Acquired Platelet Defects

Platelet Antibody Testing.

A large number of assays have been developed to detect antibodies directed against or associated with platelet membrane antigens. These assays can be categorized into direct (detecting antibody associated with the patient's platelets) or indirect (detecting antibody in the patient's serum that binds to control platelets). Indirect tests may detect, in addition to autoantibodies, alloantibodies (e.g., HLA related), particularly in multiparous women and individuals who have received multiple transfusions, and thus give false-positive results. Autoantibodies tend to be associated with the patient's platelets and are usually present in low concentration in serum. Therefore a direct test is the preferred test for autoimmune thrombocytopenias such as ITP.

A number of direct assays measure immunoglobulin on platelets, regardless of whether the immunoglobulin is specifically or nonspecifically bound to the platelet surface. These assays generally detect platelet-associated IgG (PAIgG), but they can also be engineered to detect IgM or IgA. PAIgG is increased in immune disorders such as ITP and in nonimmune thrombocytopenic disorders such as leukemia and myelodysplastic syndrome. Thus the specificity of these tests is limited. Moreover, the sensitivity of these tests is limited because autoantibodies, such as those responsible for ITP, represent only a small fraction of the total PAIgG. Megakaryocytes nonspecifically take up plasma proteins, including IgG and albumin, and incorporate these proteins into the alpha granules of platelets, especially in disease states associated with increased thrombopoiesis. Therefore increased PAIgG can result from elevated antibody production for any reason.

More recent assays use antigen capture techniques to detect platelet glycoprotein–specific antibodies, such as those that recognize GPIIb/IIIa and GPIa/IX/V, and have high specificity. Unfortunately, the sensitivity is generally too low for these assays to be used for the routine serologic diagnosis of most immune thrombocytopenias. A newer method is based on flow cytometric detection of autoantibodies reacting with specific platelet proteins immobilized on microbeads.

Macrophage and Platelet Fcγ Receptors.

Receptors for the Fc domain of IgG (Fcγ receptors) are found on a variety of cell types, including macrophages and platelets. These receptors have been shown to play a critical role in immune complex–mediated platelet destruction. Fcγ receptors are diverse in structure and function and fall into two major classes: those that activate effector functions, such as phagocytosis or platelet activation, and those that inhibit effector functions.

Activating Fcγ receptors on macrophages include the low-affinity receptors FcγRIIA and FcγRIIIA and the high-affinity receptor FcγRI ( Fig. 34-2 ). As discussed later, cross-linking of the activating receptor FcγRIII promotes phagocytosis of antibody-coated platelets in certain disease states, including ITP. The only Fcγ receptor expressed on platelets is the activating receptor FcγRIIA. Binding of IgG complexes to this receptor results in receptor cross-linking, which in turn initiates platelet aggregation and activation. This process is involved in heparin-induced thrombocytopenia (HIT), the most common drug-induced thrombocytopenia.

Pathogenesis.

ITP is caused by autoantibodies that interact with membrane glycoproteins on the surface of platelets and megakaryocytes. These antibodies result in accelerated platelet destruction and may also impair thrombopoiesis. More than a third of adults with ITP have inadequate platelet production despite increased numbers of megakaryocytes in their bone marrow. Certain antiplatelet antibodies have been shown to inhibit megakaryocytopoiesis or egress of platelets from the marrow space. The GPIIb/IIIa complex is the autoantigen implicated most often as the cause of childhood and adult ITP. Autoantibodies directed against the GPIb/IX/V and GPIa/IIa complexes have also been reported. In rare instances platelet glycolipids have been implicated as autoantigen targets in chronic ITP. Autoantibodies diminish or disappear when platelet levels are restored to normal.

Factors that trigger platelet autoantibody formation in acute ITP are not well understood. Although various mechanisms by which viruses may induce autoimmune disease have been suggested, the link between the immune response initiated by infection (or vaccination) and the subsequent production of platelet autoantibodies has not been established. Proposed mechanisms include adsorption of virus to platelets, deposition of virus-containing immune complexes onto platelet membranes, or exposure of cryptic neoantigens on the platelet surface. There are data to both support and refute the hypothesis that acute ITP is triggered by antiviral antibodies that cross-react with platelet antigens.

