Chapter 117: Thrombocytopenia

Platelets are anucleate blood cells produced in the marrow by polyploid cells termed megakaryocytes and were described in the 19th century after the application of the improved compound microscope allowed these very small cellules, approximately 2 μM in diameter, to be identified. Many early investigators are associated with the discovery of blood platelets, including Donné, Hayem, Bizzozero, and Osler, but it was James Homer Wright who, in 1906, using his special stain (later called Wright stain), described the morphology of platelets with their central granular area and marginal hyaline zone and established that they were the product of the fragmentation of marrow megakaryocytes. Clot retraction was discovered long before platelets, but Hayem, through a series of studies, showed retraction to be dependent on platelets. During the mid-20th century the aggregation of platelets, their adherence to collagen of damaged tissues, their acceleration of blood coagulation, and their relationship to the bleeding time and the biochemistry underlying several of these processes were described by scientists, among whom were Paul Owren, Kenneth Brinkhaus, Edwin Chargaff, Ernst Lüsher, Marjorie Zucker, and William Duke.

Platelets circulate in close contact with the endothelium, continually monitoring its integrity. When the vessel wall is damaged, platelets bind to subendothelial proteins, initiating the process of primary hemostasis. At sites of blood loss, the platelets aggregate to form a vessel sealing plug to halt bleeding. Activated platelets at sites of injury also provide a surface for assembly of coagulation reactions, resulting in the production of fibrin and consolidation of the thrombus. Both qualitative and quantitative deficiencies of the platelets cause bleeding. Platelets also have important functions in inflammation, tissue remodeling and wound healing.¹

Approximately 1 × 10¹¹ platelets are produced per day by an adult human, a number that can be increased 20-fold or more, if necessary.² One-third of the platelets are stored in the spleen, the remaining two-thirds circulate in blood vessels.³ Disorders that increase splenic volume cause more platelets to be trapped in the spleen, lowering the concentration of circulating platelets, although alone, this redistribution rarely causes a significant bleeding diathesis.

Under normal conditions, human platelets have a mean life span in the circulation of between 7 and 10 days.^4,5 Patients with thrombocytopenia secondary to platelet destruction have a markedly decreased platelet survival.^6,7 Patients with thrombocytopenia from marrow failure have mildly decreased platelet survival, mostly because the body's fixed daily consumption of platelets accounts for a progressively larger fraction of the reduced total daily production as the platelet count drops.⁸ Platelet turnover is a measure of the net effect of platelet production and platelet destruction under steady-state conditions.⁷ Several studies using ¹¹¹In oxine–labeled platelets have established that, under normal conditions, platelet turnover in humans ranges from 40 to 50 × 10⁹/L per day.⁷ Although a high platelet turnover is expected in patients with immune thrombocytopenia (ITP), platelet production is not always increased in this disorder.⁷ Low platelet production may result from binding of the antiplatelet antibodies to megakaryocytes, inhibiting their maturation or leading to their destruction, causing an inappropriately muted marrow response to the degree of thrombocytopenia.⁹

Every day approximately 10 to 12 percent of circulating platelets are removed by the mononuclear phagocyte system, primarily by macrophages in the spleen and liver. Although the precise mechanisms of platelet clearance are not completely understood, changes that occur as the platelets circulate are thought to lead them to be recognized by macrophages. One of these changes is the progressive loss of sialic acid from platelet surface proteins. Studies in animals and humans with anticancer drugs that inhibit apoptotic pathways have also identified a role for apoptotic proteins in platelet survival and clearance. According to these studies, a classical intrinsic apoptosis pathway regulates the life span of circulating platelets, and antiapoptotic proteins, especially Bcl_-XL, maintain platelet viability by restraining apoptosis.¹⁰

THE PLATELET COUNT

The normal platelet count (defined as the values between percentiles 2.5 to 97.5 in normal individuals) is given as 150 to 400 × 10⁹/L; classically, thrombocytopenia is defined as a platelet count of less than 150 × 10⁹/L. However, a sustained lower platelet count (100 to 150 × 10⁹/L) can be seen in otherwise healthy individuals.^11,12 Long-term observation of individuals with platelet counts between 100 and 150 × 10⁹/L showed that 88 percent of these individuals had subsequently reached normal platelet counts or remained stable. In those individuals, the probability of developing ITP was 6.9 percent, an autoimmune disease other than ITP 12 percent, and myelodysplastic syndrome (MDS) 2 percent, after 64 months of followup. All patients with MDS in this cohort were found to be older than age 65 years.¹³

THROMBOCYTOPENIA

Thrombocytopenia can be classified as severe (platelet count less than 20 × 10⁹/L), moderate (platelet count 20 to 70 × 10⁹/L), or mild (above 70 × 10⁹/L).¹⁴ Although easy bruising occurs in patients with platelet counts less than 50 × 10⁹/L and spontaneous life-threatening bleeding can be expected in patients with platelet counts less than 15 × 10⁹/L, bleeding symptomatology is largely determined by comorbid conditions affecting platelets or the coagulation system, including liver cirrhosis, uremia, disseminated intravascular coagulation (DIC), or antiplatelet drug usage.

In clinical practice, platelet counting is automated, and includes several different technologies: impedance, optical, two-dimensional laser, and optical-fluorescence methods. Although automated cell counter technology has progressed considerably during recent decades, the analytic performances of these machines for platelet counts and platelet indices is still not perfect, especially in patients with severe thrombocytopenia and macrothrombocytopenia.^15–17 Each step between the sampling of blood and its analysis is important: the blood sample should be obtained by a clean venipuncture without dilution with other IV solutions or drugs. Blood/anticoagulant ratio should be as recommended. The International Council for Standardization in Hematology (ICSH) recommends use of ethylenediaminetetraacetic acid (EDTA) as the anticoagulant. Adequate mixing of the blood sample with EDTA (the final EDTA concentration should be 1.5 to 2.2 mg/mL) is crucial to prevent clumping of the platelets. Blood samples should be kept at room temperature and analyzed within 6 hours of phlebotomy. If a sample is to be analyzed more than 6 hours after it is drawn, it can be kept at 4°C for 24 hours. The blood count analyzer should be cleaned according to laboratory standards.¹⁷

Although thrombocytopenia is variably attributed to single factors such as decreased platelet production, increased platelet destruction, or abnormal splenic pooling, combinations of factors are often involved in clinical settings. For instance, the thrombocytopenia seen in patients with viral infection can result from many factors, including platelet destruction (e.g., through an autoimmune mechanism or drug toxicity) or decreased platelet production because of direct megakaryocyte infection by the virus. Table 117–1 lists the multiple causes of thrombocytopenia and classifies them by pathogenesis.

Pseudothrombocytopenia (or spurious thrombocytopenia) is a relatively uncommon phenomenon with multiple causes, including ex vivo agglutination of platelets, the presence of abnormally large platelets (improper counting), or improper preparation of blood samples.

The incidence of pseudothrombocytopenia reported in different studies ranges from 0.09 to 0.21 percent, which accounts for 15 to 30 percent of all cases of isolated thrombocytopenia.^18–25 Pseudothrombocytopenia has been reported in association with the use of EDTA as an anticoagulant, with platelet cold agglutinins,²⁶ and with myeloma.²⁷ A very interesting report demonstrates pseudothrombocytopenia caused by platelet phagocytosis ex vivo in the presence of EDTA anticoagulant.²⁸ An example of exvivo platelet clumping is shown in Chap. 1, Fig. 1–6H, accompanied by platelet–neutrophil satellitism (see “Platelet Satellitism” below).

ANTIBODY-INDUCED PLATELET AGGLUTINATION

Platelet agglutination ex vivo can be induced by antiplatelet antibodies or by activation of the platelets during collection. The responsible antibodies do not appear to be associated with a pathologic process, as they are found in normal individuals. One hypothesis put forth to explain their presence is that the antibodies are responsible for clearing aged and damaged platelets. Most antibodies implicated in pseudothrombocytopenia recognize platelet membrane glycoproteins that are modified to expose new epitopes when calcium is chelated. Typically, the artifact is most prominent in the presence of EDTA, but other anticoagulants can also cause platelet clumping, including sodium citrate, sodium oxalate, acid citrate dextrose, and heparin. The antibodies usually are of the immunoglobulin (Ig) G type; IgM and IgA antibodies also have been described.^29–31 Most antibodies react at room temperature; thus, the reaction can be prevented by keeping the blood sample at 37°C. In 20 percent of cases, however, the antibodies, usually of the IgM type, are reactive at both 22°C and 37°C.³⁰ Clumping usually is evident within 60 minutes after the blood is drawn, but may require incubations of 2 to 3 hours. Agglutination can be reproduced by incubating plasma from patients with pseudothrombocytopenia with blood from normal individuals in the presence of EDTA.

In most cases, the antibodies are directed against the integrin α_IIbβ₃ (also termed glycoprotein [GP] IIbIIIa), a conclusion supported by the observation that platelets from patients with Glanzmann thrombasthenia, who lack the integrin α_IIbβ₃ complex, fail to agglutinate in the presence of patient sera.^32–35 Moreover, pretreatment of fresh blood with anti–integrin α_IIbβ₃ dramatically reduces EDTA-induced platelet agglutination.³⁶ The responsible epitope normally is cryptic and located in the integrin α_IIb subunit. Low temperature and calcium chelation combine to change the conformation of integrin α_IIbβ₃ and expose the epitope.³³

PLATELET SATELLITISM

Antibodies directed against integrin α_IIbβ₃ may react simultaneously with the leukocyte Fcγ receptor III (FcγRIII) and attach the platelets to neutrophils and monocytes, inducing a phenomenon known as platelet-leukocyte satellitism,³² another form of pseudothrombocytopenia (Fig. 117-1). These antibodies fail to produce satellitism in the presence of platelets from patients with type I Glanzmann thrombasthenia or in the presence of neutrophils from patients with congenital absence of FcγRIII.³² Typically, the platelets form a rosette around the periphery of leukocytes. Neutrophils are most frequently involved, but the phenomenon also is occasionally observed with monocytes.^37,38 These antibodies also are naturally occurring, and their presence does not clearly correlate with any specific clinical situation, disease, or drug. As with the antibodies that induce only platelet clumping, exposure of a cryptic antigen on EDTA-treated platelets and leukocytes may trigger this phenomenon.

ANTIPHOSPHOLIPID ANTIBODIES

Some antiplatelet antibodies from patients with pseudothrombocytopenia crossreact with negatively charged phospholipids and may exhibit anticardiolipin activity.³⁰ The sera of these patients lose their ability to clump platelets when adsorbed onto either cardiolipin or activated normal platelets, supporting the hypothesis that antibody subpopulations directed against negatively charged phospholipids can bind to antigens modified by EDTA on the platelet membrane. Another possibility is that the antigens in this case are negatively charged phospholipids on the surface of platelets.

INTEGRIN α_IIBβ₃ ANTAGONISTS

Thrombocytopenia has been described in patients suffering from acute coronary syndromes treated with the abciximab and other integrin α_IIbβ₃ antagonists.^39–41 Abciximab is associated with both pseudothrombocytopenia and true thrombocytopenia. The mechanism for platelet clumping with abciximab is unknown; the drug itself likely is not crosslinking the platelets because it is monovalent. More likely, other agglutinins bind integrin α_IIbβ₃ at new epitopes induced by the combination of abciximab binding and calcium chelation. True abciximab-induced thrombocytopenia occurs in approximately 0.3 to 1 percent of patients treated with the drug.⁴² The mechanism is incompletely understood, but likely includes reaction of preformed antibodies with a neoepitope expressed after binding of abciximab to integrin α_IIbβ₃ (ligand-induced binding sites) or abciximab-induced platelet activation with subsequent platelet sequestration from the circulation. In some abciximab-treated patients, high antibody titers are detected in the plasma.

The incidence of pseudothrombocytopenia and thrombocytopenia related to abciximab was determined in four large placebo-controlled trials:⁴⁰ c7E3 Fab Antiplatelet Therapy in Unstable Refractory Angina (CAPTURE), Evaluation of 7E3 for the Prevention of Ischemic Complications (EPIC), Evaluation of Percutaneous Transluminal Coronary Angioplasty to Improve Long-term Outcome of c7E3 GPIIb-IIIa Receptor Blockade (EPILOG), and Evaluation of Platelet IIb/IIIa Inhibitor for Stenting (EPISTENT). In these studies, pseudothrombocytopenia accounted for more than one-third of low platelet counts in patients undergoing coronary interventions and treated with abciximab. These studies demonstrated that pseudothrombocytopenia is a benign laboratory condition not associated with increased bleeding, stroke, transfusion requirements, or the need for repeat revascularization.

