop in your lower leg, thigh, or pelvis, although they can also form in other parts of your body including your arms, brain, intestines, liver, or kidney.[2] When a clot forms, it may partially or completely block the flow of blood through that vein. This happens because your veins have been injured, or because the blood flowing through them has become too sluggish and slow-moving.[2]
While DVT itself may not immediately threaten your life, it becomes dangerous when pieces of the clot break free and travel through your bloodstream. When these traveling clots, called emboli, reach your lungs and get stuck in the blood vessels there, this creates a condition called pulmonary embolism, or PE. A pulmonary embolism can be life-threatening and requires immediate medical attention.[1] When DVT and pulmonary embolism occur together, doctors refer to this as venous thromboembolism, or VTE.[1]
How Common Is Deep Vein Thrombosis?
Deep vein thrombosis is more common than many people realize. Each year, approximately one to three out of every 1,000 adults in the United States develop DVT or pulmonary embolism.[2] Tragically, up to 300,000 people die each year in the United States as a result of DVT and PE combined.[2]
DVT ranks as the third most common vascular disease, trailing only behind heart attacks and strokes in frequency.[2] The condition represents the third most common cause of death from cardiovascular disease after these two conditions.[3]
While DVT can occur at any age, it is less common in children and adolescents but becomes more frequent as people get older, especially after age 60.[2] The risk increases significantly with age. Interestingly, more than half of all DVT cases happen as a result of being in the hospital, either from a medical illness or following surgery. This is because patients are often lying in bed most of the time instead of moving around as they normally would, which allows blood to pool and clot more easily.[2]
Understanding the Causes of Deep Vein Thrombosis
Blood clots don’t form randomly. DVT develops when certain conditions come together to disrupt the normal flow and balance of blood in your veins. There are three main situations that can lead to clot formation: reduced blood flow, increased pressure in the veins, and damage to the vein walls themselves.[3]
Reduced blood flow is perhaps the most common cause. When you remain still for long periods, blood can pool in your legs rather than flowing back to your heart as it should. This can happen during bed rest after surgery, during a long hospital stay, or even during extended travel by plane, car, or train. General anesthesia during operations can also slow blood flow considerably. When blood moves too slowly, the cells that form clots have more opportunity to stick together.[3]
Increased pressure within your veins can also create problems. This might occur when something mechanically compresses a vein, such as a growing tumor or during pregnancy when the expanding uterus puts pressure on the veins in your pelvis. Certain anatomical variations or blockages that increase resistance to blood flowing out of a vein can also raise pressure inside that vein, making clots more likely.[3]
Direct injury to the vein wall is another important cause. Trauma, surgery, or even the insertion of catheters into veins can damage the delicate inner lining of blood vessels. A previous DVT can also leave lasting damage. Some people who inject drugs intravenously repeatedly damage their veins, increasing their risk considerably.[3]
Sometimes, the blood itself becomes more prone to clotting. This can happen in conditions like polycythemia rubra vera (where you have too many red blood cells), when platelet counts are very high, or when you become dehydrated and your blood becomes thicker and more concentrated.[3]
Risk Factors for Developing DVT
While anyone can develop deep vein thrombosis, certain factors significantly increase your chances of developing this condition. Understanding these risk factors helps you and your healthcare provider assess your personal risk and take preventive measures when needed.
Age is an important factor. People over 60 years old face a higher risk of developing DVT.[7] Being overweight or obese increases pressure on the veins in your pelvis and legs, making blood flow more difficult and clots more likely.[7]
If you smoke, you’re at increased risk because smoking affects blood clotting and circulation.[7] Having had DVT before puts you at significant risk of developing another clot in the future. In fact, if you’ve had one DVT, the risk of having another blood clot within five years can be around 5 percent.[16]
Women who take birth control pills or hormone replacement therapy face increased risk because these medications contain estrogen, a hormone that affects blood clotting.[7] Pregnancy and the six weeks following childbirth also increase risk due to hormonal changes and pressure on pelvic veins.[7]
Several medical conditions raise your risk considerably. Cancer and heart failure both increase the likelihood of developing blood clots.[7] People with varicose veins are at higher risk, as are those who become dehydrated.[7]
Certain situations temporarily increase your risk. Being in the hospital or having recently left hospital care, especially if you cannot move around much after an operation, significantly raises your chances of developing DVT. This is why more than half of all DVT cases occur in people who are hospitalized or have recently been hospitalized.[2] Being confined to bed for any reason, taking a long journey lasting more than three hours by plane, car, or train, and undergoing major surgery all increase risk.[7]
Some people have inherited conditions called thrombophilias that make their blood more likely to clot. These genetic factors can greatly increase the risk of DVT.[10] Sometimes, however, DVT happens for no obvious reason at all.[7]
Recognizing the Symptoms of DVT
Knowing the symptoms of deep vein thrombosis can be challenging because up to 30 percent of people with DVT experience no symptoms at all. In other cases, the symptoms may be very mild and easy to dismiss.[2] However, when symptoms do appear, they’re important to recognize.
