Understanding thrombosis: signs, causes, and treatment
Thrombosis is a common yet potentially serious medical condition involving the formation of a blood clot within a blood vessel. When this occurs, it can partially or completely block blood flow, leading to a range of complications depending on the location and severity of the clot. In this article, a consultant haematologist explains why early recognition and treatment of thrombosis are essential to prevent long-term damage or life-threatening events.
What is thrombosis?
Thrombosis occurs when blood clots form abnormally within blood vessels. This can happen in veins (venous thrombosis) or arteries (arterial thrombosis). While blood clotting is a normal and necessary process to stop bleeding when we are injured, problems arise when clots form where and when they are not needed.
Blood clots have different names and manifestations depending on their location:
- Venous thrombosis, where a blood clot forms inside a vein. This can occur as a deep vein thrombosis (DVT), which usually occurs in the deep veins of the legs, causing redness, swelling and pain in the affected leg. It can also present as a pulmonary embolism, where a blood clot forms in the blood vessels in the lungs. Rarely, veins in other body parts can be affected.
- Thrombophlebitis, which is where a blood clot in a superficial vein causes irritation in the form of swelling,sensitivity, and redness.
- Arterial thrombosis, where a blood clot forms in arteries, such as those of the brain or heart, which can manifest as stroke or heart attack. These are usually treated differently to venous thrombosis, and will not be discussed in this article.
What are the signs of thrombosis?
Symptoms of thrombosis vary depending on where the clot is located. Some clots may be entirely asymptomatic, while others can present with clear and urgent warning signs.
Common signs include:
- Swelling, pain, and tenderness in one leg (usually in the calf)
- Warmth or redness over the affected area
- Sudden shortness of breath, chest pain, coughing (sometimes with blood)
It is important to seek immediate medical attention if any of these symptoms develop.
Where do clots occur in the body?
Thrombosis can occur in almost any part of the body, but most commonly affects:
- Leg veins – DVT is the most frequent presentation
- Lungs – Pulmonary embolism, often secondary to a DVT
- Abdominal or pelvic veins – Less common, but can occur particularly after surgery or in cancer patients
Rare forms of thrombosis may affect veins in the arms, gut (mesenteric veins), or brain (cerebral venous sinus thrombosis).
Causes of thrombosis
In many cases, thrombosis is triggered by known risk factors, including:
- Prolonged immobility (e.g., long flights, hospital stays)
- Surgery or trauma
- Cancer
- Pregnancy and the postnatal period
- Hormonal treatments such as the contraceptive pill or HRT
- Obesity and smoking
However, some patients develop unprovoked clots, meaning no clear cause is identified. In such cases, further investigations may be warranted to look for underlying conditions, such as inherited clotting disorders or undiagnosed malignancy.
It’s also worth noting that thrombosis can affect people of any age, though the risk increases with age and with certain medical conditions.
How are clots diagnosed?
Diagnosis typically begins with a thorough clinical assessment, taking into account symptoms and risk factors. Investigations may include:
- Ultrasound scan, specifically a Doppler ultrasound, to view the flow of blood throughout the body
- CT pulmonary angiogram (CTPA), which is commonly used to detect pulmonary embolism
- Blood tests, such as D-dimer, which can indicate the presence of an abnormal clot
- In some cases, MRI scans or venography may be used for less common clot locations
Once diagnosed, treatment usually begins immediately to reduce the risk of complications. Further investigations may be warranted for underlying causes and organs affected.
Treatment and follow-up
The mainstay of thrombosis treatment is anticoagulation therapy – medicines that help prevent further clot formation. These include:
- Direct oral anticoagulants (DOACs) – increasingly the first choice due to ease of use
- Warfarin – still used in certain cases where close monitoring is needed
- Low molecular weight heparin (LMWH) – often used in cancer patients or during pregnancy
The duration of treatment depends on the cause and location of the clot. A typical course may last three to six months, but some patients — particularly those with ongoing risk factors or unprovoked clots — may require lifelong anticoagulation.
Follow-up at the end of the initial treatment period is important to ensure treatment is working effectively, to monitor for side effects, and to assess the ongoing risk of recurrence.