Dysplasminogensand Abnormal Fibrlnolysis
In rare cases, abnormal plasminogens (dysplasminogens), which are defective in their activation to plasmin, are associated with thrombosis. Patients with such a disorder have a low plasma plasminogen level on functional assays.68 Increased levels of plasminogen activator inhibitor-1 (PAI-1) and decreased plasma fibrinolytic activity have been reported in patients with preeclampsia.69 Acquired impairment of fibrinolytic activity may be associated with postoperative thrombosis.70 However, more studies are required to establish the role of abnormal fibri-nolysis in recurrent clinical thrombosis.71 Antigenic assays for tissue plasminogen activator (t-PA) and PAI-1 are available in some commercial laboratories, but specific functional assays are available only in research laboratories.
Elevated Fibrinogen, Factor VII, and Factor VIII Levels
A high plasma fibrinogen level is an independent risk factor for coronary artery disease.72 An elevated factor VII level has also been associated with the development of heart disease.73 A factor VIII level above the 90th percentile of normal is associated with an approximately fivefold increased risk of a first episode of DVT74,75; it also increases the risk of recurrence.76 Additional studies are required to establish the clinical utility of measuring these parameters in patients with thrombosis.
Acquired Hypercoagulable States
Antiphospholipid Antibody Syndrome
The antiphospholipid antibody syndrome is caused by au-toantibodies to proteins associated with negatively charged phospholipids. The terms antiphospholipid and anticardiolipin are used synonymously. Antiphospholipid antibodies also include lupus anticoagulant, which is an inhibitor that was first identified in patients with systemic lupus erythematosus. Many patients who have this inhibitor do not have lupus, and it is sometimes called lupuslike anticoagulant.
Epidemiology
Antiphospholipid antibody syndrome occurs secondary to systemic lupus erythematosus and, less commonly, to rheumatoid arthritis, temporal arteritis, and other connective tissue disorders. It is also associated with HIV-1 and hepatitis C infections, lymphoproliferative diseases, and certain drugs (e.g., phe-nothiazine and procainamide). When no risk factor can be identified, the syndrome is regarded as primary. In a large cohort study of 1,000 patients with antiphospholipid antibody syndrome, 53% of patients were classified as having primary an-tiphospholipid antibody syndrome, and 47% had secondary an-tiphospholipid antibody syndrome.77
Pathophysiology
The two most common protein targets for the antiphospho-lipid antibodies appear to be ^-glycoprotein I (|2-GPI) and prothrombin. |2-GPI is a plasma protein that binds anionic phos-pholipids with high affinity. It has weak anticoagulant function in vitro. | 2-GPI can induce cardiolipin from its usual bilaminar form to a hexagonal form that is highly immunogenic.78 The an-ticardiolipin antibody enzyme-linked immunosorbent assay (ELISA) usually detects antibodies directed against the cardi-olipin/|2-GPI complex. Lupus anticoagulant antibodies have been purified that specifically react with prothrombin but not thrombin. These purified antibodies can enhance the binding of prothrombin to the cultured endothelial cell surface.79 Other pro-tein-phospholipid targets may also be involved. Antiphos-phatidylethanolamine antibodies are found in many patients with antiphospholipid antibody syndrome, and some of these antibodies inhibit activated protein C function.80 Antibodies to heparin and heparan sulfate, which inhibit the heparin-depen-dent neutralization of thrombin by AT, have been found.81 On the basis of this heterogeneity of antiphospholipid antibodies, it seems likely that multiple mechanisms are involved in the pathogenesis of thrombosis in this syndrome.
Clinical Presentation
Thrombotic events occur in approximately 30% of patients with antiphospholipid antibodies (overall incidence, 2.5 events per 100 patient-years).82 In the cohort study cited above, 37% of patients presented with venous thrombosis, 27% with arterial thrombosis, 15% with both venous and arterial thrombosis, and 12% with fetal loss only77 [see 15:IV Systemic Lupus Erythematosus].
Diagnosis
The diagnosis of antiphospholipid antibody syndrome should be considered in any patient who presents with an idio-pathic arterial or venous thrombosis or in a woman with a history of recurrent miscarriages. The diagnosis is confirmed by the presence of anticardiolipin antibodies on ELISA (see above) or lupus anticoagulant on clotting assays.