MISCELLANEOUS ASSOCIATIONS

Some studies suggest that platelet agglutinins occur more frequently in hospitalized patients and in association with medical conditions such as autoimmune diseases, malignancy, liver disease, and sepsis.^25,43–46 However, others found no association with any particular pathology or with use of specific drugs.³⁰

One study showed that antibodies from patients with pseudothrombocytopenia can induce agglutination of donor platelets in the presence of EDTA. This agglutination was prevented by warming the donor platelets to 37°C or by pretreating the platelets with aspirin, prostaglandin E₁, apyrase, and monoclonal antibodies against integrin α_IIbβ₃ that block the binding site for fibrinogen and von Willebrand factor (VWF), or arg-gly-asp (RGD) peptide, which binds the site on integrin α_IIbβ₃ that recognizes cytoadhesive proteins.³³ Whether the same reaction occurs in vivo is not known, but in that case the antibodies should have a slow reactivity, or else bleeding would sometimes occur.

MANAGEMENT OF THE PATIENTS WITH PSEUDOTHROMBOCYTOPENIA

An (unexpected) low platelet count reported by automated cell counters should be confirmed by microscopic examination of the blood film. Automated cell counters identify platelets merely based on their small volumes in comparison to those of other blood cells, generally defined as volumes between 2 and 20 fL. Because platelet clumps tend to exceed 20 fL, the clumps may be counted as leukocytes,¹⁸ and even if counted as platelets, several platelets are counted as one. Thus, pseudothrombocytopenia may be accompanied by pseudoleukocytosis.^5,21,24,47 The greater the delay in processing of anticoagulated blood, the greater is the degree of platelet clumping and the greater the potential for artifact.²¹ Platelet clumping can be prevented by collecting the sample in EDTA and maintaining its temperature at 37°C. Even with these measures, however, clumping will still occur in approximately 20 percent of cases.³⁰

Another alternative is use of sodium citrate, which chelates calcium more weakly than does EDTA but still causes platelet clumping in approximately 10 to 20 percent of cases with EDTA-induced clumping. In some patients, an accurate platelet count can be obtained only by sampling blood directly into ammonium oxalate and manually counting the platelets using a Bruker chamber.³⁰ Flow cytometry may help for determining exact platelet number by immunostaining of the platelets.

Platelet agglutinins are not associated with bleeding or thrombosis, so they appear to have no clinical implications, except that they may lead to unnecessary therapy because of misdiagnosis. Transplacental transmission of agglutinins has been documented, but the pseudothrombocytopenia induced by these antibodies in the neonate resolves spontaneously.^48,49 No complications have been reported when platelet agglutinins are discovered during pregnancy.^48,50 Transfusion of blood products from patients with pseudothrombocytopenia produces an acceptable corrected count increment in the recipient, again supporting its benign nature.²³ Thus, the clinical importance of pseudothrombocytopenia concerns conditions with which it is confused rather than any pathology associated with the condition. It is important that this syndrome be recognized promptly to avoid unnecessary diagnostic tests and treatment.

Megakaryopoiesis and thrombopoiesis are regulated by a number of hematopoietic growth factors and transcription factors (Chap. 113). Any genetic defect affecting platelet production, function or morphology may cause inherited platelet disorders (IPDs; Chap. 121). In recent decades, knowledge of normal megakaryocyte and platelet physiology has grown enormously,⁵¹ aided in part by the study of IPDs.^52,53

IPDs are a very heterogeneous group of disorders. Some disorders, such as Bernard-Soulier syndrome, appear to be restricted to platelets,⁵⁴ whereas others appear as a part of a complex pathology, as seen in thrombocytopenia with absent radii (TAR) syndrome (Fig. 117-2). In some IPDs, the platelet count may be normal despite severely impaired platelet function, such as in Glanzmann thrombasthenia. Other disorders are accompanied by abnormal platelet numbers, usually thrombocytopenia. Table 117–2 summarizes the inherited thrombocytopenias.

Severe forms of IPDs that present as a bleeding tendency early in childhood are rare. IPD patients usually present with mucocutaneous bleeding, such as with purpura, epistaxis, and/or gingival bleeding. Menorrhagia and bleeding during pregnancy and labor are common problems in female patients. Spontaneous life-threatening bleeding is rare, including intracranial hemorrhage, massive gastrointestinal or genitourinary bleeding. Recent molecular investigations of IPD patients and their families with bleeding diathesis demonstrated that most IPDs cause mild bleeding tendencies, and IPDs may be more prevalent than previously thought.⁵⁵ In these milder cases, a bleeding diathesis may only be diagnosed after an episode of excessive bleeding, such as during surgery or following trauma.

Diagnosis of IPD presents a significant challenge because of the heterogeneity of clinical and laboratory findings of the patients with the same disorder, even in the same family. IPD patients with isolated macrothrombocytopenia share common clinical and basic laboratory features with certain acquired platelet disorders and are sometimes misdiagnosed. It is very important to distinguish IPD patients from those with acquired platelet disorders, such as ITP, to avoid unnecessary or potentially harmful treatments. Helpful in this regard is information obtained during the history, including a family history of bleeding and consanguinity in the family, because the majority of IPDs are inherited as autosomal recessive traits. Because some IPDs are associated with increased risk of developing myeloid malignancies, the patient and family should be asked about a family history of myeloid malignancies. The presence of skeletal, facial, ocular, audiologic, neurologic, renal, cardiac, and immune problems associated with platelet disorders may also suggest IPD.^51,56

Laboratory evaluation of a potential IPD should start with a careful blood film investigation, which could be helpful for patients with MYH9-related diseases (giant platelets and Döhle-like inclusion bodies within leukocytes; Fig. 117-3), Bernard-Soulier syndrome (macrothrombocytopenia), Gray platelet syndrome (pale platelets), and sitosterolemia (giant platelets surrounded by a circle of vacuoles, stomatocytosis). Platelet function analyzer (PFA-100) occlusion times are usually found to be prolonged. The skin bleeding time is not recommended for screening, because it is invasive and poorly reproducible. Although the PFA-100 test is very sensitive in detecting Bernard-Soulier syndrome and platelet-type von Willebrand disease (VWD), it may be normal in patients with variant forms of these disorders, or patients with storage pool deficiencies.⁵⁷ Light transmission aggregometry (LTA) using different concentrations of adenosine diphosphate (ADP), collagen, ristocetin, epinephrine, and arachidonic acid is accepted as a gold standard in diagnosing IPDs, but, again, may be normal in variant forms of IPDs and in some patients with storage pool diseases. Measurement of platelet nucleotide content and release is recommended in patients with platelet granule deficiencies. Flow cytometric analysis is very informative in patients with platelet surface GP deficiencies such as Bernard-Soulier syndrome. Marrow biopsy is needed in patients who have pancytopenia or severe thrombocytopenia, as in Fanconi anemia and congenital amegakaryocytic thrombocytopenia (CAMT), respectively. The cause of CAMT is almost always due to non-sense or severe mis-sense mutation of the TPO receptor, c-MPL, and rarely due to mutation of the THPO gene itself. Unfortunately, these tests may help diagnose only a small portion of the IPD patients. Further tests are only available in specialized centers, and include electron microscopy, Western blotting, and others. Electron microscopy is able to define characteristic ultrastructural abnormalities; Western blotting, enzyme-linked immunosorbent assay (ELISA), or radioimmunoassay can be used for qualitative and quantitative analysis of specific platelet proteins.^51,56 Even with these expensive, complicated, and time-consuming tests, the results are inconclusive in nearly half of patients being evaluated for IPD.⁵⁶ Genetic analysis is often able to determine the underlying molecular pathology, but the very large number of candidate genes limits the traditional target gene approach. Within the past decade, next-generation sequencing techniques have not only improved the speed and cost of genetic investigations, but have also begun to generate very interesting data about the genetic causes of IPD.^52,53

Iron, vitamin B₁₂, and folic acid deficiencies are the nutrient deficiencies most widely recognized to impair blood cell production. Severe nutritional deficiencies primarily cause anemia, rarely causing bicytopenia or pancytopenia. Isolated thrombocytopenia is rare in patients with nutritional deficiencies.

Iron is present in all human cells, and mediates electron transfer reactions. Iron is a key component of hemoglobin, and iron deficiency causes a hypochromic and microcytic anemia (Chap. 42). Iron-deficiency anemia (IDA) generally develops after acute or chronic bleeding, and is usually accompanied by thrombocytosis rather than thrombocytopenia. Thrombocytopenia associated with IDA is relatively rare, reported in only 2.3 percent and 2.4 percent of pediatric and adult IDA patients, respectively.^58,59

Cobalamin (vitamin B₁₂) and folate are both required for DNA synthesis and repair, but humans can synthesize neither vitamin. Dietary deficiencies, impaired absorption, or inhibition with drugs (as seen in methotrexate therapy) of these vitamins can cause megaloblastic anemia (Chap. 41). Mild thrombocytopenia occurs in approximately 20 percent of patients with megaloblastic anemia resulting from vitamin B₁₂ deficiency in the United States.⁶⁰ The frequency may be higher in patients with folic acid deficiency because of the high frequency of concomitant alcohol abuse (Chap. 41). One large study of 139 patients examined the rates of cytopenias associated with megaloblastic anemia in India.⁶¹ In this study, 76 percent had isolated vitamin B₁₂ deficiency, 7 percent had isolated folate deficiency, 9 percent had a combined deficiency, and 8 percent had normal vitamin levels. All were anemic by definition, and 80 percent had thrombocytopenia with mild to moderate depression of the platelet count. More than half of those with thrombocytopenia were also neutropenic. The authors of this study suggested that the cytopenias tended to progress from isolated anemia, to anemia plus thrombocytopenia, to pancytopenia, with the degree of cytopenia related to the severity of vitamin deficiency. Occasionally, thrombocytopenia is severe in patients with megaloblastic anemia and, when accompanied by fever, hepatomegaly, and splenomegaly, and may suggest a diagnosis of acute leukemia. In these syndromes, the primary mechanism of thrombocytopenia is ineffective platelet production⁶²; marrow megakaryocyte number usually is normal or increased. Abnormalities of megakaryocyte morphology are much less distinctive than the characteristic erythroid and myeloid defects, but often nuclear abnormalities are seen, with nuclei of larger size and dispersed nuclear segments, rather than single polyploid nuclei.⁶³ Thrombocytopenia may be seen in association with vitamin B₁₂ deficiency when the latter results from autoantibodies against parietal cells or intrinsic factor and is associated with ITP.^64,65 Various other autoimmune disorders can coexist with pernicious anemia, including autoimmune vitiligo and autoimmune thyroiditis.⁶⁶ Abnormalities of platelet function are sometimes seen associated with vitamin B₁₂ deficiency.^67,68 Diminished platelet aggregation and reduced release of ADP and ATP from granule stores in response to different agonists have been reported, and vitamin deficiency has been suggested to induce an acquired storage pool disease (Chap. 41).⁶⁸

Copper deficiency is usually seen in patients who have undergone gastric bypass surgery, and may cause anemia, leukopenia, and thrombocytopenia associated with neurologic deficits resembling vitamin B₁₂ deficiency. Patients with copper deficiency also may be misdiagnosed as having MDS, because increased ring sideroblasts and dysplastic precursor cells can be seen on marrow smears.^69,70

Acute and chronic alcohol (ethanol) consumption affects hematopoiesis and blood cell survival both directly and indirectly. Alcohol is one of the leading causes of thrombocytopenia in Western countries. Acute ethanol intoxication in healthy volunteers induces thrombocytopenia.⁷¹ Platelet counts in these cases are usually mildly decreased (generally more than 100 × 10⁹/L); severe thrombocytopenia is quite rare. Acute ethanol-induced thrombocytopenia usually resolves within 5 to 21 days with cessation of ethanol ingestion, sometimes with a transient rebound thrombocytosis that may reach up to 1,000,000 × 10⁹/L.⁷² Although the mechanism of acute alcohol-related thrombocytopenia is not clear, it has been suggested that metabolites of ethanol, especially acetaldehyde, impair the late stages of platelet production and increase platelet destruction.⁷³ Thus, thrombocytopenia associated with acute alcohol ingestion would be expected to be more frequent in those with poor nutrition (delayed oxidation of acetaldehyde) and those with partial acetaldehyde dehydrogenase defiance. Thrombocytopenia induced by alcohol ingestion is accompanied by a decreased number of marrow megakaryocytes. Vacuolated proerythroblasts and granulocyte precursors are sometimes seen, as are multinuclear erythroblasts and megaloblasts.⁷⁴ Vacuolization of the periphery of mature megakaryocytes has been reported.⁷⁵ Alcoholism (chronic ethanol consumption, which is defined as consumption of more than 80 g of ethanol per day), on the other hand, may cause thrombocytopenia by other mechanisms, such as alcoholic liver cirrhosis (both splenomegaly and thrombopoietin deficiency), folic acid deficiency, and alcohol-induced marrow suppression.^74–78

Thrombocytopenia attributable to pure aplasia or hypoplasia of megakaryocytes is rare.⁷⁹ More common are instances in which amegakaryocytic thrombocytopenia anticipates the development of full-blown MDS or aplastic anemia and is associated with subtle abnormalities of other lineages, such as macrocytosis and dyserythropoiesis.^80–84 Most commonly the disorder is caused by autoimmune suppression of megakaryocyte development, either idiopathic,⁸⁵ associated with autoimmune disorders such as systemic lupus erythematosus (SLE)⁸⁶ and eosinophilic fasciitis, or associated with infections such as hepatitis C.⁸⁷ Antibodies against thrombopoietin (TPO)⁸⁸ have been described to cause the disorder, as have antibodies against the TPO receptor.⁸⁹ Patients may achieve durable remission with therapies designed to blunt the autoimmune response, such as cyclosporine or antithymocyte globulin (ATG).⁹⁰

IMMUNE THROMBOCYTOPENIA

Table 117–3 summarizes the various types of ITP.

