Acquired causes of Thrombosis

Antiphospholipid Antibodies Syndrome

Antiphospholipid syndrome (APS) is a rare autoimmune disorder characterized by recurring blood clots (thromboses). Blood clots can form in any blood vessel of the body. The specific symptoms and severity of APS vary greatly from person to person depending upon the exact location of a blood clot and the organ system affected. APS may occur as an isolated disorder (primary antiphospholipid syndrome) or may occur along with another autoimmune disorder such as systemic lupus erythematosus (secondary antiphospholipid syndrome).

APS is characterized by the presence of antiphospholipid antibodies in the body. Antibodies are specialized proteins produced by the body’s immune system to fight infection. In individuals with APS, certain antibodies mistakenly attack healthy tissue. In APS, antibodies mistakenly attack certain proteins that bind to phospholipids, which are fat molecules that are involved in the proper function of cell membranes. Phospholipids are found throughout the body. The reason these antibodies attack these proteins and the process by which they cause blood clots to form is not known.

Other Acquired Causes of Thrombosis

Pregnancy

Pregnancy does not cause blood clots, but pregnancy does increase a woman’s chance of developing a blood clot by about fourfold. A woman’s risk is even higher immediately after delivery: in the first six weeks after delivery, a new mother’s chance of developing a blood clot is five times higher than during her pregnancy. This tendency to form blood clots post delivery likely evolved to protect women from hemorrhage at the time of miscarriage or childbirth. Nonetheless, the chance that a young, healthy woman will develop a blood clot during pregnancy is still low, about 1 to 2 in 1,000.

Eighty percent of blood clots that arise during pregnancy occur in the veins. Four of every five venous clots involves DVT, and one of every five involves PE. The other 20 percent of blood clots occur in arteries. Five of every six arterial blood clots leads to stroke, and one of every six causes a heart attack.

Hormone Therapy

Birth control pills are the leading method of birth control (contraception) in the United States. Although they do not cause blood clots, most birth control pills do increase a woman’s chance of developing a blood clot by about three to four times.

Most oral contraceptives contain an estrogen and a progestin (synthetic progesterone). Estrogen and progesterone have many effects on a woman’s body. They are the hormones that sustain pregnancy and, when given in the form of birth control pills, imitate pregnancy, thereby preventing pregnancy. These hormones also increase the levels of clotting factors and are assumed to be responsible for women’s increased risk of blood clots during pregnancy.

For the average woman taking birth control pills, the absolute risk of a blood clot is very small: Only 1 in 1,000 women per year who are taking birth control pills will develop such a clot. For a woman with thrombophilia or a history of thrombosis, however, this risk is significantly higher. The new patches (transdermal contraceptives) may increase this risk even more. The amount of estrogen absorbed from the patches has been reported to be 60 percent higher than the amount delivered by the pills. Little information about the risk of blood clots with birth control rings is available. Like patches and most birth control pills, these devices also contain an estrogen and a progestin; thus they probably carry a risk of thrombosis similar to that of birth control pills or patches.