The general criteria for the diagnosis of lupus anticoagulant are (1) prolongation of at least one phospholipid-dependent clotting assay; (2) proof, by mixing studies, that the prolongation is caused by an inhibitor and not a clotting factor deficiency; and (3) confirmation that the inhibitor is phospholipid dependent [see Table 5]. The clotting tests commonly used are activated PTT (aPTT) and dilute Russell viper venom time (RVVT). The reagents in aPTT are variably sensitive to the lupus anticoagulant and are influenced by concentrations of some plasma clotting factors (e.g., factor VIII). Therefore, an aPTT reagent that is sensitive to the lupus anticoagulant should be used in the screening test. Dilute RVVT is much more sensitive than aPTT but is a manual test and not as well standardized. Other tests, such as kaolin clotting time and the tissue thromboplastin inhibition test, are useful when available. The presence of an inhibitor necessitates a mixing study to demonstrate lack of correction with normal plasma. Correction of the prolongation by addition of phos-pholipid in the form of platelet lysates or as hexagonal-phase phospholipid will confirm the diagnosis. Clotting factor assays can be carried out in equivocal cases. A lupus anticoagulant will cause functional deficiency of several phospholipid-dependent clotting factors, not just one particular factor.
Table 5 Proposed Clinical and Laboratory Criteria for the Antiphospholipid Antibody Syndrome93
|
Clinical features |
Pregnancy morbidity (any of the following): |
|
More than one unexplained fetal death at greater than 10 wk |
|
|
Delivery at less than 34 wk, with severe pregnancy-induced hypertension |
|
|
Three or more pregnancy losses at less than 10 wk |
|
|
Thrombosis: |
|
|
Venous (superficial thrombophlebitis; deep vein thrombosis; pulmonary embolism; cerebral and retinal vein thrombosis; renal, splanchnic, and mesenteric vein thrombosis) |
|
|
Arterial (ischemic cerebral infarction, transient cerebral ischemia, amaurosis fugax, migraine, carotid and vertebrobasilar artery thrombosis, aortic arch syndrome, peripheral arterial thrombosis and embolism, renal and mesenteric artery thrombosis, livedo reticularis) |
|
|
Laboratory features |
Lupuslike anticoagulant: |
|
Activated partial thromboplastin time, dilute Russell viper venom time, kaolin clotting time, tissue thromboplastin inhibition test |
|
|
Anticardiolipin antibodies (either of the following): IgG anticardiolipin antibodies (> 20 GPL) IgM anticardiolipin antibodies (> 20 MPL) |
|
|
Thrombocytopenia (platelet count < 100,000/ ^l) |
|
Table 6 Classification of Antiphospholipid Antibodies147 |
|
Autoimmune causes |
|
Primary (do not fulfill criteria for systemic lupus erythematosus) |
|
Secondary (fulfill criteria for systemic lupus erythematosus or other connective tissue diseases) |
|
Drug-induced (e.g., phenothiazines, quinidine, quinine, synthetic penicillins, hydralazine) |
|
Alloimmune causes Infections (viral, bacterial, fungal) |
|
Malignancies (e.g., hairy-cell leukemia, lymphoproliferative disease) |
Anticardiolipin antibodies are reported as IgG (in IgG phospholipid [GPL] units) and IgM (in IgM phospholipid [MPL] units). The prevalence of elevated anticardiolipin IgG and IgM antibodies in normal populations is approximately 5%; with repeated testing, the prevalence is less than 2%.83 High titers of anticardiolipin IgG antibodies (> 33 GPL) are associated with an approximately fivefold increase in overall thrombotic risk.82,84 The importance of low titers of IgG antibodies (< 20 GPL), isolated IgM antibodies, and IgA antibodies has not been estab-lished.85,86 Both functional and antigenic assays should be ordered in the evaluation of a patient, because these two assays do not completely overlap. In one study of antiphospholipid antibody syndrome, 88% of patients had anticardiolipin antibodies (IgG, IgM, or both) and 54% of patients had lupus anticoagulant. Lupus anticoagulant was typically found in association with an-ticardiolipin antibodies, but it occurred in isolation in about 12% of patients.77 Certain infections and drug exposures may lead to a transient appearance of antiphospholipid antibodies, which disappear after the resolution of infection or discontinuance of the drug [see Table 6]. Therefore, laboratory tests should be repeated at least once (6 weeks after the first tests) to confirm the diagnosis. Conversely, approximately 20% of patients with low titers of anticardiolipin IgG antibodies will have higher titers upon repeat testing. Retesting is also warranted in patients with new or recurrent thrombosis.85
Treatment
The current therapeutic recommendations for antiphospho-lipid antibody syndrome are mostly based on observational studies that support an association between antiphospholipid antibodies and thrombosis, particularly recurrent thrombosis.82-86 In the acute treatment of DVT in patients with antiphospholipid antibody syndrome, monitoring the effect of unfractionated hep-arin can be problematic because lupus anticoagulant prolongs the aPTT. The use of LMWH circumvents this problem because LMWH does not require dose titration and monitoring. The patient should be treated with LMWH and warfarin in the usual fashion, with an overlap of at least 5 days before discontinuing LMWH.