Some evidence suggests that T lymphocytes also play a role in the pathogenesis of ITP. Dysregulated helper T cells can promote the expansion of antiplatelet antibody–producing B-cell clones. Cytotoxic T cells from some ITP patients have the capacity to destroy platelets ex vivo. CD3+ T lymphocytes from ITP patients are resistant to glucocorticoid-induced apoptosis, thus suggesting that disturbed apoptosis may contribute to defective clearance of autoreactive T lymphocytes.

Genetic factors have been proposed to influence the development of ITP. Studies have failed to detect linkage of the disease to particular HLA genotypes. Polymorphisms in genes encoding phagocyte Fcγ receptors or proinflammatory cytokines may influence the development of ITP. Immunodeficiency states that have been associated with chronic ITP are discussed later.

Clinical and Laboratory Features.

The typical manifestation of acute ITP is the abrupt onset of bruising and bleeding in an otherwise healthy child. Frequently, there is a history of a viral illness in the weeks preceding the onset of bruising. Seasonal fluctuation in the diagonsis of ITP has been noted, with a peak during spring and a nadir in the autumn. Petechiae and ecchymoses are evident in most patients. Epistaxis and oral mucosal bleeding are seen in fewer than a third of patients. Hematuria, hematochezia, or melena is evident in fewer than 10%. Menorrhagia may be observed in adolescent women with ITP. Although children with ITP may have extremely low platelet counts, bleeding episodes are less severe in these patients than in those with hypoproductive thrombocytopenia. This finding has been attributed to enhanced platelet production and young, large, hemostatically effective circulating platelets. A palpable spleen is present in about 10% of reported cases of childhood ITP; this finding alone should not justify performing imaging studies on the affected child's abdomen. Malaise, bone pain, and adenopathy are uncommon and should raise concern for another cause, such as acute leukemia.

The peak age at diagnosis is 2 to 6 years. Although acute ITP may be diagnosed in children of any age, adolescents and infants are more likely to have chronic ITP develop in combination with some other immune disorder. In children ITP is seen equally in males and females, whereas in adults ITP is seen predominantly in females by a 2-to-1 ratio ( Table 34-1 ).

Roughly 80% of children have platelet counts below 20,000/µL, often less than 10,000/µL. Leukocyte and red cell counts are usually normal, although anemia may be seen in as many as 15% of these children, especially those with a significant history of epistaxis, hematuria, or gastrointestinal bleeding. A review of the peripheral blood film is mandatory in every child suspected of having ITP; features inconsistent with a diagnosis of ITP should prompt further investigation. Bone marrow aspiration or biopsy reveals normal or increased numbers of megakaryocytes. Increased numbers of eosinophils and their precursors may be noted, although this is not predictive of outcome.

The need for bone marrow examination in children with typical features of acute ITP is a subject of debate. There is a consensus that bone marrow aspiration is not necessary if the initial management is observation alone or administration of intravenous immunoglobulin (IVIG) or anti-D. Controversy exists regarding whether bone marrow aspiration should be performed in all children before glucocorticoid therapy is started to exclude the possibility of acute leukemia (see Box 34-2 ).

Additional tests that may be considered in the initial evaluation and management of suspected ITP include blood group and Coombs test (required if anti-D therapy is contemplated), quantitative immunoglobulin levels (before IVIG therapy), and antinuclear antibody test. Retrospective studies have shown that the antinuclear antibody test is positive in approximately 30% of pediatric patients with otherwise uncomplicated ITP and may predict a subset of patients at risk for the development of further autoimmune symptoms. Tests not considered useful unless specific reasons are identified in the patient's history and physical examination include screening coagulation tests, liver and renal function tests, serum complement levels, thyroid function tests, and testing for human immunodeficiency virus (HIV) or Helicobacter pylori infection.

Options for the Initial Management of Immune Thrombocytopenic Purpura.

Childhood acute ITP is usually a benign, self-limited disorder that requires minimal or no therapy in the majority of cases. There is no convincing evidence that medical therapy alters the natural history of the disease. Indications for treatment vary among practitioners and are a source of debate. One set of practice guidelines put forth by the American Society of Hematology (ASH) recommends that children with ITP and platelet counts less than 20,000/µL plus significant mucosal membrane bleeding or those with platelet counts less than 10,000/µL and minor purpura be treated with IVIG or a glucocorticoid. However, many pediatric hematologists take exception to this recommendation. In treatment guidelines put forth by other expert panels, a child's condition, rather than the platelet count, steers management; children with bruising but without mucosal or more severe hemorrhage may be treated by observation alone, irrespective of the platelet count.

