A clot in a vein of the deep venous system — what in medicine we call deep vein thrombosis or DVT — is a clot that blocks a deep vein. Recognizing its signs in time is important, because early treatment reduces complications.
What deep vein thrombosis is
It is the formation of a clot (thrombus) inside a deep vein, usually in the leg, though it can also occur in the arms or in veins of the abdomen. In the leg, the clot almost always affects the veins of the calf or thigh. That clot partially or completely blocks the passage of blood, which pools behind it and causes the swelling and pain.
It differs from phlebitis or superficial vein thrombosis, which affects a vein near the surface and is usually less serious. DVT matters more because of its risk of pulmonary complication.
Symptoms: how to recognize it
Deep vein thrombosis usually causes symptoms in a single leg:
- Swelling of the leg or calf, appearing suddenly.
- Pain, especially in the calf.
- Warmth and sometimes redness or discoloration of the skin.
- A feeling of tightness or heaviness that doesn't improve when the leg is raised.
Not all of these are always present, and some thromboses cause few symptoms. That's why, if a leg becomes swollen and painful without a clear cause, it's wise to seek medical advice.
When it is an emergency
The reason a DVT should not wait is pulmonary embolism: a fragment of the clot can break off, travel through the blood, and block an artery in the lung. Go to the emergency room immediately if you notice:
- Sudden shortness of breath or difficulty breathing.
- Chest pain, especially when breathing deeply.
- Palpitations, dizziness or feeling faint, or coughing up blood.
When a leg becomes swollen and painful suddenly, DVT must be ruled out. The first step is usually a blood test for D-dimer, which, if negative, rules out that possibility with a very high probability. Doppler ultrasound is the test of choice, and if it is a thrombosis, starting anticoagulation early makes the difference.
Why a clot forms
Risk is increased by prolonged immobility (a cast, hospitalization, very long trips, being bedridden), recent surgery, certain hormonal treatments, pregnancy and the postpartum period, cancer, thrombophilia (a predisposition to clotting), and a previous history of thrombosis. Sometimes it appears without an obvious cause.
Diagnosis and treatment
Diagnosis relies on the physical exam, a blood test (the D-dimer) and, above all, on Doppler ultrasound, which confirms the clot and its location. Treatment is generally based on anticoagulants to stop the clot from growing and to prevent embolism; it is maintained for months depending on the case. You can find the details on the page about venous thrombosis.
In summary
A blood clot in the leg shows up as swelling and pain in a single leg, and its main risk is pulmonary embolism. With these symptoms, it's wise to seek assessment the same day; if shortness of breath or chest pain appears, go to the emergency room. With early diagnosis by Doppler ultrasound and anticoagulant treatment, most thromboses have a good outcome.