The most common symptom is swelling in the affected leg or arm. This swelling can develop suddenly and may involve the entire limb or just part of it.[2] Pain or tenderness in your leg or arm is another frequent symptom. This pain often starts in the calf and can feel like cramping or soreness. Sometimes the pain only appears when you’re standing or walking.[1]
The area where the clot has formed may feel warmer than the surrounding skin.[2] You might notice changes in skin color over the affected area. Your skin may become red or discolored, appearing purple, bluish, or darkened. On brown or black skin, these color changes may be harder to see.[7]
The veins near the surface of your skin may become larger and more visible than normal. When you touch these swollen veins, they might feel hard or sore.[7] These symptoms can also occur in your arm or abdomen if that’s where the blood clot is located.[7]
If you develop any of these symptoms, you should seek medical attention promptly. Even if symptoms seem mild, it’s important to have them evaluated because DVT can lead to serious complications.
Preventing Deep Vein Thrombosis
The good news is that there are many practical steps you can take to lower your chances of developing deep vein thrombosis. Prevention is particularly important if you have risk factors for DVT or if you’re in a situation that temporarily increases your risk.
Staying active is one of the most effective prevention strategies. Regular physical activity keeps your blood flowing well through your veins. Try to walk regularly and avoid sitting still for long periods. If you have a desk job, make it a habit to get up and move around every hour or so.[7] Even simple movements like flexing your ankles and toes can help when you must remain seated.[7]
Maintaining a healthy weight is important because excess weight puts extra pressure on your veins.[7] Staying well-hydrated helps too, because dehydration makes your blood thicker and more likely to clot. Drink plenty of fluids throughout the day, especially water.[7]
When you’re sitting, avoid crossing your legs, as this can restrict blood flow.[7] If you smoke, quitting will reduce your risk of DVT along with many other health benefits. Limiting alcohol consumption is also advisable.[7]
If you’re planning a long journey lasting three hours or more by plane, train, or car, take some precautions. Wear loose, comfortable clothing that doesn’t restrict blood flow. Drink plenty of water and avoid alcohol during the journey. Walk around whenever possible, and when you can’t move around, do simple exercises like moving your toes up and down and rotating your ankles in circles.[7]
If you’re going into hospital, your healthcare team should assess your risk of developing DVT. If they determine you’re at higher risk, you may receive preventive treatment such as blood-thinning medication or special compression stockings. These are knee-high elastic socks that help improve your blood circulation. Treatment may continue after you leave the hospital because blood clots can form weeks after your hospital stay.[7]
While in the hospital, you can help protect yourself by staying active and walking around if you’re able. If you must stay in bed, do the exercises your healthcare team gives you, such as moving your toes and ankles.[7]
How DVT Changes Normal Body Functions
To understand what happens in deep vein thrombosis, it helps to know how blood normally flows through your body. Your heart pumps blood out through arteries to deliver oxygen and nutrients to all your tissues. After the blood has delivered its cargo, it returns to your heart through veins. In your legs, this means blood must flow upward against gravity, which is no small task.
Your veins have special one-way valves that help blood move in the right direction and prevent it from flowing backward. Muscles in your legs also help by squeezing the veins when you walk or move, pushing blood upward toward your heart. This is sometimes called the “muscle pump” effect.[2]
When DVT develops, a clot forms inside a deep vein and may partially or completely block blood flow through that vessel. The clot acts like a dam, preventing blood from moving forward as it should. Blood begins to back up behind the clot, creating increased pressure in the vein. This increased pressure forces fluid out of the blood vessel and into the surrounding tissues, causing swelling.[2]
The blockage also means that less oxygen-rich blood can reach the tissues downstream from the clot. The affected area may become painful and discolored as a result. The skin may feel warm because inflammation develops around the clot as your body tries to respond to the problem.[2]
Even after the acute clot is treated, long-term problems can develop. As many as half of people who have DVT in their legs develop a condition called post-thrombotic syndrome. This happens because the clot damages the valves and inner lining of the vein. Without properly functioning valves, blood pools more than it should, increasing pressure within the veins. This leads to chronic leg swelling, pain, skin discoloration, and in severe cases, leg ulcers that are difficult to heal.[2]
The most dangerous pathophysiological change occurs when part of the clot breaks free from the vein wall. This piece of clot travels through larger and larger veins until it reaches the heart. The heart then pumps it into the lungs, where it gets stuck in one of the blood vessels there. When this happens, the blocked vessel can no longer deliver blood to part of the lung. This section of lung tissue cannot function properly without blood flow. If the clot is large enough, it can severely limit the lungs’ ability to absorb oxygen and can put dangerous strain on the heart. In the worst cases, a large pulmonary embolism can cause the heart and lungs to fail, leading to death.[2]
