Most people who have factor V Leiden never develop signs or symptoms. However, the first indication that you have the disorder may be the development of a blood clot (thrombosis).
Some clots do no damage and disappear on their own. Others can be life-threatening. Symptoms of a blood clot depend on where it forms and whether and where it travels.
A clot developing in a deep vein
This is known as deep vein thrombosis (DVT). A DVT may not cause any symptoms. If signs and symptoms do occur, they commonly affect your legs, including your ankles and feet, and may include:
- Significant swelling
A clot that forms closer to the surface of your skin
This is referred to as superficial venous thrombosis, phlebitis or thrombophlebitis. Signs and symptoms usually include:
- Tenderness or pain, often in or around the vein with the blood clot
A clot that travels to your lungs
Known as a pulmonary embolism, this occurs when a deep vein clot breaks free and travels through the right side of your heart to your lung, where it blocks blood flow. Symptoms may include:
- Sudden shortness of breath
- Chest pain when breathing in
- A cough that produces bloody or blood-streaked sputum
- Rapid heartbeat (tachycardia)
When to see a doctor
Seek medical attention immediately if you:
- Have signs or symptoms of a pulmonary embolism, such as chest pain or discomfort.
- Have signs or symptoms of DVT, such as leg pain and swelling.
See a doctor if you:
- Have a family history of blood clots or if family members have factor V Leiden. Ask your doctor about the risks and benefits of genetic testing for the disorder.
- Have had one or more blood-clotting incidents without an apparent cause, especially if you're under 50.
A blood clot (thrombus) normally forms to stop the bleeding when an artery or vein is damaged, such as when you experience a cut. Clots form as a result of chemical reactions between specialized blood cells (platelets) and proteins in your blood (clotting factors). Substances in the blood known as anti-clotting factors control excessive formation of blood clots.
Normally, factor V is a clotting protein. But, people with factor V Leiden have a genetic mutation that causes the factor V protein to respond more slowly to being deactivated by the anti-clotting factors.
In the normal clotting process, anti-clotting proteins combine to help break up factor V to keep it from being reused and forming clots when clotting isn't needed. However, the factor V Leiden mutation keeps the anti-clotting proteins from breaking down factor V, which keeps it in the blood longer and increases the chance of clotting.
If you have factor V Leiden, you either inherited one copy of the defective gene (heterozygous), which slightly increases your risk of developing blood clots, or more rarely you inherited two copies, one from each parent (homozygous), which significantly increases your risk of developing blood clots.
A family history of factor V Leiden increases your risk of inheriting the disorder. The disorder is most common in people who are white and of European descent.
Risk factors that can increase your risks of developing complications, such as a DVT, include:
- Being pregnant
- Taking birth control pills or hormone replacement therapy
- Taking medications to treat breast cancer or prevent a recurrence of breast cancer, called selective estrogen receptor modulators (SERMS), such as tamoxifen and raloxifene (Evista)
- Being over age 60
- Being overweight or obese
- Getting a leg injury
- Having surgery or other invasive medical procedures
- Traveling for more than a few hours, especially by airplane
Factor V Leiden can be associated with a variety of serious and potentially serious clotting complications, including:
- Pregnancy complications. Although most women with factor V Leiden have normal pregnancies, the mutation has been linked with an increased risk of miscarriage and possibly other complications during pregnancy, including pregnancy-induced high blood pressure (preeclampsia), slow fetal growth and early separation of the placenta from the uterine wall (placental abruption). If you're a woman with factor V Leiden and you get pregnant, be sure your doctor monitors you carefully throughout your pregnancy.
- Deep vein thrombosis (DVT). People with factor V Leiden have an increased risk of developing DVT compared with someone without the mutation, though the overall risk of developing DVT is still low.
- Pulmonary embolism. DVT puts you at risk of a clot breaking off and traveling to your lungs or, rarely, your brain. A pulmonary embolism can be fatal, so it's important to watch for signs and symptoms of a pulmonary embolism, such as shortness of breath or chest pain, and to seek prompt medical attention.