Chapter 91: Antibody-Mediated Thrombotic Disorders: Thrombotic Thrombocytopenic Purpura and Heparin-Induced Thrombocytopenia

• Thrombotic microangiopathies are characterized by thrombocytopenia, microangiopathic hemolytic anemia, and microvascular thrombosis, leading to variable injury of the central nervous system, kidney, and other organs.

• The classic form of thrombotic microangiopathy, i.e., thrombotic thrombocytopenic purpura (TTP) is usually associated with an acquired (autoimmune) deficiency of ADAMTS13, a metalloprotease that cleaves ultralarge multimers of von Willebrand factor.

• Hemolytic uremic syndrome (HUS) refers to the thrombotic microangiopathy that mainly affects the kidney and may be diarrhea-associated (caused by enteric infection with Shiga-toxin producing gram-negative microorganisms) or atypical.

• Secondary thrombotic microangiopathies occur in association with infections, certain drugs, metastatic cancer, malignant hypertension, or after stem cell transplantation.

• Heparin-induced thrombocytopenia (HIT) is a significant complication of heparin treatment, associated with mild to moderate thrombocytopenia and a high frequency of both arterial and venous thrombosis. HIT is caused by the formation of anti-heparin/platelet factor-4 antibodies that activate platelets, leukocytes and endothelial cells.

Etiology and Pathogenesis

• Most cases of TTP are caused by autoantibodies that inhibit ADAMTS13. Congenital deficiency of ADAMTS13 (Upshaw-Schulman syndrome) is documented.

• The underlying mechanism causing TTP is unregulated von Willebrand factor-dependent platelet thrombosis.

• (Ultra)large von Willebrand factor multimers are released from endothelial cells of the vessel wall and mediate platelet adhesion by binding connective tissue at sites of vascular injury and to platelet glycoprotein Ib on the platelet surface. ADAMTS13 cleaves the von Willebrand factor multimers, thereby preventing platelet-vessel wall interactions in the absence of vascular injury.

• Deficiency of ADAMTS13 leads to spontaneous microvascular platelet thrombosis, causing microvascular obstruction and microangiopathic hemolytic anemia.

Epidemiology and Clinical Features

• The incidence of TTP in the United States has been estimated at approximately 4.5 per million per year.

• The peak incidence is between ages 30 and 50 years, the disease is rare before age 20 years. The female to male ratio averages approximately 2:1, but female preponderance is more pronounced at relatively younger age.

• Other risk factors for TTP include African ancestry and obesity, and genetic risk factors, such as a low frequency of HLA-DR53.

• The onset of TTP can be dramatically acute or insidious, developing over weeks.

• Approximately one-third of patients have symptoms of hemolytic anemia. Thrombocytopenia typically causes petechiae or purpura; oral, gastrointestinal or genitourinary bleeding is less common but can be severe.

• Systemic microvascular thrombosis can affect any organ, and the consequences are variable. Renal involvement is common, but acute renal failure occurs in fewer than 10 percent of cases. Neurologic findings can be transient or persistent and may include headache, visual disturbances, vertigo, personality change, confusion, lethargy, syncope, coma, seizures, aphasia, hemiparesis, and other focal sensory or motor deficits.

• Many patients have fever. The symptoms of TTP sometimes can be quite atypical, either at first presentation or upon relapse. Thrombocytopenia without hemolytic anemia may herald the onset of disease. In rare instances, visual disturbances, pancreatitis, stroke, or other thrombosis may precede overt thrombotic microangiopathy by days to months.

• Cardiac involvement may cause chest pain, myocardial infarction, congestive heart failure, or arrhythmias. Direct pulmonary involvement is uncommon but severe acute respiratory distress syndrome may occur, possibly secondary to cardiac failure.

• Gastrointestinal symptoms are common and can include abdominal pain, nausea, vomiting, and diarrhea. Physical examination may suggest acute pancreatitis or mesenteric ischemia.

Laboratory Features

• Because the symptoms and signs of TTP are nonspecific, the diagnosis depends on laboratory testing to document microangiopathic hemolytic anemia and thrombocytopenia, without another predisposing cause.

• Thrombocytopenia typically is severe, and one-half of patients have platelet counts below approximately 20 × 10⁹/L. Signs of hemolysis are present and direct antiglobulin test (Coombs test) is almost always negative.

• The characteristic morphologic feature of TTP on the blood film is a marked increase in schistocytes. Patients with TTP often have markedly increased schistocytes; spherocytes also may be seen.