PRIMARY IMMUNE THROMBOCYTOPENIA

ITP, formerly known as autoimmune thrombocytopenic purpura, is the most common cause of isolated thrombocytopenia in clinical practice. ITP is characterized by immune-mediated platelet destruction and impaired platelet production. ITP occurs in every age group. Childhood ITP typically is acute in onset, often developing after a viral infection or vaccination. Although thrombocytopenia may be severe, it usually resolves spontaneously, within a few weeks up to 6 months.⁹¹ In contrast to childhood ITP, adult ITP generally is a chronic disease of insidious onset and rarely resolves spontaneously.

“Purpura” was recognized by Hippocrates (c. 460 to c. 370 BC) and Galen (AD 129 to c. 200/c. 216) as a sign associated with fever. Chronic purpura was first described in details by Ibn-i Sina (Avicenna, c. 980 to c. 1037) in his famous book “The Canon of Medicine.” In 1705, Werlof suggested that purpura was related to infections and described it as “morbus maculosus haemorrhagicus.” Patients with purpura were diagnosed as having “Werlof disease” for centuries. After the discovery of platelets and their role in hemostasis, the relationship between purpura and low-platelet count was understood.⁹²

In 1915, Erich Frank renamed this disorder as “essential thrombocytopenia,” and suggested that platelet production from megakaryocytes was impaired because of a toxic substance produced by the spleen.⁹³ Kaznelson, inspired by Frank's theory, proposed splenectomy for a patient with chronic thrombocytopenic purpura. The treatment was successful, and splenectomy was first-line therapy for ITP until the introduction of glucocorticoids in 1950s.

In the first issue of the journal Blood (in 1946) Damashek and Miller reviewed the megakaryocyte count and marrow morphology of patients with “idiopathic thrombocytopenic purpura.”⁹⁴ They showed that most ITP patients had an increased number of megakaryocytes, but very few of them were producing platelets, so “actual platelet-producing tissue” might be decreased.⁹⁴

Although Marino first showed that antiplatelet antibodies might cause thrombocytopenia in animal studies in 1905, the Harrington-Hollingsworth experiment (1951) was an important milestone in the understanding of autoantibody-directed platelet destruction in the pathophysiology of ITP. In this pioneering work, normal volunteers (including Harrington himself, who received the highest dose) were infused with the plasma from patients with ITP, resulting in severe thrombocytopenia in the recipients, and they postulated that ITP could be caused by antiplatelet antibodies.^95,96 Subsequently, Shulman and coworkers⁹⁷ showed that the thrombocytopenic effect of ITP plasma was dose-dependent and associated with the globulin fraction. In the 1950s, glucocorticoids began to be used to treat ITP, and they became first-line therapy for adults. Shortly thereafter other immunosuppressive agents were introduced for the treatment of chronic ITP.⁹²

In the early 1970s, two groups showed that platelets from chronic ITP patients had elevated levels of platelet-associated immunoglobulin G (PAIgG).^98,99 In 1982, the first platelet target was identified: autoantibodies from patients with ITP failed to bind platelets deficient in the integrin α_IIbβ₃ complex (i.e., from patients with Glanzmann thrombasthenia).¹⁰⁰ In the late 1980s, two specific assays for the target antigens were described: the immunobead assay¹⁰¹ and the monoclonal antibody-specific immobilization of platelet antigens (MAIPA) assay.¹⁰² These assays showed that the majority of antiplatelet antibodies in patients with ITP are directed against integrin α_IIββ_{3(GPIIβ-IIIα)}(approximately 80 percent), and the remainder are against the GPIb-IX-V complex and other platelet GPs such as GPIV and integrin α₂β₁ (GPIa-IIa).^103,104 Some sera contain antibodies that recognize several antigens. Most antiplatelet autoantibodies are IgG; the remainder are IgM and IgA. Unfortunately, elevated levels of PAIgG later were found in patients with non-ITP. Therefore, PAIgG could not be used as a specific laboratory test for ITP in the same way that the direct antiglobulin test is used for the diagnosis of autoimmune hemolytic anemia.^105,106 To date there is still no specific laboratory test for ITP, the diagnosis of ITP being based on exclusion of other causes.

Antibody-coated platelets bind tissue macrophages through Fcγ receptors, leading to their destruction primarily in the spleen and, to a lesser extent, in the liver and marrow.^97,107,108 In 1981, Imbach reported successful treatment of pediatric ITP with intravenous immunoglobulin (IVIG) and suggested that the mechanism could involve blockade of macrophage Fc receptors. IVIG became first-line therapy in children, and now is also used in adults when a prompt increase is the platelet count is desired.¹⁰⁹

Early studies of PAIgG reported that the antibodies in ITP were polyclonal.¹¹⁰ However, later studies showed that at least some ITP patients had clonal B-cell proliferation, as determined by DNA analysis for immunoglobulin heavy- and light-chain rearrangements and by flow cytometry of B cells from blood and spleen for surface Ig light chains.^111,112 This led to the use in ITP of the chimeric anti-CD20 monoclonal antibody, rituximab, which was designed for the treatment of CD20-positive B-cell lymphomas. The rapid elimination of B cells with rituximab encouraged the use of this agent in the treatment of ITP.

Numerous abnormalities in cell-mediated immunity have been described in patients with ITP, including abnormalities in antigen-presenting cells, T lymphocytes, and cytokine release. Under normal conditions, antigen-presenting cells recognize and process foreign antigens and express the antigens on their surface in association with major histocompatibility complex (MHC) molecules. MHC–antigen complexes activate resting (naïve) CD4+ T cells to differentiate into a variety of phenotypes such as T-helper 1 (Th1) and T-helper 2 (Th2), Th17, and T-regulatory (Treg) cells. Th1 cells are involved in cell-mediated immunity and host defense against intracellular bacteria and protozoa. Th2 cells are involved in humoral immunity and host defense against extracellular parasites. Th17 cells are involved in host defense against extracellular bacteria and fungi. Treg cells (formerly known as suppressor T cells) play an important role in self-tolerance by inhibiting autoimmune responses. Abnormal T-cell responses drive the differentiation of autoreactive B-cell clones and autoantibody secretion. In patients with ITP, both Th1 and Th17 cells have been found to be upregulated, whereas the number and the suppressor functions of the Treg cells were found to be decreased.^113–115 This imbalance is believed to induce an autoimmune responses against the platelets. It is unclear whether these abnormalities are causative or represent an epiphenomenon.^114,115 In addition, CD8+ cytotoxic T cells might be involved in the pathogenesis of ITP through cell-mediated destruction of platelets and megakaryocytes and through suppression of megakaryocytes, impairing platelet production.^115–117

Antiplatelet autoantibodies may also activate platelet destruction by activating complement through the classical complement pathway. Increased platelet-associated C3, C4, and C9 have been demonstrated on the platelets from patients with ITP.^118,119 In vitro studies show that, in the presence of antiplatelet antibodies, C3 and C4 can bind platelets, increase the phagocytosis of the platelets by macrophages, and can cause their lysis by stimulating assembly of the membrane attack complex.^120,121

Early studies demonstrated that platelet survival is shortened in ITP patients and returns to normal after splenectomy-induced remission.¹²² Platelet transfusion only transiently increases a patient's platelet count, and the transfused platelets also have a shortened survival, reflecting the fact that the major problem in ITP is platelet destruction. However, later studies showed that platelet life span was not short enough to account for the observed thrombocytopenia on the basis of destruction alone, again suggesting a concomitant defect in platelet production.¹²³ Potential mechanisms for this observation were provided by later studies that autoantibodies against platelet surface GPs might interfere with the maturation of megakaryocytes, resulting in reduced platelet production, contributing to the severity of thrombocytopenia in some ITP patients.¹²⁴ Antibodies that target the GPIb–IX–V complex may induce thrombocytopenia by decreasing platelet production, as GPIb autoantibodies inhibit megakaryopoiesis in vitro,¹²⁴ and GPIb monoclonal antibodies inhibit proplatelet formation in vitro.¹²⁵

In 1958, a hematopoietic growth factor regulating platelet production was proposed and named TPO by Kelemen.¹²⁶ Although interleukin (IL)-3, IL-6, IL-11, granulocyte-macrophage colony-stimulating factor, and c-KIT ligand increase megakaryocyte or platelet counts in vivo and in vitro, animal studies of these factors proved that they are not the main regulator of megakaryopoiesis.¹²⁷ In 1994, TPO was first characterized by five independent groups. TPO binds to its receptor MPL (formerly known as c-MPL), enhances megakaryocyte colony formation, and increases the size, number, and ploidy of megakaryocytes, and platelet production (Chap. 113).^128–130 TPO is synthesized in greatest quantity in the liver but is found in other organs (kidney, muscle, and marrow stromal cells).¹²⁸ TPO is also required to maintain the viability of hematopoietic stem cells.¹³¹ The regulation of TPO production is complex. Hepatic production of TPO is both constitutive (in the steady state) and inducible (by inflammation), and the concentration of TPO to which megakaryocytes are exposed is also determined by the platelet concentration. Platelets, bearing TPO receptors, remove the hormone from the circulation, at least partially accounting for the inverse relationship between TPO and platelet levels. TPO levels are markedly elevated in patients with thrombocytopenia associated with megakaryocytic hypoplasia, including disorders such as aplastic anemia or acute leukemia. In most reports, ITP patients have normal or slightly elevated TPO levels whether measured in plasma or serum, but the levels are always lower than the concentrations found in thrombocytopenias resulting from megakaryocytic hypoplasia.^{128–130,132,133} Initial studies with recombinant and pegylated TPO molecules showed successful responses in patients with thrombocytopenia, but development of autoantibodies against these molecules restricted their use in clinical settings. Based on the success of creating erythropoietin receptor agonist peptides, a number of screening efforts were undertaken to design small peptides or organic molecules that might bind to the TPO receptor and stimulate thrombopoiesis. One such molecule contains four copies of a 14-amino-acid peptide grafted onto an Ig Fc domain, forming a “peptibody” termed romiplostim. This agent, which binds to a region of the TPO receptor that overlaps that bound by authentic TPO, was shown to increase platelet counts in patients with ITP who had failed other modalities,¹³⁴ and was approved by the FDA for this indication in 2008. Another small organic thrombopoietic molecule, eltrombopag, was developed almost simultaneously¹³⁵ and approved in 2008 by FDA for the same indications.¹²⁷ This agent activates TPO receptor signaling by binding to the transmembrane domain of the receptor, a site quite distinct from the binding site for TPO and romiplostim. Both TPO-receptor agonists are currently being evaluated for additional clinical indications in clinical trials.¹³⁶

Some patients with ITP appear to display a genetic predisposition. ITP has been documented in monozygotic twins¹³⁷ and shown to be highly prevalent in some families.¹³⁸ In addition to contributing to the development of ITP, like in other autoimmune disorders heredity may also affect the response to ITP therapy. Human leukocyte antigen (HLA) class I and class II allele frequencies in patients with ITP have been studied by several investigators, with inconsistent results. Some investigators reported an increased frequency of HLA-Aw32, -DRw2, and -DRB1^*0410.^{108,139–141} Investigation has focused on genetic differences associated with dysregulation of immune tolerance and humoral immunity, but results have been inconclusive. For example, genetic polymorphisms of cytotoxic T-lymphocyte antigen (CTLA)-4, tumor necrosis factor, and Fcγ receptors IIA and IIIA have been suggested to influence the development of ITP and the response to therapy,^141–143 but as yet no strong association has been found.

Accumulating data indicate that the pathophysiology of ITP is more complex than previously thought, with ITP comprising a heterogenous group of disorders with different etiologies and responding to different treatment modalities. The identification of the different subsets of ITP patients will help to better define treatment options.