Retrospective analysis shows that patients with the antiphos-pholipid antibody syndrome and a history of thrombosis have a high rate of recurrent thrombosis (in the range of 50% to 70%) if they are not given prolonged warfarin therapy.87,88 The site of the first thrombotic event (i.e., arterial or venous) tends to predict the site of the recurrent event.88 High-intensity warfarin therapy, to an international normalized ratio (INR) of 3.0 to 3.5, had been advocated for these patients. This recommendation was based primarily on a retrospective analysis,87 however, and two prospective clinical trials have now demonstrated that most of these patients can be adequately treated with conventional levels of anticoagulation (i.e., an INR of 2.0 to 3.0).89 On the other hand, there are clearly some patients with antiphospholipid antibody syndrome who experience recurrent thrombosis with conventional anticoagulation and hence require a higher level of treatment.
The optimal duration of oral anticoagulation therapy for an-tiphospholipid antibody syndrome has not been fully established. Recurrent venous thrombosis or ischemic stroke usually justifies long-term warfarin. In a patient with a first episode of DVT who is found to have antiphospholipid antibodies, warfarin therapy is indicated for at least 6 months and perhaps for life.90 The severity of the specific thrombotic episode, the coexistence of any reversible thrombotic risk factors, and the risks of long-term oral anticoagulation therapy should also be considered. It should be noted that the lupus anticoagulant may occasionally increase the prothrombin time (PT) and, in turn, the INR, thus posing a problem for the monitoring of warfarin ther-apy.91 When PT and INR increase, use of a lupus anticoagulant-insensitive thromboplastin reagent is helpful.
In asymptomatic patients with anticardiolipin antibodies or lupus anticoagulant but no history of thrombosis, anticoagula-tion is not required.
Pregnancy Loss in Antiphospholipid Antibody Syndrome
Several prospective studies confirm an association between recurrent miscarriages and antiphospholipid antibodies. The antibodies presumably cause pregnancy loss by promoting placen-tal thrombosis.92 Antiphospholipid antibodies should be measured in patients with a history of unexplained second- or third-trimester loss, fetal demise, early-onset severe preeclampsia, and intrauterine growth retardation.93 In contrast, antiphospholipid antibodies are not associated with sporadic early pregnancy loss,94 which is frequently the result of genetic abnormalities in the fetus. The relationship of antiphospholipid antibodies with infertility is uncertain at present.
The management of pregnant women with antiphospholipid antibody syndrome is difficult because of the syndrome’s association with thrombosis and the increased risk of bleeding with antithrombotic therapy. In a prospective, randomized, placebo-controlled trial, a combination of prednisone and aspirin was demonstrated to be ineffective in promoting live birth; in fact, it increased the risk of prematurity.95 On the other hand, two prospective trials have demonstrated that heparin and low-dose aspirin (81 mg a day) provide a significantly better pregnancy outcome than low-dose aspirin alone, with viable infants being delivered in 70% to 80% of cases.96,97 Furthermore, low-dose hep-arin (given initially as 5,000 units subcutaneously twice daily and adjusted to maintain the aPTT within the upper limits of the normal range) seems to be as effective as higher-dose heparin combined with low-dose aspirin.98 Treatment should begin as soon as pregnancy is confirmed. LMWH is preferable to unfrac-tionated heparin for long-term use because LMWH can be given once or twice daily and may reduce the risk of osteopenia and heparin-induced thrombocytopenia (see below). Enoxa-parin (40 mg once daily) and aspirin (100 mg daily) has been given from week 12 of gestation until 6 weeks postpartum with good results.99
Heparin-Induced Thrombocytopenia and Thrombosis
Epidemiology
Heparin-induced thrombocytopenia (HIT) is a relatively common antibody-mediated drug reaction, occurring in about 1% of patients receiving porcine heparin and 5% of patients receiving bovine heparin.100 The incidence of HIT is much lower in patients treated with LMWH. In a subset of patients, HIT progresses to a potentially fatal disorder characterized by venous and arterial thrombosis. Interestingly, both the frequency of HIT antibody formation and the clinical manifestations of HIT vary considerably in different patient populations. The incidence of HIT antibody formation is much higher after cardiac surgery than after orthopedic surgery (50% versus 15%); however, the incidence of clinically significant postoperative HIT appears to be lower in cardiac surgical patients than in orthopedic patients, in whom the incidence is 5%.101,102
Pathophysiology
The pathogenesis of HIT is attributable to the presence of an IgG antibody that recognizes a complex of heparin and platelet factor 4 (PF4) [see Figure 3].103 PF4 is a cationic protein found in platelet a-granules; when released from the granules, PF4 binds to the negatively charged heparin molecule with high affinity. The IgG antibody binds to the PF4-heparin complex on platelet membranes, forming a ternary complex that in turn binds to the platelet membrane FcyRII receptor. This binding activates the platelets, leading to further release of PF4 and formation of PF4-heparin complex. The immune complex-coated platelets are cleared rapidly by the reticuloendothelial system, giving rise to thrombocytopenia. The thrombotic complications in HIT are caused by activation of platelets by the immune complex, which leads to the formation of platelet microparticles and enhanced thrombin generation.104 PF4 also binds to heparinlike sulfated glycosaminoglycans (e.g., heparan sulfate) on the endothelial cell surface. In vitro evidence indicates that the antibody in HIT is able to bind to endothelial cells. The cells may then become activated, giving rise to thrombosis.105 Given that only a subset of patients who form HIT antibodies experience clinical HIT,102 the induction of HIT antibodies and the development of thrombo-cytopenia and subsequent thrombosis should be regarded as a continuum. Concomitant thrombotic risk factors probably play a major role in determining the clinical progression and manifestations of HIT.