At the heart of the treatment debate is the perceived risk for ICH, a rare but potentially life-threatening complication in children with ITP. In a review of 12 case series involving 1293 children, the incidence of ICH was 0.9%. Other surveys suggest that this may be an overestimate and that the true incidence of ICH in children with ITP is between 0.1% and 0.5%. The subgroup of children with ITP who appear to be at greatest risk for ICH are those with platelet counts less than 20,000/µL and additional risk factors such as a history of head trauma, aspirin use, or arteriovenous malformation. However, the platelet count alone has never been shown to predict the severity of bleeding symptoms.

There is no evidence that medical therapy, such as administration of a glucocorticoid or IVIG, reduces the incidence of ICH. Indeed, retrospective studies have shown that ICH may occur despite previous or concomitant therapy with IVIG or a glucocorticoid. The low incidence of ICH in children with ITP precludes a randomized clinical trial to determine whether treatment reduces the risk.

The results of randomized trials for various treatment approaches have been summarized previously. Regardless of whether pharmacologic therapy is used, detailed education and careful follow-up should be provided to the patient and family. The child's activities should be limited, and aspirin-containing medications should be avoided. Although hospitalization is appropriate for the treatment of a child with a severe bleeding episode, there is no evidence to support routine hospitalization of patients with otherwise uncomplicated ITP.

First-Line Medical Therapies.

Glucocorticoids: Glucocorticoids are presumed to act through several mechanisms, including inhibition of both phagocytosis and antibody synthesis, improved platelet production, and increased microvascular endothelial stability. The latter effect may explain why symptomatic bleeding episodes often subside before the platelet count increases.

In randomized trials prednisone therapy has been shown to induce normalization of the platelet count more promptly than placebo does. Although various doses have been used, the traditional glucocorticoid regimen is prednisone at 2 mg/kg/day (maximum of 60 to 80 mg) for approximately 21 days. A regimen of 4 mg/kg/day for 7 days and then tapered to day 21 is equally effective and appears to cause fewer side effects. Prednisone at a dose of 4 mg/kg/day orally for 4 days with no tapering is also effective. An alternative to these regimens is megadose pulse therapy (methylprednisolone, 30 mg/kg/day intravenously or orally for 3 days). Side effects of glucocorticoid therapy include cushingoid facies, weight gain, fluid retention, acne, hyperglycemia, hypertension, moodiness, pseudotumor cerebri, cataracts, growth retardation, avascular necrosis, and osteoporosis.

IVIG: Imbach and colleagues first reported the successful use of IVIG for the treatment of acute ITP in a small series of children. This was followed by many reports that documented the ability of IVIG to cause a rapid increase in the platelet count in patients with acute and chronic ITP.

IVIG slows the clearance of antibody-coated blood cells from the circulation by inhibiting the phagocytic activity of cells of the reticuloendothelial system. Studies in the early 1980s suggested that this effect is mediated primarily through the Fc portion of IgG. Specially treated preparations of intravenous IgG lacking the Fc portion of the molecule were inferior to unmodified IgG at increasing the platelet count in ITP patients. The proposed mechanism for this effect was Fc receptor blockade. Subsequent studies of FcγRIIB-deficient mice suggested that IVIG elicits its effect by activating these inhibitory receptors and not simply by Fc receptor blockade ( Fig. 34-3 ). However, recent publications cast doubt on the conclusion that IVIG acts through activation of FcγRIIB. Although some data support the notion that anti-idiotypic antibodies may also be present in commercial preparations and contribute to the action of IVIG, the clinical importance of this mechanism remains unproved.

The traditional dose of IVIG is 2 g/kg divided over a period of 2 to 5 days. However, several studies suggest that lower doses are effective. In a randomized trial, pediatric patients who received 0.8 g/kg IVIG had at least as rapid a response rate as those who received 2 g/kg over a 2-day period. In more recent randomized trials, favorable results have been seen with even lower doses of IVIG (250 mg/kg/day for 2 days). Among pediatric patients in whom a response is seen, IVIG produces a more rapid increase in platelet count than do traditional doses of a glucocorticoid (2 mg/kg/day of prednisone), as documented in controlled trials.