Definition and Classification

Although ITP has been recognized for centuries, there is as yet no consensus on either the definition or management of the disease. In 1996, the American Society of Hematology (ASH)¹⁴⁴ published practice guidelines for the diagnosis and management of ITP. In 2003, the British Committee for Standards in Haematology published its own guidelines.¹⁴⁵ In spite of these guidelines, the heterogeneity of the definitions and clinical criteria used in different studies has made it difficult to interpret the data regarding the incidence, pathogenesis, and treatment of ITP. In 2008, the International Working Group (IWG) proposed a standardization of terminology, definitions, and outcome criteria for ITP patients.¹⁴⁶ In 2010, an international consensus report on the investigation and management of ITP was published.¹⁴⁷ Shortly thereafter, in 2011, ASH updated its 1996 ITP guidelines.¹⁴⁸

The IWG definition proposed use of the term “immune thrombocytopenia” instead of “idiopathic thrombocytopenic purpura” as the basis for the ITP acronym, because the immune nature of ITP is clear but most ITP patients do not have purpura. A platelet count of 100 × 10⁹/L was proposed as the threshold level to entertain the diagnosis of ITP, because a sustained lower platelet count (100 to 150 × 10⁹/L) can be seen in otherwise healthy individuals,^11,12 and long-term observation of these indicate that 88 percent reach normal platelet counts or remain stable.¹³ ITP is classified based on the absence or presence of other diseases as “primary” or ‘“secondary.” “Primary ITP”’ denotes the absence of any other identified pathology. All other autoimmune thrombocytopenias are classified as “secondary ITP” (see Table 117–3), and the associated primary disorder is indicated in parenthesis, for example “secondary ITP (SLE-associated)” or “secondary ITP (drug-induced).” Heparin-induced thrombocytopenia (HIT) or alloimmune thrombocytopenias are not classified as ITP, and maintain their standard classifications.¹⁴⁶

The IWG described three phases of ITP: (1) newly diagnosed ITP (within 3 months of diagnosis); (2) persistent ITP (patients who do not achieve a stable remission between 3 and 12 months after diagnosis); and (3) chronic ITP (continuing for more than 12 months). ITP was formerly classified as mild, moderate, and severe depending on the platelet counts. However, the degree of thrombocytopenia does not always correlate with bleeding. The IWG proposed that the term “severe ITP” only be used for patients with clinically significant bleeding requiring additional therapy regardless of platelet count.¹⁴⁶

One of the major problems with comparing ITP studies had been the definition of response to therapy. The IWG proposed the following terms and criteria for response to ITP treatment: “complete response, CR” (platelet count exceeding 100 × 10⁹/L and no bleeding symptoms), “response, R” (platelet count higher than 30 × 10⁹/L or at least a twofold increase from the baseline count and no bleeding symptoms), “no response, NR” (platelet count below 30 × 10⁹/L or less than a twofold increase from the baseline count, or presence of bleeding symptoms). “Duration of response” is measured from the time between first measured CR or R to relapse. “Corticosteroid dependence” is defined as the need for ongoing or repeated glucocorticoid use for at least 2 months to maintain CR or R. Patients who relapsed after splenectomy (failure to maintain CR or R) and required therapy are classified as “refractory ITP.” “On-demand therapy” is a term used for therapies employed to temporarily increase the platelet count in special situations such as trauma or surgery. “Adjunctive therapies” are treatments that are not designed to increase platelet counts, but that may decrease bleeding symptoms by other means, for example, treatment with oral contraceptives or antifibrinolytic drugs.¹⁴⁶

Incidence

ITP is relatively common, but demographic studies have yielded a wide range of incidence rates largely because of differences in the age and gender distribution of the populations studied and differences in cutoff platelet counts used to define the disease. ITP can affect males and females of any age. In one detailed study, the reported incidence of ITP was 3.9 per 100,000 per year. Although the overall incidence was higher in women than in men, a male predominance was seen in patients younger than 18 years of age and older than 65 years of age.¹⁴⁹

Clinical Features

ITP is of acute onset in children, often developing after vaccination or after a viral illness, and resolves spontaneously in 90 percent of cases. In adults, however, ITP usually is a chronic disease. Table 117–4 highlights the differences in ITP in children and adults. Approximately 25 percent of adult ITP patients are diagnosed incidentally on routine complete blood counts. Symptoms and signs of ITP depend not only on the platelet count, but also on the nature of coexisting conditions that can increase the tendency to bleed, such as uremia, trauma, and ingestion of drugs that affect platelet function (Table 117–5). Approximately one-third of patients have platelet counts greater than 30 × 10⁹/L at diagnosis and no significant bleeding.¹⁵⁰ Common bleeding signs include purpura (ecchymoses and petechiae), epistaxis, menorrhagia, and gingival bleeding. Hematuria, hemoptysis, and gastrointestinal bleeding are less common. Intracerebral hemorrhage is rare and generally occurs in patients with platelet counts less than 10 × 10⁹/L and usually is associated with trauma or vascular lesions. The incidence of life-threatening complications is highest in patients older than age 60 years.^150–154 The majority of ITP patients have a good prognosis, the mortality rate being only slightly higher than that of the general population. However, ITP patients who present with severe thrombocytopenia (<30 × 10⁹/L) and do not respond to any therapy within 2 years, have a fourfold increased risk of death compared to the general population.¹⁵⁵

The purpuric lesions seen in ITP are not palpable, do not blanch with pressure, and often develop on distal regions of the extremities and on skin areas exposed to pressure (e.g., around tight belts and stockings and at tourniquet sites). Hemorrhagic bullae, which may develop in the buccal mucosa, generally reflect acute, severe thrombocytopenia. Bleeding after surgery, trauma, or tooth extraction is common.

Besides the physical findings associated with platelet-type bleeding, the history and physical examination are usually unremarkable, except for the possibility of similar symptoms in other family members. Family history is especially important to discriminate familial thrombocytopenic syndromes from ITP. The spleen usually is not enlarged but may be palpable in some patients, a finding considered to occur with the same incidence as in normal adults.¹⁵⁶ Constitutional symptoms, such as fever, significant weight loss, marked splenomegaly, hepatomegaly, and lymphadenopathy provide evidence that the thrombocytopenia has another cause. The presence of skeletal, cardiac, renal abnormalities, hearing loss, albinism. or immune deficiencies in patients with thrombocytopenia should trigger suspicion of IPDs.

Fatigue is one of the common, but often neglected, complaints of patients with primary ITP. In a survey including UK and U.S. ITP cohorts, the prevalence of fatigue was found to be significantly higher in adult primary ITP patients (39 percent and 22 percent for the UK and U.S. cohorts, respectively) compared with healthy controls.¹⁵⁷ Fatigue has also been described in 20 percent of pediatric patients with ITP; fatigue resolved with the elevation of platelet counts.¹⁵⁸ Although glucocorticoids and immunosuppressive agents may induce fatigue, fatigue can occur in untreated ITP patients. The mechanism of fatigue in patients with ITP is unknown.

Patients with ITP are at slightly increased risk of venous and arterial thrombosis.¹⁵⁹ A recent retrospective study evaluating 986 patients with ITP showed the cumulative incidences of venous and arterial thrombosis to be 1.4 percent and 3.2 percent, respectively. This study found that increased thrombotic risk was associated with splenectomy, older age (>60 years), with the presence of more than two thrombotic risk factors at the time of diagnosis, and with glucocorticoid therapy.¹⁶⁰

Laboratory Features

In ITP patients the blood film usually demonstrates isolated thrombocytopenia without erythrocyte or leukocyte abnormalities. Platelet anisocytosis is a common finding. Mean platelet volume and platelet distribution width are increased. Platelets may be abnormally large or abnormally small. The former reflect accelerated platelet production,¹⁶¹ and the latter represent platelet fragments associated with platelet destruction.¹⁶² The observation of giant platelets should trigger consideration of IPDs, which often are misdiagnosed as ITP.¹⁶³ The bleeding time correlates inversely with platelet count if the count is less than 50 × 10⁹/L, but may be normal in patients with mild or moderate thrombocytopenia,¹⁶⁴ making it an unreliable test for use in such patients. The ultrastructure of ITP platelets viewed by electron microscopy is similar to that of normal platelets.¹⁶⁵

Hemoglobin concentration and hematocrit are generally normal in patients with ITP. Anemia that is not easily explained (e.g., resulting from iron deficiency in bleeding patients or associated with thalassemia minor in endemic areas) must be investigated further. Autoimmune hemolytic anemia with a positive direct antiglobulin (Coombs) test and reticulocytosis may accompany ITP; this association is termed Evans syndrome.¹⁶⁶ Neither erythrocyte poikilocytosis nor schistocytes should be present. Total leukocyte counts and differential are generally normal. Although atypical lymphocytes and eosinophilia may occur in children with ITP, leukocytosis and leukopenia with immature cells are not consistent with the diagnosis.

Marrow examination, which is not always required to make a diagnosis of ITP in adults, generally reveals a normal or increased number of megakaryocytes of normal morphology, although a decreased number of megakaryocytes does not rule out ITP. Erythropoiesis and myelopoiesis are normal. The international consensus report states that a marrow examination should usually be reserved for patients older than age 60 years, for those with systemic symptoms or other signs, and for those for whom splenectomy is contemplated. Biopsy for morphologic examination should be carried out, along with aspirate for flow cytometric and cytogenetic analysis.¹⁴⁷ The ASH 2011 guidelines, however, conclude that a marrow examination is unnecessary when the presentation is typical, even if the patients are older or being considered for splenectomy.¹⁴⁸

In ITP patients, initial workup should be targeted to exclude secondary causes of thrombocytopenia (see Table 117–3). Testing for viral etiology (hepatitis C virus [HCV], HIV, and in endemic areas hepatitis B virus [HBV]) and Helicobacter pylori is also recommended.^147,148 Quantitative immunoglobulin assessment should be considered for pediatric cases to rule out common variable immunodeficiency (CVID).¹⁴⁷ Mild thrombocytopenia has been reported in patients with hypo- or hyperthyroidism, which returns to normal after appropriate therapy. Thyroid-stimulating hormone (TSH) and antithyroid antibodies may help to evaluate thyroid status in those patients.¹⁴⁷ Other tests to consider include blood group analysis and a pregnancy test for female patients of childbearing age, antiphospholipid antibodies, antinuclear antibody (ANA), viral polymerase chain reaction (PCR) for parvovirus, and cytomegalovirus (CMV). The results of these tests can change the treatment strategy.¹⁴⁷ On the other hand, the ASH 2011 guidelines do not recommend routine testing for antiphospholipid antibodies and ANAs in the initial workup of ITP,¹⁴⁸ unless signs or symptoms of an autoimmune disorder are present in the patient. Other tests, such as TPO levels, reticulated platelets, PAIgG, platelet survival studies, bleeding time, and serum complement levels are not recommended for the diagnosis and management of ITP patients in either of these guidelines.^147,148

Therapy and Course

What little is known of the natural course of moderate or severe ITP derives from before the glucocorticoid era, and suggests that left untreated, ITP in adults typically is a chronic disease, in contrast to ITP in children. In adults, the rate of spontaneous remission is reported as 9 percent,¹⁶⁷ and can occur even after 3 years in patients who present with severe thrombocytopenia.¹⁶⁸ Although ITP is a benign disease, side effects of the therapies can cause serious morbidity and even mortality. Treatment for patients with ITP should be based on bleeding signs and symptoms and on the presence of factors that increase the bleeding risk (see Table 117–5). Possible side effects of the drugs and other treatments used in ITP should always be considered.

Initial Management

Observation Because a significant portion of ITP patients are diagnosed incidentally in routine evaluation, signs and symptoms of bleeding are important in determining whether any treatment is required. The primary therapeutic goal is not simply to increase the platelet count, but to reach a safe platelet count where the risk of bleeding is minimal. Patients with no bleeding and consistent platelet counts in excess of 30 × 10⁹/L do not require treatment and can be observed periodically. These patients are at low risk for clinically important bleeding. Simple observation is not recommended for patients with platelet counts lower than 10 × 10⁹/L, in those with platelet counts between 10 and 30 × 10⁹/L and significant mucosal bleeding, or in those with risk factors for bleeding (see Table 117–5).¹⁶⁹ The presence of extensive purpura or hemorrhagic bullae in mucosal tissues (wet purpura) should be regarded as a harbinger of life-threatening bleeding and treated as such. Because ITP patients often have large platelets that may not be recognized by automated cell counters, a blood film should be evaluated before starting therapy in ITP patients with very-low platelet counts who are not bleeding. Identification of secondary ITP cases is very important, and management of these patients should include treatment of the underlying pathology, if possible.

