In healthy people, homeostatic balance exists between procoagulant (clotting) forces and anticoagulant and fibrinolytic forces. Numerous genetic, acquired, and environmental factors can tip the balance in favor of coagulation, leading to the pathologic formation of thrombi in veins (eg, deep venous thrombosis [DVT]), arteries (eg, myocardial infarction, ischemic stroke), or cardiac chambers. Thrombi can obstruct blood flow at the site of formation or detach and embolize to block a distant blood vessel (eg, pulmonary embolism, embolic stroke).
Etiology
Genetic defects that increase the propensity for venous thromboembolism include
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Factor V Leiden mutation, which causes resistance to activated protein C (APC)
Acquired defects also predispose to venous and arterial thrombosis (see table Acquired Causes of Thromboembolism).
Other disorders and environmental factors can increase the risk of thrombosis, especially if a genetic abnormality is also present.
Acquired Causes of Thromboembolism
Condition |
Comments |
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Autoimmune disorder with increased risk of venous or arterial thrombi |
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Increases risk of arterial thrombi Higher risk in patients with preexisting stenosis When atherosclerotic plaques rupture, they expose or release tissue factor, activate coagulation, initiate local platelet adhesion and aggregation, and cause thrombosis |
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Cancer (promyelocytic leukemia; lung, breast, prostate, pancreas, stomach, and colon tumors) |
May activate coagulation by secreting a factor X–activating protease, by expressing/exposing tissue factor on membrane surfaces, or both |
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Associated with platelet aggregation and increased risk of thrombosis |
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|
Possible cause Due to folate, vitamin B12, or vitamin B6 deficiency |
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Infection, if severe (eg, sepsis) |
Increases risk of venous thrombosis Increases expression/exposure of tissue factor by monocytes and macrophages Decreases formation of activated protein C |
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Oral contraceptives that contain estrogen |
Low risk with low-dose regimens More frequent in patients who have a genetic abnormality that predisposes to venous thromboembolism, and in women who smoke |
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Tissue injury |
Due to trauma or surgery |
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Venous stasis |
Due to surgery, orthopedic or paralytic immobilization, heart failure, pregnancy, or obesity |
Symptoms and Signs
Common manifestations of a thrombotic disorder include unexplained deep venous thrombosis and pulmonary embolism (PE). Superficial thrombophlebitis can also develop. Other consequences may include arterial thrombosis (eg, causing stroke or mesenteric ischemia). Symptoms depend on the location of the clot, as in the following examples:
Most of the inherited disorders do not begin to cause an increased risk of clotting until young adulthood, although clots can form at any age. Women may have a history of multiple spontaneous abortions.
Diagnosis
Diagnoses are summarized elsewhere in THE MANUAL specific to the location of the thrombus (eg, deep venous thrombosis, pulmonary embolism, ischemic stroke). Patients who have a thromboembolic event in the absence of an overt clinical explanation may have one of the genetic or acquired predisposing entities described below. It is often valuable to enlist the input of a hematologist in order to sort out the daunting array of diagnostic possibilities.
Predisposing factors
Predisposing factors should always be considered. In some cases, the condition is clinically obvious (eg, recent surgery or trauma, prolonged immobilization, cancer, generalized atherosclerosis). If no predisposing factor is readily apparent, further evaluation should be conducted in patients with
As many as half of all patients with spontaneous DVT have a genetic predisposition.
Testing for predisposing congenital factors includes specific assays that measure the quantity or activity of natural anticoagulant molecules in plasma, and screening for specific gene defects, as follows:
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Clotting assay for lupus anticoagulant
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Clotting assay for resistance to activated protein C
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Genetic test for factor V Leiden
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Genetic test for prothrombin gene mutation (G20210A)
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Functional assay of antithrombin
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Functional assay of protein C
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Functional assay of protein S
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Antigenic assays of total and free protein S.
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Measurement of plasma homocysteine levels
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Immunoassays for antiphospholipid antibodies
Treatment
Treatment is summarized elsewhere in THE MANUAL specific to the location of the thrombus.
Anticoagulation is often required, typically beginning with parenteral heparin or low molecular weight heparin, and then followed by oral warfarin or one of the direct oral anticoagulants (DOACs). Drugs include factor Xa inhibitors (rivaroxaban, apixaban, edoxaban) and a direct thrombin inhibitor (dabigatran). Unlike warfarin, the direct oral anticoagulants do not require regular laboratory monitoring. DOACs can be reversed either by time (they have brief in vivo half-lives) or in some cases by recently developed agents (idaricizumab for dabigatran; andexanet for rivaroxaban or apixaban [1, 2]).
A major current disadvantage of DOACs is their expense.
Treatment references
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1. Cuker A, Burnett A, Triller D, et al: Reversal of direct oral anticoagulants: Guidance from the Anticoagulation Forum. Am J Hematol 94:697–709, 2019.
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2. Levy JH, Douketis J, and Weitz JI: Reversal agents for non-vitamin K antagonist oral anticoagulants. Nat Rev Cardiol 15: 273–281, 2018.
Drugs Mentioned In This Article
| Drug Name | Select Trade |
|---|---|
rivaroxaban |
XARELTO |
warfarin |
COUMADIN |
edoxaban |
SAVAYSA |
apixaban |
ELIQUIS |
heparin |
PANHEPRIN |