• Almost all patients have normal values for prothrombin time (PT), and activated partial thromboplastin time (aPTT), reflecting a minor role of intravascular coagulation in TTP. Mildly elevated fibrin degradation products have been reported in some patients.

• Severe congenital ADAMTS13 deficiency (level <5%) is characteristic of congenital TTP. Severe acquired ADAMTS13 deficiency appears to be specific for TTP, although the sensitivity of the association is debated and the frequency of severe ADAMTS13 deficiency in TTP depends on how patients are ascertained.

Differential Diagnosis

• Many diseases associated with secondary thrombotic microangiopathy can produce overlapping clinical and laboratory findings. As a consequence, making a diagnosis of TTP can be a challenge and a wide differential diagnosis often must be considered (Table 91–1).

• Schistocytes occur in a variety of conditions besides TTP, although the level seldom enters the 1 to 18 percent range typical of TTP. Severe Coombs-negative hemolysis and marked schistocytosis sometimes occur in patients with defective mechanical heart valves.

• Conditions resulting in disseminated intravascular coagulation sometimes cause microangiopathic changes and thrombocytopenia with little change in blood coagulation tests, which can suggest a diagnosis of TTP. Infections may trigger disease in patients with severe ADAMTS13 deficiency, but more commonly, infections cause secondary thrombotic microangiopathy by other mechanisms.

• Recipients of solid-organ transplantations can develop thrombotic microangiopathy, often dominated by renal involvement associated with immunosuppression by cyclosporine or tacrolimus. These drugs appear to damage renal endothelial cells directly and can cause neurotoxicity, adding another feature suggestive of TTP.

• Similarly, hematopoietic stem cell transplantation recipients may develop thrombotic microangiopathy associated with high-dose chemotherapy or radiation, immunosuppressive drugs, graft-versus-host disease, or infections.

• Thrombotic microangiopathy occurs in a small fraction of patients with almost any cancer but most commonly with adenocarcinoma of the pancreas, lung, prostate, stomach, colon, ovary, breast, or unknown primary site. In most cases, the cancer is widely metastatic.

• The differential diagnosis of thrombotic microangiopathy in pregnancy includes preeclampsia, eclampsia, HELLP (hemolysis, elevated liver enzymes, and low platelet count) syndrome, acute fatty liver of pregnancy, abruptio placenta, amniotic fluid embolism, and retained products of conception. In addition, pregnancy can sometimes trigger disease in patients with congenital or acquired ADAMTS13 deficiency; in most case series of TTP, between 12 and 31 percent of patients are pregnant women, usually in the third trimester or immediately postpartum.

• Autoimmune thrombocytopenia may be confused with TTP if other causes of microangiopathic hemolytic anemia are present. Asymptomatic thrombocytopenia also may sometimes be the only finding in TTP, as demonstrated by a previous or subsequent episode of disease. Patients have been described in whom TTP and autoimmune thrombocytopenia appeared to occur simultaneously or sequentially. Evans syndrome (autoimmune hemolytic anemia with autoimmune thrombocytopenia) usually can be distinguished from TTP by a positive Coombs test and the prominence of spherocytes relative to schistocytes in the blood film. HIT may sometimes resemble TTP, with thrombocytopenia and disseminated arterial and venous thrombosis (see "Heparin-Induced Thrombocytopenia" below).

• Systemic lupus erythematosus (SLE) can cause autoimmune hemolysis and thrombocytopenia, and lupus vasculitis can cause microangiopathic changes, renal insufficiency, and neurologic defects consistent with TTP. Vasculitis associated with other autoimmune disorders can pose a similar diagnostic problem.

• Thrombotic microangiopathy can develop in patients with antiphospholipid syndrome (APS), with or without concurrent SLE (see Chap. 85). The clinical features resemble HUS, catastrophic APS, malignant hypertension, TTP, or HELLP syndrome. One-third of patients present during pregnancy or in the postpartum period.

• Among the drugs that have been associated with thrombotic microangiopathy, the antiplatelet drugs ticlopidine and clopidogrel are unusual because they appear to induce autoantibody inhibitors of ADAMTS13, effectively causing TTP. Thrombotic microangiopathy occurs in 200 to 625 per 1 million users of ticlopidine, usually between 2 and 12 weeks after starting therapy. The incidence of TTP with clopidogrel is lower and is estimated to be 10 per 1 million users. A comprehensive list of other drugs causing TTP can be found in Table 91–2. Drugs commonly implicated include selected antineoplastic agents, cyclosporine A, tacrolimus, and quinine.

• Malignant hypertension is associated with microangiopathic hemolytic anemia, thrombocytopenia, neurologic symptoms, and renal insufficiency, and therefore may resemble TTP.