Emergency Treatment of Acute Bleeding Resulting from Severe Thrombocytopenia Bleeding symptoms generally are not severe in adult patients with ITP, even with very-low platelet counts. However, life-threatening bleeding can occur, especially after trauma. Emergency treatment should be instituted in patients with intracranial or gastrointestinal bleeding, massive hematuria or internal hematoma, or those in need of emergency surgical intervention or about to go into labor. Patients who experience significant bleeding should be hospitalized and monitored closely. Recommended treatment includes IVIG and parenteral glucocorticoids in combination. IVIG is given as 1 g/kg per day for 2 days, and high-dose parenteral glucocorticoid therapy includes high-dose prednisone or methylprednisolone (1 g/day for 1 to 3 days). In most patients, IVIG increases the platelet count within 2 to 3 days.^147,148 Although platelet transfusions may not increase the platelet counts because the transfused platelets are destroyed rapidly, they nevertheless may contribute to the formation of platelet plugs at sites of bleeding and improve hemostasis. Platelet transfusion following IVIG infusion may increase the platelet count because IVIG may improve platelet survival.^147,148,170 Aminocaproic acid, which inhibits fibrinolysis, can be used to reduce bleeding¹⁷⁰ and is safe except in the presence of hematuria, in which case it can cause thrombi of the glomeruli, renal pelves, and ureters. This agent does not affect platelet count or function. Aminocaproic acid is usually administered intravenously (initial dose 0.1 g/kg over 30 minutes, then given either by continuous infusion at 0.5 to 1.0 g/h or as an equivalent intermittent dose every 2 to 4 hours). Aminocaproic acid also can be administered orally in a similar dose in emergency situations because it is absorbed very rapidly from the gastrointestinal tract.^147,148 Vincristine can be used in combination with glucocorticoids and IVIG in older patients.¹⁰⁸ Other hemostatic therapies, such as recombinant factor VIIa and fibrinogen infusions, have been reported to be effective in some ITP patients with life-threatening bleeding, but the risk-to-benefit ratio needs to be evaluated in controlled studies.^171,172 Emergency splenectomy has been reported to be successful in refractory ITP with bleeding, but reports of its use in this situation are rare.¹⁷³ Because of this, this therapy should only be considered in the most dire circumstances. Although there are some case reports describing successful results with plasmapheresis, this treatment is not recommended in current ITP guidelines.^147,148

Glucocorticoid Therapy Glucocorticoids are accepted as the standard therapy for initial treatment in adult patients with ITP.^147,148 Glucocorticoids increase the platelet count in several ways, including by inhibiting phagocytosis of antibody-coated platelets by macrophages, decreasing autoantibody production, and improving marrow platelet production.^174,175 These agents also appear to reduce capillary leakage, thereby decreasing blood loss.¹⁷⁶ The major drawback of glucocorticoid therapy is that often the adverse effects of the treatment are worse than the disease itself. Important side effects, which can be severe, include facial swelling (chipmunk or moon facies), weight gain, folliculitis, hyperglycemia, hypertension, cataracts, osteoporosis, aseptic bone necrosis, opportunistic infections, and behavioral disturbances.^177,178

Still under investigation is which glucocorticoid and dosing regimen is best for raising the platelet count. Prednisolone, dexamethasone and methylprednisolone are all used. Generally, oral prednisone 1 to 2 mg/kg per day (or methylprednisolone at equivalent doses) is preferred as first-line therapy.^147,148 Patients usually respond to prednisone therapy within 3 weeks. In approximately two-thirds of patients, platelet counts increase to greater than 50 × 10⁹/L within 1 week, but decrease again when the prednisone dose is decreased.^152,177 Although no consensus exists regarding the duration of initial therapy, treatment should continue until platelet counts reach a safe range. In patients who respond, the recommendation is to continue glucocorticoid therapy 1 mg/kg per day for a total of 3 weeks before initiating a slow tapering of doses.¹⁴⁸ Sustained remission rates with glucocorticoid therapy are variable, reported rates ranging from 5 to 50 percent.^108,155,177 If the patient does not respond to 3 weeks of prednisone therapy, other therapeutic options should be considered.

In addition to the standard 1 to 2 mg/kg per day dose of prednisone, lower^179,180 and higher doses^181–184 of prednisone, dexamethasone, and methylprednisolone have been investigated, with good results. The major aim of the high-dose glucocorticoid regimes is to reduce duration of therapy, and therefore reduce the side effects of the glucocorticoids. Studies with dexamethasone 40 mg/day for 4 consecutive days for one course, or with the same dose for four courses given every 2 weeks have been reported to produce responses in 50 percent and 89.2 percent of newly diagnosed ITP patients, respectively.^185,186 High-dose methylprednisolone therapy has also been shown to be effective, with an 80 percent response rate.¹⁸⁷ Despite the favorable results of these studies, high-dose glucocorticoid regimens as first-line therapy still have not been validated with randomized controlled trials. ASH 2011 guidelines recommend longer courses of standard doses of glucocorticoids (prednisone 1 to 2 mg/kg per day) as a first-line treatment of ITP.¹⁴⁸

Splenectomy Splenectomy was demonstrated to be an effective treatment for patients with ITP a century ago¹⁸⁸ and after the glucocorticoid era, it has been used for decades as a standard second-line therapy. The spleen is the major site both for synthesis of antiplatelet antibodies and for destruction of antibody-coated platelets. Splenectomy will decrease antibody production and platelet destruction, and will be effective in patients in whom antibody-mediated platelet destruction rather than platelet production is the major cause of thrombocytopenia. Although splenectomy has been reported to be less preferred in recent ITP cohorts because of the emergence of new therapies such as TPO receptor agonists and rituximab,¹⁸⁹ splenectomy still produces the highest cure rates for ITP patients compared to all other therapies. Approximately 85 percent of patients with persistent or chronic ITP respond well to splenectomy, and 60 to 66 percent of the patients remain in remission after 5 years.^189–191 These high cure rates makes splenectomy an important therapeutic option in the treatment of chronic ITP. The duration of the disease prior to splenectomy does not affect the outcome of the procedure, as it can be effective even years after ITP is diagnosed.^192,193 Splenectomy can be performed during pregnancy (preferably during the second trimester), and does not affect the response rates to other treatments except anti-D therapy in chronic ITP patients. Also, the cost of splenectomy is lower than that of newer treatments such as rituximab and TPO-receptor agonists.¹⁹¹

On the other hand, splenectomy is an invasive procedure, causes the permanent loss of an organ, and increases the risk of serious bacterial infection, bleeding and thrombosis. Because ITP can remit spontaneously, splenectomy should be postponed at least 6 to 12 months after diagnosis if possible.^147,148 Splenectomy is not recommended in patients with CVID, with chronic infections such as chronic hepatitis and HIV, or with known thrombophilia.

No validated clinical or laboratory tests exist that can predict whether splenectomy will be effective in elevating platelet counts in ITP patients. Although it has been suggested that ITP patients with predominant splenic sequestration (as determined by radioisotope techniques) have better response rates than patients with predominantly nonsplenic sequestration, these data have not been validated in other studies¹⁸⁹ and the required radioisotope techniques are not widely available.

Over the past decade minimally invasive laparoscopic splenectomy has gained preference over open splenectomy. Modern laparoscopic approaches reduce mortality rates (<1 percent), even in patients with severe thrombocytopenia.¹⁹⁴ The mortality rate increases in older patients, in patients with severe thrombocytopenia, and in the presence of coexisting illnesses.^177,195 Postsplenectomy sepsis is a major cause of morbidity and mortality in ITP. Extended steroid or other immunosuppressive therapy preceding splenectomy may increase the risk of perioperative infection. To minimize the risk of sepsis, patients should be immunized at least 2 weeks before splenectomy with polyvalent pneumococcal vaccine, Haemophilus influenzae type B vaccine, and quadrivalent meningococcal polysaccharide vaccine.¹⁹⁶ Interestingly, newer studies of ITP patients undergoing splenectomy show enteric organisms to be responsible for most of the cases of postsplenectomy sepsis, probably because of the widespread vaccination of ITP patients.¹⁹¹ Splenectomized patients should be informed to be alert for the symptoms and signs of infection and be prepared for an emergency situation. Any fever should be carefully evaluated, and the patient treated with broad-spectrum antibiotics.

Splenectomy also increases the risk of thrombosis in ITP patients. In a large cohort of 9976 ITP patients, in whom 1762 underwent splenectomy; the cumulative incidences of abdominal venous thromboembolism and deep vein thrombosis/pulmonary embolism were increased in splenectomized patients compared to nonsplenectomized patients (1.6 percent vs. 1 percent for abdominal venous thrombosis, 4.3 percent vs. 1.7 percent for deep vein thrombosis–pulmonary embolism, respectively).¹⁹⁷ Several mechanisms may contribute to this enhanced risk for thrombosis, including postsplenectomy thrombocytosis and a failure to clear platelets, other cells and microparticles that express the procoagulant lipid phosphatidylserine. Perioperative measures such as antiembolic stockings and anticoagulant prophylaxis should be considered in those cases.

Both the time required to reach a normal platelet count and the magnitude of platelet recovery are accepted as useful predictors of the long-term efficacy of splenectomy. In most cases, platelet counts recover within 10 days. Patients who attain a normal platelet count within 3 days of splenectomy generally have a good long-term response.¹⁹⁸ In patients refractory to splenectomy, the presence of accessory splenic tissue should be suspected, particularly if the blood film shows no evidence of splenectomy (i.e., pitting and Howell-Jolly bodies are absent in the erythrocytes; Chap. 55). Such patients should be screened with sensitive radionuclide or magnetic resonance scans to identify residual or accessory splenic tissue.

Intravenous Immunoglobulin IVIG was first shown to be effective in childhood ITP in 1981,¹⁰⁹ then later in adult patients.¹⁹⁹ IVIG rapidly increases the platelet count in more than 75 percent of patients with chronic ITP and normalizes the platelet count in approximately 50 percent of the patients.^177,178 The effect of IVIG is similar whether or not the patient has undergone splenectomy and is transient, generally lasting only 3 to 4 weeks. Postulated mechanisms for the action of IVIG include blockade of macrophage Fc receptors, which slows clearance of antibody-coated platelets, antiidiotype neutralization of antiplatelet autoantibodies, cytokine modulation, immunomodulation (increased suppressor T-cell function and decreased autoantibody production), complement neutralization, and dendritic cell priming.^178,200,201 The recommended total dose of IVIG is 2 g/kg administered either as 0.4g/kg per day on 5 consecutive days or as 1 g/kg per day on 2 consecutive days. If the need to increase the platelet count is urgent, the preferred dosing is 1 g/kg per day for 2 days combined with glucocorticoids.¹⁴⁸ For maintenance therapy, 0.5 to 1.0 g/kg as a single dose may be used, administered every 3 to 4 weeks, or as needed. Although the annual total world consumption of IVIG exceeds 100 tons, the cost of IVIG is still high, and this also limits the use of IVIG in adults.²⁰² Adverse effects of IVIG therapy include headache, backache, nausea, fever, aseptic meningitis, alloimmune hemolysis, hepatitis, renal failure, pulmonary insufficiency, and thrombosis. Anaphylactic reactions may occur in patients with congenital IgA deficiency.¹⁷⁷ The patient may become refractory to the effect with repeated infusions of IVIG.²⁰³ IVIG is used as a first-line therapy in childhood ITP, because the thrombocytopenia is usually transient. In adult ITP, however, IVIG is usually reserved for patients with life-threatening bleeding, when a prompt increase in platelet count is needed,^147,148 or as first-line therapy when glucocorticoids are contraindicated.¹⁴⁸

Anti-(Rh)D Anti-(Rh)D is a polyclonal γ-globulin containing high titers of antibodies against the Rh_o(D) antigen of erythrocytes. It is administered intravenously for treatment of ITP. Anti-(Rh)D binds Rh-positive erythrocytes and leads to their destruction in the spleen. Because splenic Fc receptors are blocked, more antibody-coated platelets survive in the circulation.^204,205 Anti-(Rh)D also can also modulate Fcγ receptor expression and regulate the production of various cytokines, including IL-6, IL-10, and tumor necrosis factor-α.²⁰⁶ A positive direct antiglobulin test, a decrease in serum haptoglobin levels, and mild and transient hemolysis occur in all Rh-positive patients after anti-(Rh)D infusion, generally without requiring a blood transfusion.²⁰⁵ The rate of serious hemolytic reactions has been estimated as one in 1115 patients; any reaction occurs within 4 hours of administration in almost all cases.²⁰⁷ Anti-(Rh)D therapy is not effective in patients who have undergone splenectomy or in Rh-negative patients, and is not recommended in patients with a positive direct antiglobulin test.¹⁴⁸

It is recommended that anti-(Rh)D be given as a single dose of 50 to 100 mcg/kg by intravenous infusion over 3 to 5 minutes.^204,208,209 Adverse effects of anti-(Rh)D therapy resemble those observed with both γ-globulin infusion and autoimmune hemolytic anemia; symptoms include headache, asthenia, chills, fever, abdominal pain, diarrhea, vomiting, dizziness, and myalgia. Patients can experience immediate anaphylactic reactions and both type I (IgE-mediated) and type III (immune complex–mediated) hypersensitivity reactions.^{204,205,208,210,211} Although anti-(Rh)D reportedly increases platelet counts within 1 week in more than 70 percent of patients who are Rh-positive and have their spleen,²¹² and may obviate the need for splenectomy,²¹¹ a randomized, controlled trial comparing anti-(Rh)D with conventional therapy showed no differences in the rates of spontaneous remission or the need for splenectomy.²⁰⁹ Anti-(Rh)D is listed in current ASH ITP guidelines as a first-line agent when glucocorticoids are contraindicated.¹⁴⁸ Anti-(Rh)D is currently not available in Europe.