Clinical Presentation
HIT typically develops 5 to 10 days after the initiation of hep-arin therapy. However, in patients who received heparin within the previous 100 days and are being retreated, the onset can be rapid—within hours after starting heparin.106 Conversely, onset of HIT may not occur until as long as 19 days after heparin therapy is stopped.107 This delayed-onset HIT appears to be associated with a higher titer of IgG antibodies against the PF4-heparin complex.
HIT is generally defined as a platelet count below 150 x 109/L or a drop in the platelet count by more than 50% from the postoperative peak at 5 to 14 days after heparin is started. The mean platelet count in HIT is about 60 x 109/L. Severe thrombocytopenia, with platelet counts below 20 x 109/L, occurs in fewer than 10% of patients; in 10% to 15% of patients, despite the 50% drop from peak levels, the platelet count nadir is above 150 x 109/L.102
The risk of HIT-associated thrombosis was once thought to be quite small; however, it is now recognized that thrombosis occurs in about one third to one half of patients with HIT, with venous thrombosis occurring more frequently than arterial throm-bosis.104 Thrombosis can occur at any platelet count, even at a very low one.
DVT, with or without pulmonary embolism, is the most common event leading to the diagnosis of HIT. The disorder may be further complicated by limb gangrene, especially in the setting of warfarin treatment without concomitant alternative anticoagulant coverage (see below). Cerebral vein thrombosis and adrenal hemorrhagic necrosis are uncommon but well-documented complications of HIT, and early diagnosis and urgent therapy can be lifesaving. Arterial thrombosis may present as limb ischemia, stroke, myocardial infarction, or, less commonly, mesenteric thrombosis and renal arterial thrombosis. Some patients have laboratory findings that support a diagnosis of DIC. Heparin-induced skin lesions may occur at heparin injection sites and range from painful erythematous papules to extensive dermal necrosis.108
Unlike antibodies induced by quinine, quinidine, or sulfon-amides, which can persist for years, heparin-induced antibodies appear to be quite transient. They fall to undetectable levels at a median of 50 to 85 days, depending on the assay performed.
Diagnosis
The diagnosis of HIT is supported by the finding of heparin-induced platelet aggregation in the presence of the patient’s serum. However, the sensitivity of this test can be as low as 50%.109 Sometimes, the patient’s serum can cause spontaneous aggregation of donor platelets in the absence of heparin, most likely caused by the presence of immune complexes unrelated to HIT, which makes proper interpretation of the test result impossible. Heparin-induced platelet serotonin release using washed platelets has high sensitivity and specificity for HIT but is available only in a few specialized laboratories. ELISAs to detect antibodies that are reactive to the PF4-heparin complex are commercially available and have become the most commonly used test for HIT. These ELISAs have a higher sensitivity than the platelet aggregation assay and can be more easily performed in a general clinical diagnostic laboratory. However, false positive results (i.e., positive tests in the absence of HIT or thrombocytopenia) occur in 10% to 15% of medical patients and in more than 20% of patients receiving heparin for peripheral vascular surgery. A seroconversion rate as high as 50% has been reported in patients undergoing cardiopulmonary bypass surgery, limiting its usefulness in that situation.110 Because HIT can be complicated by serious thrombotic problems, however, diagnosis of HIT should be based primarily on appropriate clinical findings, and management should be started while laboratory confirmation is awaited.
Figure 3 In a proposed explanation for heparin-induced thrombocytopenia, IgG antibodies recognize platelet factor 4 (PF4)-heparin complexes. The resulting PF4-heparin-IgG immune complexes bind to Fc receptors on circulating platelets. Fc-mediated platelet activation releases PF4 from a-granules in platelets, establishing a cycle of platelet activation and formation of prothrombotic platelet microparticles. Removal of immune complex-coated platelets by the reticuloendothelial system results in thrombocytopenia. PF4 also binds to heparan sulfate on the surface of endothelial cells, leading to immune-mediated injury, thrombosis, and disseminated intravascular coagulation.