• Diarrhea-associated HUS can occur at any age but affects mainly children younger than 10 years.

• The disease occurs sporadically and in epidemics, associated with ingestion of foods or other materials contaminated with Shiga toxin-producing bacteria. Escherichia coli O157:H7 accounts for at least 80 percent of cases in many series, but HUS can also be caused by other toxin-bearing E. coli serotypes or by Shigella dysenteriae type 1.

• Within 3 days of ingesting the bacteria, patients develop painful diarrhea, without fever, that usually evolves to bloody diarrhea within a few days. HUS may develop during the subsequent 2 weeks, with the acute onset of microangiopathic hemolytic anemia, thrombocytopenia, and renal injury. ADAMTS13 levels are normal in diarrhea-associated HUS.

• Diarrhea-negative HUS or atypical HUS is much less common than diarrhea-associated HUS. At least half of cases appear to be caused by inherited defects in complement regulatory proteins and activating components, including mutations in complement factor H, factor I, factor H-related proteins 1 and 3 (CFHR1, CFHR3), and C4 binding protein (C4BP). In addition, autoantibodies to factor H have been identified in some patients with atypical HUS, often in association with mutations in CFHR1 and CFHR3.

• The clinical presentation of diarrhea-negative HUS may be sporadic, recessive, or dominant. Many patients develop HUS in childhood, but some have their first episode in adulthood or remain asymptomatic. Occasional patients have long intervals between exacerbations, which may appear to be precipitated by infections, other illness, or pregnancy. The disease often recurs in patients with transplanted kidneys, probably because kidney transplantation does not alter the underlying complement defect.

• The mainstay of therapy for TTP is plasma exchange (see Table 91–3), which removes antibody inhibitors and replenishes ADAMTS13.

• With the exception of factor H deficiency, and possibly APS and quinine-induced disease, no compelling evidence indicates that plasma therapy is effective for thrombotic microangiopathy caused by a mechanism other than ADAMTS13 deficiency. Regardless of mechanism, however, the clinical features are variable and overlapping. Consequently, plasma exchange may sometimes be used to treat apparent HUS or secondary thrombotic microangiopathy, particularly in adults, based on the possibility that such patients may have an atypical presentation of TTP that will respond.

• After diagnosing TTP, or determining that the diagnosis is sufficiently likely to justify treatment, plasma exchange therapy should be started immediately. The optimal dose of plasma is not known, but a common practice is to perform plasma exchange once daily at a volume of 40 or 60 mL/kg, equivalent to 1 or 1.5 plasma volumes.

• Prompt treatment is essential and if plasma exchange must be delayed more than a few hours, plasma should be given by simple infusion at 20 to 40 mL/kg total dose per day, consistent with the patient's ability to tolerate the fluid load.

• Plasma exchange should be continued daily until the patient has a complete response, as shown by a platelet count greater than 150 × 10⁹/L, lactic acid dehydrogenase (LDH) within the normal range, and resolution of nonfocal neurologic symptoms.

• The optimal schedule for tapering and discontinuing therapy has not been determined. A typical (however, not evidence-based) strategy is to continue plasma exchange until a complete response is sustained for a minimum of 2 days, and then reduce the frequency of plasma exchange to every other day (or twice per week) for several days. If the disease remains quiescent, then treatment can be stopped and the patient monitored closely for recurrence.

• The long-term mortality of TTP treated with plasma exchange ranges from 10 to 20 percent. Most deaths occur within a few days after presentation, and almost all occur within the first month.

• The duration of illness is quite variable. Complete response occurs after an average of 9 to 16 days of plasma exchange, and almost all responders are encompassed by a range of 2 to 40 days.

• Recurrences of disease more than 30 days after a complete response, occur in up to one-third of patients. Most relapses occur during the first year, but have been documented up to 13 years after diagnosis.

• TTP often is an autoimmune disease and the use of glucocorticoids is logical, although a beneficial effect has not been demonstrated conclusively. Common practice is to give prednisone or equivalent at a total daily dose of 1 or 2 mg/kg, in one or two doses, for the duration of plasma exchange, followed by tapering. An alternative regimen is methylprednisolone 1 g intravenously daily for 3 days.

• The use of antiplatelet agents in TTP is controversial. Aspirin and dipyridamole often are combined with plasma exchange but have not been shown conclusively to modify the course of TTP. Low-dose aspirin (e.g., 80 mg/day) has been suggested for thromboprophylaxis, once the platelet count exceeds 50 × 10⁹/L.