Rituximab B lymphocytes play many roles in the pathophysiology of ITP, including producing antibodies, presenting antigens, and regulating the functions of T cells and dendritic cells. B cells are targeted therapeutically with rituximab, a chimeric monoclonal antibody against CD20, which binds B cells and causes Fc-mediated lysis, thereby depleting these cells from blood, lymph nodes, and marrow. Rituximab rapidly depletes B cells in patients with autoimmune diseases, with the effect usually lasting 6 to 12 months.^{114,213–217}

The optimal dosing regimen and duration of therapy have not been determined for patients with ITP. Usual rituximab doses are in the range of 100 to 375 mg/m². Most studies have used weekly infusion for 4 consecutive weeks at, the dose used to treat B-cell lymphoma (375 mg/m²). Studies with low-dose rituximab (100 mg weekly for 4 weeks) showed similar activity to the standard dose.²¹⁸ Published studies with rituximab, however, have generally not been controlled and are extremely heterogeneous in terms of rituximab dosing and response criteria. Approximately 40 to 60 percent of the ITP patients demonstrate a response to rituximab at 1 year, and 20 to 25 percent of those have a long-term response (at 5 years).^215,219 Splenectomy does not affect response rates to rituximab therapy.^135,216 In ITP patients who have relapsed more than 1 year after rituximab therapy, retreatment with the drug will induce similar responses in 75 percent of patients who responded initially.²²⁰ In spite of the apparent benefit of rituximab, its use is still considered “off-label” for ITP.

Different patterns of response have been reported in ITP patients treated with rituximab. Although the majority of patients responded within 4 to 6 weeks (early responders), response was delayed for several months in some patients (late responders). In ITP patients who responded to rituximab, the increase in platelet count was associated with reduction in the quantity of platelet-associated autoantibodies. Rituximab also indirectly affects T cells, as depletion of autoreactive B cells prevents T-cell activation. Interestingly, despite the depletion of peripheral B cells, platelet-associated autoantibodies were still found in the plasma of ITP patients who do not respond to rituximab.²²¹ A study analyzing the spleens of ITP patients who did not respond to rituximab therapy demonstrated the presence in the spleen of long-lived plasma cells that produced antiplatelet antibodies for as long as 6 months after rituximab therapy ended. However, this class of cells was not found in the spleens of patients who had not received rituximab. The authors of this study suggested that depletion of peripheral B cells by rituximab promotes the differentiation of long-lived plasma cells in the spleen of ITP patients, which might be responsible for the persistence of antiplatelet antibodies.²²²

In a meta-analysis of 306 ITP patients treated with rituximab, adverse reactions were reported as mild-to-moderate in 66 patients (21.6 percent) and life-threatening in 10 patients (3.7 percent); nine patients (2.9 percent) died.²¹⁵ Although some of these deaths were attributed to ITP-related complications and not to rituximab itself, this mortality rate is higher than expected. Infusion-related reactions in rituximab therapy can be severe, and rarely fatal. Premedication with methylprednisolone is recommend to avoid these reactions.²¹⁹ The risk of infection can increase as a result of depletion of B cells, decreased antibody production, and, rarely, neutropenia.²¹⁹ Treatment can also reactivate latent viruses, especially hepatitis B. Alteration of T- and B-cell populations, and decreased antibody titers against HBV may stimulate HBV replication, and rarely cause fatal fulminant hepatitis. All patients should be screened for HBV before rituximab therapy.²²³ Although preventive lamivudine or entecavir can be used in HBV-positive ITP patients, it is instead recommended that alternative therapies be used.²¹⁹ Other viral reactivation syndromes are less common; progressive multifocal leukoencephalopathy (caused by reactivation of polyomavirus JC) is extremely rare.

Thrombopoietin Receptor Agonists The observation that platelet production in patients with ITP is impaired, the massive megakaryopoiesis seen in the marrow of mice and humans treated with recombinant TPO (far greater than seen in patients with ITP), and the unexpectedly normal or only modestly elevated TPO levels in patients with ITP suggested the potential benefit of megakaryocyte-stimulation therapy in patients with refractory ITP. Early use of an altered form of a recombinant TPO molecule to stimulate platelet production in normal platelet donors was halted because of its stimulation of autoantibodies that cleared endogenous TPO. Because of this untoward effect, the use of recombinant TPO was abandoned, and a search for molecules that might bind to and stimulate the TPO receptor ensued. Since then, the TPO receptor agonists romiplostim and eltrombopag have been clinically shown to stimulate platelet production.^224,225

Romiplostim This drug is a peptibody that carries four copies of a 14-amino-acid TPO-receptor–binding peptide fused to an immunoglobulin scaffold, and binds to the TPO-binding site of the TPO receptor with high affinity. The TPO receptor agonist induces megakaryocyte proliferation and differentiation by activating Janus-type tyrosine kinase (JAK)–signal transducer and activator of transcription (STAT) and mitogen-activated protein (MAP) kinase pathways.²²⁴ The insertion of dimeric peptide into the IgG₁ heavy chain increases the half-life of the molecule.²²⁵ Romiplostim has no homology with endogenous TPO, thus the risk of the development of antibodies against TPO is very low. Romiplostim and TPO may also increase platelet responses to agonists. Weekly subcutaneous injection of romiplostim at doses of 1 to 3 mcg/kg produced a dose-dependent increase in the platelet count, starting from day 5, with peak platelet levels reached by days 12 to 15, and platelet counts returning to baseline by day 28.^131,135,224 Therapy for ITP is usually initiated at a dose of 1 mcg/kg per week, and the dose is then increased by 1 mcg/kg to a maximum of 10 mcg/kg until the patient reaches target platelet counts (>50 × 10⁹/L). Higher starting doses up to the maximum dose can be used in emergency situations. If the platelet count does not increase to safe levels after 4 weeks of romiplostim treatment at the maximum dose, the drug should be discontinued. Because platelet responses are highly variable, patients should be evaluated periodically, and the dose adjusted based on the platelet counts. Although discontinuation of romiplostim is recommended when the platelet count exceeds 400 × 10⁹/L, it should be kept in mind that platelet counts can drop to extremely low levels. Close monitoring of the platelet counts is therefore crucial. Romiplostim can be used in patients with hepatic or renal insufficiency, but is not recommended in pregnant patients because it can cross the placenta. Two parallel placebo-controlled trials examined response rates to romiplostim in both splenectomized and nonsplenectomized patients treated for 24 weeks.¹³⁴ Durable platelet responses and overall platelet responses were achieved by 38 percent and 79 percent of splenectomized patients, and by 61 percent and 88 percent of nonsplenectomized patients who were given the drug. A newer study evaluating long-term (up to 5 years) results of romiplostim therapy showed that a platelet count of greater than 50 × 10⁹/L was achieved at least once by 95 percent of treated ITP patients.²²⁶ Recent evaluation of large databases of patients who were administered Romiplostim has revealed a reasonable number in whom discontinuation of the drug was not met with relapse of thrombocytopenia, or at least not severe thrombocytopenia.²²⁷

Eltrombopag This agent is a small (442 Da) nonpeptide molecule that binds to the transmembrane domain of the TPO receptor and triggers megakaryocyte growth and differentiation, increasing platelet production. Eltrombopag has some distinctive features compared to recombinant human thrombopoietin (rhTPO) and romiplostim: Eltrombopag does not compete with TPO binding, and while it induces the phosphorylation of STAT proteins, it does not affect the AKT pathway.²²⁸ Eltrombopag has no effect on platelet activation in response to agonists.²²⁵ In healthy volunteers, daily doses given for 10 days elevated platelet counts beginning at 8 days and peaking at 16 days. Eltrombopag is used orally at daily doses of 25 to 75 mg, and should be given 2 hours before or after meals because food can affect its absorption. Ethnic differences in eltrombopag pharmacokinetics have been described. Lower initial doses and slower titration is preferred in East Asian patients.²²⁹ Divalent cations such as calcium interfere with absorption of the drug, so it should not be taken with dairy products or antacids. Eltrombopag can also interfere with the uptake and metabolism of statins, increasing their plasma concentrations. Eltrombopag is metabolized in the liver, and causes liver function abnormalities in approximately 13 percent of patients administered the drug. Reduced initial doses are recommended in patients with liver disease.²²⁵ Eltrombopag increases platelet counts (50 × 10⁹/L) in 80 percent of splenectomized and 88 percent of nonsplenectomized chronic ITP patients.²³⁰ A newer study evaluated repeated short-term doses of eltrombopag (50 mg daily for up to 6 weeks followed by up to 4 weeks off therapy over three cycles) and suggested that eltrombopag can be used as on-demand therapy and repeated courses would be effective and safe.²³¹

Newer Thrombopoietin Receptor Agonists Other congeners are currently being evaluated. An oral, nonpeptide TPO-receptor agonist, avatrombopag, binds to the transmembrane domain of TPO receptor, and increases platelet counts. Lack of significant food interaction is an important feature of this new drug.¹³⁶ Its use is pending FDA approval.

Common side effects of TPO receptor agonists include mild headache, arthralgia, nasopharyngitis, fatigue, diarrhea, and nausea. These side effects are generally mild and usually of insufficient severity to cause the discontinuation of the drugs. Abnormalities of liver function tests (elevated alanine aminotransferase [ALT], aspartate aminotransferase [AST], and bilirubin levels) occur in approximately 2 percent of ITP patients receiving eltrombopag therapy but not with romiplostim.²²⁵ Autoantibodies against romiplostim may develop but rarely have neutralizing activity.

TPO-receptor agonists can induce extreme thrombocytosis, sometimes exceeding 1000 × 10⁹/L. Careful dose titration is very important, because cessation of the TPO-receptor agonists cause rebound thrombocytopenia in approximately 10 percent of ITP patients.²²⁵ Rates of thromboembolic events were reported as 6.5 percent and 4 percent with extended romiplostim and eltrombopag treatment, respectively.^225,231 The authors of these studies concluded that thromboembolic events are not associated with the dose of TPO receptor agonists or platelet counts, and at least one acquired and inherited thrombotic risk factor was present in most of the patients who experienced thrombosis while they were taking TPO-receptor agonists.^226,231 Nevertheless, the frequency of thrombosis in these studies was slightly higher than observed in other ITP studies.¹⁶⁰ Secondary myelofibrosis (increased marrow reticulin) is sometimes associated with therapy with TPO receptor agonists, and is usually reversible. Concerns have also been expressed that these drugs might accelerate the progression of hematologic and solid malignancies. Under normal circumstances, expression of the TPO receptor (mpl) is highly restricted to hematological tissues including marrow, spleen, placenta, brain and fetal liver cells. TPO receptor expression has been demonstrated on the leukemic cells of patients with acute myelogenous leukemia (AML) and MDS, but not in lymphoid malignancies, myeloproliferative neoplasms, or other nonhematologic malignancies.²³² Although romiplostim therapy was discontinued in a study of its use in patients with low-/intermediate-risk MDS and thrombocytopenia because of increased blast and AML rates (interim hazard ratio: 2.51), long-term analysis of the study showed similar survival and AML rates in the romiplostim and control groups.²³³ The question of whether use of TPO-receptor agonists increases the risk of leukemia warrants further study.

Azathioprine This purine analogue is converted to 6-mercaptopurine following gastrointestinal absorption and works by suppressing the immune response. At least 4 months of azathioprine therapy at doses ranging from 50 to 250 mg/day are necessary to evaluate therapeutic efficacy. One study reported that azathioprine produced a sustained normalization of the platelet counts in up to 45 percent of patients with refractory ITP.²³⁴ Azathioprine can be used in pregnancy if necessary (see “Thrombocytopenia During Pregnancy” below). As with other immunosuppressive drugs, major adverse effects are marrow suppression and possible increased risk of secondary malignancy.^177,235

Cyclophosphamide This alkylating drug can be used orally (50 to 200 mg/day) or parenterally (1.0 to 1.5 g/m² IV every 4 weeks) in patients with refractory ITP.^236,237 It increases platelet counts in 60 to 80 percent of patients with ITP, and 20 to 40 percent of those patients will remain in remission for 2 to 3 years¹⁷⁷ after receiving 2 to 3 months of therapy. Its beneficial action is linked to its immunosuppression. The major complications of cyclophosphamide therapy are marrow suppression, hemorrhagic cystitis, infertility, alopecia, and secondary malignancy.

Cyclosporine Cyclosporine is an immunosuppressive drug inhibiting T-cell function, and is primarily used to prevent rejection in patients with organ transplantation. Although cyclosporine may induce a durable remission in patients with ITP when used at relatively low doses (2.5 to 3.0 mg/kg/day),²³⁸ experience with cyclosporine in ITP patients is usually based on small case series. Cyclosporine has several side effects, some potentially serious, including fever, increased risk of opportunistic infections, gingival hyperplasia, diarrhea, peptic ulcer, pancreatitis, renal dysfunction, elevated liver enzymes, hypertension, peripheral neuropathy, convulsions, hirsutism, and increased risk of secondary malignancy.