Several other general conclusions have emerged from studies on IVIG in pediatric patients. First, both splenectomized and nonsplenectomized patients may respond to IVIG. Second, responses to IVIG are generally reproducible. Third, the duration of the response is brief, approximately 2 to 4 weeks.

Enthusiasm for the use of IVIG is offset by cost considerations. IVIG is considerably more expensive than glucocorticoids or anti-D. One study concluded that IVIG may be cost-effective if it reduces hospital stay or if it prevents the need for a splenectomy.

Complications associated with IVIG are common and occur in 15% to 75% of patients. Frequent side effects include flulike symptoms such as headache, nausea, lightheadedness, and fever. Some of these adverse effects can be alleviated by pretreatment with analgesics or antihistamines. Approximately 10% of patients treated with IVIG (2 g/kg) experience aseptic meningitis manifested as a severe, protracted headache and photophobia. These symptoms, which can cause considerable anxiety in patients, parents, and physicians, may spur additional diagnostic studies (e.g., computed tomography) or result in prolonged hospitalization. A rare but serious side effect of IVIG infusion is anaphylaxis, which can occur in patients who are totally deficient in IgA. Most preparations of IVIG contain small amounts of IgA, and IgE antibodies responsible for anaphylaxis form after an initial exposure to IgA in these preparations.

Anti-D: Anti-D is a plasma-derived immunoglobulin prepared from donors selected for a high titer of anti-Rh ₀ (D) antibody. Anti-D elicits a rise in platelet count, along with mild to moderate anemia, in most patients with ITP. A role for anti-D in the treatment of chronic ITP is now well established. Its utility in acute ITP has been studied less extensively, but it is also effective in this clinical setting.

Anti-D can be used to treat only Rh ₀ (D)-positive patients with ITP because Rh ₀ (D)-negative patients do not show a response. The presumed mechanism of action is phagocytic cell blockade. Patients with intact spleens are more likely to respond to anti-D than splenectomized patients are.

The generally recommended dose is 50 to 75 µg/kg as a short intravenous infusion or subcutaneous injection. Uncontrolled studies have demonstrated that anti-D treatment increases the platelet count in approximately 80% of Rh ₀ (D)-positive children. The therapeutic effect of anti-D lasts for 1 to 5 weeks. A controlled trial showed that anti-D therapy was less effective than IVIG or a glucocorticoid in terms of days required to attain a platelet count of 20,000/µL. Anti-D is less expensive than IVIG, with an estimated cost savings of 35% per episode of ITP. The shorter administration time required for anti-D therapy also makes the medication more convenient to use than IVIG.

Adverse reactions of headache, nausea, chills, dizziness, and fever have been reported in 3% of infusions, and these adverse reactions were classified as severe in only a minority of cases. Some degree of hemolysis, the main adverse reaction with anti-D, is inevitable because of binding of anti-D antibody to Rh ₀ (D)-positive erythrocytes. There is laboratory evidence of hemolysis in most patients. The average decline in hemoglobin ranges from 0.5 to 1 g/dL, and most cases of hemolysis do not require medical intervention. In a small subset of patients, more significant hemolysis has been observed, which has tempered enthusiasm for use of the drug. Postmarketing surveillance by the U.S. Food and Drug Administration documented 15 cases of hemoglobinemia or hemoglobinuria after anti-D therapy. Of these patients six required transfusion, eight experienced an onset or exacerbation of renal insufficiency, and two underwent dialysis. The incidence of intravascular hemolysis was estimated to range from 0.1% to 1.5%. Why certain patients experience severe intravascular hemolysis after anti-D therapy is unclear. Subcutaneous delivery of anti-D may be associated with a lower incidence of severe hemolytic reactions.

Relapses or Treatment Failures.

Many patients treated with a glucocorticoid, IVIG, or anti-D will become thrombocytopenic again after a few weeks. These patients are likely to respond again to the therapy used initially. Patients who require therapy and whose thrombocytopenia does not respond to initial treatment with a glucocorticoid, IVIG, or anti-D are usually treated with one of the alternative frontline therapies. No clinical studies document the response rate in these instances. Patients with persistent mild hemorrhage may benefit from low-dose glucocorticoid therapy.