• Transfusion of platelets may sometimes correlate with the acute deterioration and death in TTP, although direct harm is difficult to establish. Consequently, platelet transfusions are relatively contraindicated and should be reserved for the treatment of life-threatening hemorrhage, preferably after plasma exchange treatment has been initiated.

• TTP that is refractory to plasma exchange may respond to immunosuppression. Anecdotal experience suggests that vincristine may be beneficial, although its efficacy is difficult to assess. Dosing schedules have included 2 mg intravenously on day 1 followed by 1 mg on days 4 and 7, or 2 mg intravenously per week for 2 to 14 weeks. Cyclosporine has been used to treat TTP and may be effective in refractory disease. Apparent responses, with normalization of ADAMTS13 activity, have been observed with cyclosporine 2 to 3 mg/kg daily in two divided doses as an adjunct to plasma exchange, or without plasma exchange for early recurrences of TTP.

• Rituximab (375 mg/m² weekly for two to eight doses) resulted in an approximately 95 percent remission in patients with therapy-resistant TTP within 1 to 3 weeks of starting treatment, including a normal ADAMTS13 level and disappearance of anti–ADAMTS13 antibodies (if present).

• Other immunosuppressive regimens have included oral or intravenous cyclophosphamide, oral azathioprine, combination chemotherapy with cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP), and autologous stem cell transplantation.

• Many reports suggest that splenectomy can result in lasting remissions or reduce the frequency of relapses for some patients with TTP who are refractory to plasma exchange or immunosuppressive therapy, presumably by removing a major site of anti–ADAMTS13 antibody production.

• The cornerstone for treatment of secondary thrombotic microangiopathy is management of the underlying disorder. In most cases, this is sufficient to ameliorate the manifestations of the thrombotic microangiopathy.

Epidemiology

• The frequency of HIT depends on the nature of the heparin used, dose and duration of heparin exposure, and clinical setting.

• The frequency of HIT in nonsurgical settings is clearly higher in patients treated with unfractionated, high-molecular-weight heparin (1–5%) than in patients treated with low-molecular-weight heparin (0.2–1%). Bovine-derived heparin may be associated with a higher incidence of HIT than porcine heparin. Newer, synthetic pentasaccharide anticoagulants may have a much lower or no risk of inducing HIT.

• HIT may be prevented by limiting the exposure time to heparin, and avoiding heparin flushes through intravenous lines. Heparin-bonded catheters can underlie the development of HIT.

• The greatest clinical risk factors for developing HIT are the patient's age and the nature of the patient's medical condition. HIT occurs rarely or never in pediatric patients, especially neonates. Patients being treated for medical conditions have a lower risk of developing HIT than do patients who are undergoing surgical procedures. Among surgical patients, those undergoing coronary artery bypass grafting, orthopedic procedures, or isolated limb perfusion are particularly vulnerable to developing HIT.

• Determination of the incidence of thrombosis in various settings has been hampered by the infrequency of HIT, and the need to carefully document both the diagnosis and the thrombotic complications. Nevertheless, some prospective studies suggest that the incidence of thrombosis is between 35 and 58 percent in patients with documented HIT. The ratio of arterial to venous thrombi is high (0.7:1).

Etiology and Pathogenesis

• HIT is an immune complex disorder of heparin therapy involving heparin–platelet factor (PF)-4 complexes. Such antibodies are not demonstrable in other forms of thrombocytopenia.

• Binding of these antibodies to platelets lead to platelet activation through the FcγRIIA and the formation of procoagulant platelet microparticles that contribute to the thrombosis.

• As part of this activation, HIT antibodies also bind to endothelial cells likely via PF4-surface glycosaminoglycan complexes. This binding may further increase vascular activation, augmenting local thrombosis.

Clinical Features

• The diagnosis of HIT is difficult to establish in a complicated patient who can have multiple causes for developing thrombocytopenia or thrombosis. A scoring system based on the severity of thrombocytopenia, timing of onset of symptoms, occurrence of thrombosis, and potential thrombocytopenia from other causes ("4 Ts") was developed to help maintain focus on potentially affected patients (Table 91–4).

• Typically, patients develop thrombocytopenia 5 to 10 days after the onset of heparin therapy unless exposure occurred within the preceding 3 months.

• Bleeding manifestations secondary to the thrombocytopenia, such as petechiae, nosebleeds, and oozing from catheter sites, are not seen in HIT.

• Symptoms of venous thrombosis include those related to deep venous thrombosis of the lower or upper extremity and pulmonary embolism, adrenal infarctions, and cerebral venous thrombosis. Major venous obstruction can lead to limb gangrene.