Danazol This synthetic androgen, with reduced virilizing effects compared to other androgens, has been used to treat patients with refractory ITP. Given at doses of 400 to 800 mg/day for at least 6 months, reported response rates range from 10 to 80 percent.^177,235 Danazol is postulated to decrease Fc receptor numbers on phagocytic cells by antagonizing the effects of estrogens.¹⁵³ Danazol should not be given to pregnant women or patients with liver disease. Common side effects of danazol therapy are weight gain, fluid retention, seborrhea, hirsutism, secondary amenorrhea, vocal changes, acne, hepatic toxicity, headache, lethargy, cholesterol spectrum abnormalities (i.e., reduced high-density lipoprotein [HDL] cholesterol) and myalgia. Because liver dysfunction is common with these doses of danazol therapy, liver function should be evaluated monthly.^153,177,235

Dapsone Dapsone possesses antibacterial and antiinflammatory effects; it is primarily used for leprosy, malaria, and some types of dermatitis. When used at a dose of 75 to 100 mg/day, dapsone may increase platelet counts in patients with persistent, refractory, or chronic ITP.^147,239,240 The median time to response is long, up to 2 months. Partial and CR rates are approximately 50 percent and 20 percent, respectively, but platelet counts return to baseline levels after discontinuation of the therapy.^239,240 The mechanism of dapsone action in ITP is not known. The most important side effects are nausea, headache, skin rashes, hepatitis, cholestasis, dose-dependent hemolysis, and methemoglobinemia. Dapsone should not be given to patients with glucose-6-phosphate dehydrogenase deficiency.

Vinca Alkaloids Both vincristine and vinblastine transiently increase the platelet count in approximately 70 percent of ITP patients within 5 to 21 days, but produce sustained remissions in only 10 percent of treated patients.^{108,153,177,235} The recommended dose of vincristine is 1 to 2 mg and of vinblastine is 0.1 mg/kg (maximum: 10 mg), both given by bolus injection at 1-week intervals for a minimum of three courses. It has been proposed that vinca alkaloids bind to platelet microtubules and thereby are transported to the spleen, where they subsequently inhibit the phagocytic functions of splenic macrophages. They may also stimulate megakaryopoiesis. Peripheral neuropathy, neutropenia, jaw pain, alopecia, and constipation are complications of treatment with vinca alkaloids.^{235,241–243}

Other Therapies ITP patients with H. pylori infection should receive eradication therapy.¹⁴⁸ Many other therapies, including interferon-α,²⁴⁴ immunoadsorption with staphylococcal protein A,²⁴⁵ ascorbic acid,²⁴⁶ colchicine,²⁴⁷ and plasmapheresis,²⁴⁸ have been studied for refractory ITP cases, but none has been clearly demonstrated to be effective.

Accessory Therapies Adjunctive therapies include agents designed to reduce bleeding without necessarily affecting the platelet count. Aminocaproic acid or tranexamic acid, both of which inhibit fibrinolysis, can be used for excessive mucosal bleeding. Local bleeding can be controlled by compression and use of gelatin sponges, fibrin sealants, or antifibrinolytic-embedded gauze. Avoiding the use of antiplatelet drugs, contact sports, and activities that increase bleeding risk, and educating patients about maintaining dental hygiene are very important. Menorrhagia is a common problem in patients with chronic ITP; gynecologic evaluation of uterine problems is crucial. Oral contraceptives and hormonal intrauterine devices together with antifibrinolytic drugs may help to reduce excessive menstrual bleeding in these patients.

SECONDARY IMMUNE THROMBOCYTOPENIA

Secondary ITP is defined as immune-mediated platelet destruction in the presence of other conditions, including infections, lymphoproliferative disorders, solid tumors, SLE, or the antiphospholipid syndrome (APS) (Fig. 117-4).²⁴⁹ ITP can sometimes be the presenting sign of the illness, or may develop during the course of the disease or with certain therapies. Thrombocytopenia in a patient with chronic disease may develop for other reasons, and the diagnosis of immune-mediated platelet destruction may require more detailed tests. Generally, thrombocytopenia is not severe in patients with secondary ITP, but bleeding risk may be enhanced at a particular platelet count because of the underlying disorder. The treatment strategy should be tailored to the individual patient.

IMMUNE THROMBOCYTOPENIA IN PATIENTS WITH ANTIPHOSPHOLIPID SYNDROME, SYSTEMIC LUPUS ERYTHEMATOSUS AND OTHER CONNECTIVE TISSUE DISORDERS

Thrombocytopenia in the Antiphospholipid Syndrome

APS is characterized by recurrent arterial and/or venous thrombosis and well-defined morbidity during pregnancy in the presence of antiphospholipid antibodies (APLAs) (Chap. 132).²⁵⁰ APS may affect any organ in the body, including the heart, brain, kidney, skin, lung, and placenta. This syndrome predominantly affects females (female-to-male ratio 5:1), especially during the childbearing years.²⁵¹ APLAs (lupus anticoagulant; anticardiolipin antibodies; anti–β₂-GPI antibodies) represent a heterogeneous family of antibodies that react with anionic phospholipids and phospholipid–protein complexes. Despite overwhelming evidence that APLAs are associated with thrombosis, the mechanisms remain uncertain. Many have been proposed, including endothelial cell damage and apoptosis, inhibition of prostacyclin release from endothelial cells, inhibition of the protein C–protein S anticoagulant system, induction of tissue factor, activation of platelets and the complement system, interference with antithrombin, impairment of fibrinolytic activity, and inhibition of annexin V binding to membrane phospholipids, eliminating the antithrombotic effect of annexin V.^252–255 APS is considered one of the most common causes of acquired thrombophilia.^256,257

Thrombocytopenia is reported in approximately 20 to 40 percent of patients with APS, usually is mild (70 to 120 × 10⁹/L), and does not require clinical intervention. Severe thrombocytopenia (platelet counts <50 × 10⁹/L) occurs in 5 to 10 percent of patients.^258–260 Although thrombocytopenia was a clinical criterion used to define the syndrome in the initial classification of APS,²⁶¹ it was not included in the most recently proposed classification.²⁶² Because ITP patients who present with APLAs are at increased risk for thrombosis,²⁶³ measurement of APLA, especially lupus anticoagulant, in patients diagnosed with ITP may identify a subgroup at high risk for developing APS. The pathogenesis of thrombocytopenia in APS is not clear. Potential mechanisms explaining thrombocytopenia in APS patients include APLA-related direct platelet destruction, immune platelet destruction by antibodies against platelet GPs, complement-mediated platelet destruction, and platelet aggregation and consumption. Evidence indicates APLAs bind platelet membranes and cause platelet destruction, but the link is not definitive. Some investigators suggest that antibodies against platelet GPs, rather than APLAs, are responsible for thrombocytopenia in patients with APS. Antibodies against the integrin α_IIββ₃ or GPIb–IX–V complexes are found in approximately 40 percent of thrombocytopenic patients with APS.²⁶⁴ Such antibodies do not cross-react with antibodies against phospholipids or β₂-GPI.²⁶⁵ Immunosuppressive treatment in these patients increases the platelet count and reduces the titers of anti-GP antibodies but not the titers of APLAs.²⁶⁶ These data suggest that thrombocytopenia is a secondary immune phenomenon that develops concomitantly with APS. Against this conclusion, platelet antigens in thrombocytopenic patients with APS were found to be different from those in ITP and the antibodies to display virtually no reactivity with membrane GPs.²⁶⁷ CD40 ligand on platelets is another possible antibody target. Anti-CD40 ligand antibodies have been found in patients with APS (13 percent) and ITP (12 percent), but not in healthy controls; and it was suggested that these antibodies cause thrombocytopenia.²⁶⁸ Platelet activation, aggregation, and consumption (APS-associated thrombotic microangiopathy) may also contribute to thrombocytopenia.²⁶⁰ Another issue of clinical importance in evaluating thrombocytopenia associated with APS is the risk for future development of thrombosis. In one study in which APS patients were divided into three groups according to platelet counts as normal, moderately thrombocytopenic (50 to 100 × 10⁹/L), or severely thrombocytopenic (<50 × 10⁹/L), the rates of future thrombosis were 40 percent, 32 percent, and 9 percent, respectively.²⁶⁹ These data show that moderate thrombocytopenia does not prevent thrombosis in patients with APS. Antithrombotic prophylaxis should be considered in these patients whenever it is possible.^258,269

Although thrombocytopenia is a common finding in patients with APS, bleeding complications are rare, even with severe thrombocytopenia. Bleeding in an APS patient with moderate thrombocytopenia should trigger evaluation for the presence of antiprothrombin antibodies²⁷⁰ and other disorders that may affect hemostasis, such as DIC, liver insufficiency, and uremia. Severe thrombocytopenia may require therapy, with treatment strategies similar to those used for patients with ITP. Glucocorticoids are effective in only 15 percent of patients.²⁵⁸ IVIG and immunosuppressive drugs such as azathioprine and cyclophosphamide can be used in patients with severe bleeding and “catastrophic” APS. In general, splenectomy should be postponed as long as possible, and is only preferred in patients with severe bleeding. Splenectomy may produce sustained remission in approximately two-thirds of patients as in patients with primary ITP.^167,271,272 Because of their increased risk of thrombosis, patients should be prophylactically anticoagulated in the immediate postoperative period. Rituximab has been used to treat refractory thrombocytopenia in patients with APS, with a wide range of results.^273–275 Although there is no consensus on dosing and schedule with rituximab therapy, it is generally administered as in patients with ITP (see ITP therapy in “Therapy and Course” above). TPO receptor agonists may increase thrombosis risk in patients with APS and SLE and these diagnoses in a patient with ITP were accepted as exclusion criteria in some randomized controlled studies of TPO-receptor agonists.¹³⁶ Two case reports described acute renal failure (one was a result of thrombotic microangiopathy) after use of eltrombopag.^276,277

Thrombocytopenia in Patients with Systemic Lupus Erythematosus and Other Connective Tissue Disorders

SLE is a complex autoimmune disease that primarily afflicts women of childbearing age. The autoimmune attack in SLE is not organ specific; it may affect any tissue in the body. The diagnostic criteria for SLE are based on a classification system proposed by the American College of Rheumatology.^278,279 The presence of hematologic findings (leukopenia, thrombocytopenia, or hemolytic anemia) is one of the criteria in the diagnosis and classification of SLE. Thrombocytopenia is common in patients with SLE, occurring in 20 to 40 percent of patients, and may be a presenting symptom.²⁸⁰ Immunologic destruction of platelets is also seen in several other autoimmune conditions, including polyarteritis nodosa, rheumatoid arthritis, mixed connective tissue disease, and Sjögren syndrome, albeit at much lower rates than in SLE.

The causes of thrombocytopenia in SLE are many and include platelet destruction (ITP, DIC, thrombotic thrombocytopenic purpura [TTP] or hemolytic uremic syndrome [HUS], sepsis, drugs), ineffective hematopoiesis (megaloblastic anemia), abnormal platelet pooling (hypersplenism), marrow hypoplasia (from drugs and infections), and dilutional thrombocytopenia related to therapy. Severe thrombocytopenia is relatively rare, occurring in 5 percent of patients.²⁸⁰ Although clinically significant bleeding is uncommon even in patients with severe thrombocytopenia, fatal gastrointestinal, cerebral, and pulmonary bleeding have been reported. Among the many potential contributors to thrombocytopenia in SLE patients, platelet destruction by autoantibodies is the major mechanism. Antiplatelet antibodies are present in up to 60 percent of SLE patients.^281,282 The presence of antiplatelet antibodies is correlated with low platelet counts and increased disease severity.²⁸² Besides the antiplatelet antibodies, APLAs (see “Thrombocytopenia in the Antiphospholipid Syndrome” above) and circulating immune complexes that bind platelets may nonspecifically accelerate platelet destruction.²⁸³ Specific antiplatelet antibodies, especially those against integrin, have an important role in the pathogenesis of thrombocytopenia in SLE patients.^281,282,284 In general, marrow megakaryocytes are normal or increased, and platelet production is not affected in SLE patients with thrombocytopenia. However, decreased numbers of megakaryocytes and even amegakaryocytic thrombocytopenia have been reported.^86,285 High levels of TPO in the plasma, and both anti-TPO and anti-TPO receptor antibodies have been reported in SLE patients,^286,287 the latter associated with a decrease in marrow megakaryocytes and thrombocytopenia.²⁸⁷ Thrombocytopenia in SLE is associated with serious organ pathology, leading to neuropsychiatric disease,²⁸⁸ renal disease,^289,290 and APS,²⁹¹ and is an independent indicator of poor prognosis.^290,292,293 A study of selected SLE families in which at least one affected member was thrombocytopenic reported genetic linkage to loci at chromosomes 11p13 and 1q22–23.²⁹⁴ A severe lupus phenotype was much more common in patients with thrombocytopenia and their affected family members than in patients from families with no thrombocytopenic patients. Therefore, thrombocytopenia in a family member may herald severe lupus in familial SLE.

