Antiphospholipid syndrome (APS) is an autoimmune disorder, meaning that the body's immune system makes proteins known as antibodies that mistakenly attack its own cells or tissues. The syndrome is associated with risk of inappropriate blood clot formation, so it is considered an excessive clotting disorder (thrombophilia).
Antibodies normally defend the body against infections. But in APS, antibodies attack the lipid-proteins found in the outermost layer of cells (cell membranes) and platelets. These autoantibodies interfere with the blood clotting process in a way that is not fully understood. APS is associated with blood clots (thrombotic episodes), a low number of platelets (thrombocytopenia), and with pregnancy complications such as pre-eclampsia and recurrent miscarriages.
The primary antiphospholipid antibodies associated with APS include lupus anticoagulant, cardiolipin antibody, and beta-2 glycoprotein I (β2GP1) antibody. These antibodies increase an affected person's risk of developing recurrent inappropriate blood clots in both veins and arteries. Individuals with APS may experience one or more thrombotic episodes. Symptoms and complications may range from mild to critical. If blood clots form, they can obstruct blood flow and can damage tissues and organs. If they are carried to the lungs, heart, brain or kidneys, they can cause a pulmonary embolism, heart attack, stroke, and/or kidney damage. A small subset of people with APS may have widespread thrombotic disease with damage to many of the large internal organs of the body, referred to as "catastrophic" APS.
Individuals with antiphospholipid antibodies may have APS and another co-existing autoimmune disorder such as lupus (systemic lupus erythematosus or SLE) or may have one or more of the antibodies present with no associated symptoms. Low levels of antiphospholipid antibodies may be associated with HIV, Lyme disease, and some cancers. APS can also be seen in the elderly and temporarily during infections and with some medications, such as the psychiatric drugs phenothiazines or the antibiotic amoxicillin and the heart rhythm regulator procainamide. Having a family member with antiphospholipid syndrome may also increase a person's risk.
Antiphospholipid syndrome may affect anyone, but women of child-bearing age or those with another autoimmune disorder are more commonly affected. Antiphospholipid antibodies are found in 1% to 5% of young, healthy people and up to 10% of the general public. According to the March of Dimes, APS is the most common acquired excessive clotting disorder, affecting up to 5% of pregnant women.