• Arterial thrombi in this disease can be striking and were the first feature that led to the recognition of HIT as a distinct clinical entity. Common thrombotic complications include stroke, myocardial infarction, bowel infarction from mesenteric artery thrombosis, and renal infarction.

Laboratory Features

• Thrombocytopenia is the key laboratory finding in HIT. Most often it is moderate, ranging from 20 to 100 × 10⁹/L, or is represented by a 50 to 70 percent decline in platelet count. Rarely is the thrombocytopenia more severe or absent, as observed in some cases with skin necrosis.

• Although thrombin generation increases in HIT, patients rarely have decompensated disseminated intravascular coagulation.

• Two assay prototypes for confirming the diagnosis of HIT are available. One measures the Ig antibodies to the heparin–PF4 complex (antigen assay), and the other measures heparin-dependent antibodies that activate platelets (activation assay) in plasma or sera.

• Antigen assays are easily established in the laboratory and have a rapid turnaround time. The specificity of this test for the diagnosis of HIT is, however, limited. A recently described cause for a false-positive antigen test is the presence of anti-PF4 rather than anti-PF4/heparin antibodies.

• The activation assays are not commercially established because specific platelet donors are needed each time. Donor platelets can vary greatly in their sensitivity to activation by HIT sera. One of the earliest and best-established activation assays, serotonin-release assay, involves ¹⁴C-serotonin release from platelets induced by HIT antibodies and heparin. An activation assay will have greater specificity than the antigen assay, although specificity may vary based on the experience of the reference center. Its major usefulness is in post factum documentation of the cause of thrombocytopenia.

• Other activation tests include platelet activation in a heparin-induced platelet activation assay (HIPA), luminography, and microparticle generation.

• A practical approach is that in patients with a high clinical risk of having HIT, heparin should be stopped and alternative treatment is started even before laboratory results are available. A positive antigen test and particularly a progressive increase in the number of platelets over the following days are confirmatory for HIT. A negative antigen test does not rule out HIT and should be repeated after 24 hours while the patient is undergoing alternative anticoagulant therapy. If the repeat assay is negative and platelet count does not increase, alternative diagnoses should be considered.

• Many patients with HIT often have complicated medical and surgical conditions, many of which can also cause thrombocytopenia. A differential diagnosis is listed in Table 91–5.

Treatment of HIT

• An established diagnosis or high clinical suspicion of HIT should lead to immediate termination of heparin treatment. Since patients remain at high risk for thrombotic complications, alternative anticoagulant treatment should be instituted.

• Coumadin should not be used as the sole initial treatment of HIT because of the increased risk of untoward thrombosis, in particular skin gangrene. Low-molecular-weight heparin should not replace high-molecular-weight heparin because of cross-reactivity. Synthetic pentasaccharides may be considered as alternative anticoagulant treatment, although there is a single publication describing cross-reactivity with HIT antibodies.

• Other accepted drugs that are used in the treatment of patients with HIT are danaparoid sodium (no longer available in the United States), recombinant hirudin (lepirudin), and argatroban.

• Danaparoid is a mixture of low-molecular-weight heparanoids consisting of heparan sulfate, dermatan sulfate, and chondroitin sulfate. It has much greater anti-factor Xa activity than anti-factor IIa activity. Danaparoid is an effective anticoagulant that inhibits HIT antibody-induced platelet aggregation in vitro. Studies suggest danaparoid use improves platelet count recovery and decreases the incidence of serious thrombosis and death in patients with HIT.

• Lepirudin and argatroban directly inhibit thrombin. Both drugs are given intravenously and have rapid onset of action. Lepirudin binds to two sites on thrombin, the catalytic site and a fibrinogen-binding site, whereas argatroban binds only to the active site. Lepirudin prolongs the aPTT, so this test can be used to monitor effective dosing.

• Lepirudin is excreted in the urine, and its half life is dramatically increased in patients with renal failure. Lepirudin induces antilepirudin antibodies in approximately half of patients who receive the drug. These antibodies rarely alter biologic activity, but do tend to prolong the drug's half-life, necessitating careful monitoring by aPTT.

• Argatroban is synthesized from arginine and is rapidly metabolized in the liver. It affects both the aPTT and PT.

• Use of lepirudin and argatroban in HIT is efficacious; the incidence of thrombotic complication is reduced, perhaps by half, and the time to platelet count recovery is shortened. However, bleeding complications can occur.

• As with danaparoid, lepirudin or argatroban should be given until patients recover from thrombocytopenia before adding and then switching to a prolonged course of an oral anticoagulant.