As in many bleeding disorders, several conditions can alter the balance within the normal hemostasis process and cause excessive thrombosis. Abnormalities that predispose a person to thrombotic events include decreased clotting inhibitors within the circulation (which enhances coagulation), altered hepatic function (which may decrease production of clotting factors or clearance of activated coagulation factors), lack of fibrinolytic enzymes, and tortuous vessels (which promote platelet aggrega-tion). Thrombosis can be caused by more than one predisposing factor. Several conditions can result from thrombosis, such as myocardial infarction , cerebral vascular accident, and peripheral arterial occlusion. Several inherited or acquired deficiency conditions, including hyperhomocystinemia, anti-thrombin III (AT III) deficiency, Protein C deficiency, acti-vated Protein C (APC) resistance, factor V Leiden, and Protein S deficiency can predispose a patient to repeated episodes of throm-bosis; they are referred to as hypercoagulable states or throm-bophilia. Table 33-8 delineates these disorders, their abnormal laboratory values, and the need for family testing.
Thrombosis requires anticoagulation therapy. The duration of therapy varies with the location and extent of the thrombosis, pre-cipitating events (eg, trauma, immobilization), and concurrent risk factors (eg, use of oral contraceptives, tortuous blood vessels, history of thrombotic events).