There are no well-established treatment strategies for severe thrombocytopenia in patients with SLE. Because SLE ranges in severity from milder forms with easily controlled symptoms and signs to severe forms that can be fatal, the treatment of severe thrombocytopenia should be tailored to the individual patient. Patients with severe thrombocytopenia are generally treated with glucocorticoids as first-line therapy, but sustained remission is infrequent. Because most patients with severe thrombocytopenia also have nephritis and neurologic symptoms, they receive immunosuppressive therapy either alone or combination with glucocorticoids.^295–298 IVIG is reserved for use in patients with severe bleeding.^299,300 It is well-known that B lymphocytes play an important role in the pathogenesis of SLE. Although lymphopenia is common in patients with active SLE, autoantibody-producing B cells have been shown to be expanded, and B cells were found to be more sensitive to inflammatory cytokines.³⁰¹ B-cell targeted therapy—rituximab—is effective in the treatment of refractory SLE patients, especially those with nephritis and severe thrombocytopenia.³⁰¹ A retrospective study evaluating the long-term effects of rituximab therapy in 65 patients with refractory ITP associated with SLE and mixed connective tissue disease reported an overall response rate of 80 percent.³⁰² Although case series indicate that splenectomy yields sustained remission in 61 percent of SLE patients with severe thrombocytopenia²⁹⁶ and is relatively safe in terms of perioperative complications,³⁰³ splenectomy may increase the risk of thrombotic complications in SLE patients,³⁰⁴ and may also increase the risk of infection if the patients require further immunosuppressive therapy.

THROMBOCYTOPENIA IN INFECTIOUS DISEASES

The first recorded observation of purpura was made in patients with fever, and purpura was accepted as a sign of severe infections for centuries. Thrombocytopenia can be seen in patients with viral, bacterial, fungal and parasitic infections. Infection can decrease platelet levels in several ways: by decreasing production in the marrow, by increased immune destruction, or by inducing microangiopathy as seen in patients with infection induced DIC or HUS. In addition, drugs used for the treatment of an infection can contribute to thrombocytopenia (see “Drug-Induced Thrombocytopenia” below).

Viral infections are an important cause of secondary ITP. ITP can be seen after a viral infection, especially in children, and usually resolves within 2 to 8 weeks. In patients with viral infections such as rubella, mumps, and infectious mononucleosis, thrombocytopenia can be present with other clinical signs and symptoms. Adult patients with isolated thrombocytopenia with no obvious causes should be screened for HIV, HCV and, in endemic areas, for HBV. Because other clinical symptoms and signs associated with infection with these viruses may not be present initially, and it may not be possible to distinguish these cases from primary ITP.

HIV is a leading cause of isolated thrombocytopenia in Western countries. Thrombocytopenia associated with HIV infection has numerous causes, many of which can be present simultaneously. These include accelerated platelet destruction primarily related to immune complexes, decreased platelet production, especially in advanced disease, splenic sequestration, and, rarely, platelet consumption associated with TTP. Medications, concurrent infections such as hepatitis C, and hematologic malignancies may contribute to the development of thrombocytopenia (Chap. 83).^305–308

HCV is another important cause of thrombocytopenia in adults. It is a hepatotrophic RNA virus of the Flaviviridae family. HCV infection is chronic in approximately 85 percent of the infected individuals and progresses to cirrhosis in 20 percent of these individuals. The World Health Organization (WHO) estimates that approximately 3 percent of the world's population is infected with HCV, the prevalence ranging from 0.5 to 2 percent in Western countries to 20 percent in some underdeveloped countries.³⁰⁹ HCV causes thrombocytopenia through different mechanisms, including hypersplenism, decreased TPO level associated with liver insufficiency, the effect of drugs (pegylated interferon [IFN] and ribavirin), and immune-mediated platelet destruction.³¹⁰ Immune dysregulation in HCV is associated with several autoimmune disorders, including arthritis, Sjögren syndrome, cryoglobulinemia, and immune cytopenias.³¹¹ As a potential mechanism of immune destruction, one study demonstrated binding of both free and IgG-complexed HCV to platelets.³¹² In secondary ITP associated with HCV infection, antiviral therapy with pegylated IFN and ribavirin will decrease viral load and may also treat thrombocytopenia. However, platelet counts can be unaffected or even decrease after these therapies. Severe thrombocytopenia interferes with optimal HCV treatment, and may increase bleeding risk. In this situation, the ASH 2011 guideline recommends IVIG as a first-line therapy, because glucocorticoids may increase viral load.¹⁴⁸ Glucocorticoids and splenectomy both appear to be effective treatments for thrombocytopenia, but their use should be balanced against other considerations after discussion with a hepatologist. TPO receptor agonists may increase the risk of abdominal thrombosis in HCV patients with liver cirrhosis.³¹³

The potential role of H. pylori in the pathogenesis of chronic ITP is controversial. Japanese and Italian studies showed that eradication of H. pylori with antibiotics resulted in marked platelet count increases in patients with ITP. However, this success was not reproduced in American and other European studies.³¹⁴ It appears that response rates are higher in countries where H. pylori infection is endemic. ITP patients treated for H. pylori had higher platelet counts than untreated ITP patients, even if the therapy was unsuccessful in eradicating the infection.³¹⁵ It has therefore been speculated that the antibiotic therapy, rather than eradication of H. pylori, may be the factor improving platelet counts. However, meta-analysis found that H. pylori eradication therapy was much more likely to increase platelet counts in patients with H. pylori infection than in uninfected patients,³¹⁶ strengthening the case for a causal relationship between infection and thrombocytopenia. On the other hand, eradication was shown to be less effective in patients with severe thrombocytopenia.³¹⁰ The recent ASH ITP guideline suggests that ITP patients be screened for H. pylori and for eradication therapy to be used if testing is positive.¹⁴⁸

Thrombocytopenia is the second most common hematologic problem in pregnancy, after anemia. Table 117–6 lists the major causes of thrombocytopenia in pregnancy (Chap. 7). Platelet counts tend to decrease during normal pregnancy, and mild thrombocytopenia (platelet counts ranging from 120 to 150 × 10⁹/L) occurs with moderate frequency, especially during the third trimester.^317,318 Bleeding symptoms are generally mild, even in patients with severe thrombocytopenia, probably because of the procoagulant state of pregnancy. Nevertheless, it is important to investigate the cause of thrombocytopenia and exclude the disorders associated with significant morbidity such as eclampsia and hemolysis, elevated liver enzymes, low platelets (HELLP) syndrome (Table 117–6). A medical history should include previous blood counts, history of other diseases, nutritional status, and intake of drugs and herbal supplements. It is important to be alert to constitutional symptoms including fever, and, especially, weight loss; neurologic abnormalities, arthritis, rash, and icterus. Key steps in the evaluation of thrombocytopenia in a pregnant woman include blood pressure measurement, evaluation of coagulation parameters, liver and kidney function tests, and examination of the blood film. Physical examination of the abdomen may be difficult in the third trimester and abdominal ultrasound may be required to detect organomegaly. If there are no suspicious clinical or laboratory findings, marrow aspiration is considered unnecessary.^318,319

GESTATIONAL THROMBOCYTOPENIA

Gestational thrombocytopenia is detected in 5 to 7 percent of otherwise healthy pregnant women, accounting for 64 to 80 percent of patients with thrombocytopenia at term.^320–322 Gestational thrombocytopenia is a benign disorder and is not associated with an increased risk of bleeding. Platelet counts are greater than 70 × 10⁹/L^{317,318,320,321} and return to normal after delivery.

The pathogenesis of gestational thrombocytopenia is unknown. Several mechanisms have been proposed, including hemodilution, a compensated state of subclinical coagulopathy, endothelial cell injury, and immune destruction. Some authors have proposed platelet consumption by the placenta and hormonal depression of megakaryopoiesis as causes of gestational thrombocytopenia, as suggested by the rapid return of the platelet count to normal after delivery and by the transient normalization of the platelet count during pregnancy in some cases of essential thrombocythemia.^322–325 Discriminating gestational thrombocytopenia from ITP can be difficult because ITP is also common in young women, and is often exacerbated by pregnancy. Neither condition can be definitively diagnosed by currently available tests. The diagnosis of ITP is favored if the patient had a previous episode of ITP unassociated with pregnancy or if the thrombocytopenia is severe and associated with bleeding that occurs in the first trimester. In healthy pregnant women, a platelet count greater than 70 × 10⁹/L late in pregnancy does not require intensive investigation, because bleeding is not likely in the woman or her newborn child.³²⁶

IMMUNE THROMBOCYTOPENIA INPREGNANCY

ITP is responsible for 4 to 5 percent of all cases of pregnancy-associated thrombocytopenia.^320,322 Pregnancy itself may induce ITP, or exacerbate preexisting ITP, but generally the platelet count returns to the prepregnancy level after delivery. Diagnosis of ITP in a pregnant woman requires the exclusion of other causes of thrombocytopenia as in a nonpregnant woman, but also requires the evaluation of other pregnancy-related causes (see Table 117–6). However, the management of ITP during pregnancy is different than in nonpregnant women. First, many of the drugs used to treat ITP may complicate pregnancy-related problems such as gestational diabetes, hypertension, and psychiatric disorders. Second, the fetus can also be affected by ITP and its treatment. Antiplatelet antibodies can cross the placenta, decrease the fetal platelet count, and sometimes cause bleeding.³²¹ ITP drugs can affect fetal development and growth, a fact to be considered in selecting therapy during pregnancy. And third, all pregnancies will end with delivery of the baby, a process that may happen unexpectedly. Preparation for delivery in a pregnant ITP patient requires close collaboration between the hematologist, the obstetrician, and the neonatologist.

In the management of pregnancy-related ITP, bleeding symptoms and platelet counts should be considered.^147,148 Although previous guidelines have defined threshold platelet levels for treatment during pregnancy and labor, these numbers are arbitrary and not based on randomized controlled studies. Generally, observation without therapy is appropriate if the platelet count is greater than 30 × 10⁹/L and the patient has no bleeding symptoms. Therapy is required for a pregnant woman who is bleeding, has a platelet count less than 20 × 10⁹/L in any trimester, or has a platelet count of 20 to 30 × 10⁹/L in the third trimester.^147,319 Platelet counts should be increased to safe levels (generally >30 × 10⁹/L) if invasive procedures are planned. Glucocorticoids are the preferred initial therapy for these patients. Because of their side effects, glucocorticoids should be used at the minimal dose that will keep platelet counts in a safe range. The recommended starting dose of prednisone is 10 mg/day, which can be modified as appropriate.^147,319 Fetal side effects will be minimal with a low-dose glucocorticoid regimen, because approximately 90 percent of the glucocorticoid dose is metabolized in the placenta.³¹⁸ IVIG is indicated in pregnant patients who do not respond to or tolerate glucocorticoid treatment, or when it is necessary to rapidly increase the platelet count. A dose of 1 g/kg per day for 2 days, or 400 mg/kg per day for 5 days can be used alone, or combined with low-dose prednisone. If the initial therapy with glucocorticoids and IVIG fails, all second-line therapies generate some concern. Anti-(Rh)D can cause severe hemolytic reactions in both the mother and the fetus, and should be used only in patients refractory to glucocorticoids and IVIG.^148,319 Experience with azathioprine and cyclosporine in pregnancy is largely based on the case series from patients with rheumatologic disorders and solid-organ transplantation. These studies reported that exposure to these drugs during pregnancy was not associated with an increase in the risk of negative pregnancy outcomes and had no significant toxicity to the fetus.³²⁷ Splenectomy can be used in pregnant ITP patients who are unresponsive or intolerant to available drugs and at significant risk of bleeding. If splenectomy is necessary, it is preferable that it be performed during the second trimester.^147,191

Rituximab is not an optimal drug for use during pregnancy. It can cross the placenta, and transfer from mother to fetus increases with gestational age. The half-life of the drug is also very long; rituximab can be found in blood 6 months after of an infusion. In a review evaluating 231 pregnancies with rituximab exposure reported in the literature, most of the patients had SLE, rheumatoid arthritis. and B-cell lymphoma, with rituximab being used in combination with other drugs. This retrospective study showed low risk of premature births, hematologic abnormalities and birth defects. However, because of the lack of controlled studies, it is recommended that women avoid pregnancy for 1 year after rituximab infusion.³²⁸

TPO receptor agonists were found to cause fetal loss and reduced fetal body weight in animal studies, and there is no data on humans.³²⁹ Vinca alkaloids, cyclophosphamide, and danazol are not recommended during pregnancy.

The optimal mode of delivery in pregnant ITP patients has not been determined. Because earlier studies reported that thrombocytopenic neonates have an increased risk for intracranial hemorrhage, some physicians recommend delivering the baby by cesarean section in women with ITP to avoid injuries to the fetus during passage through the birth canal.³³⁰ However, because of the rarity of intracerebral hemorrhage, there are no data proving the effectiveness of cesarean delivery in reducing the occurrence of intracerebral hemorrhage in the thrombocytopenic fetus.³²³ Measurement of platelet counts in infants before delivery, such as by percutaneous umbilical cord blood sampling or fetal scalp vein sampling after cervical dilatation, is not recommended routinely because the risk of bleeding during these procedures is high.^331–333 The mother's platelet count at delivery does not correlate with the infant's platelet count. In ITP patients who gave birth more than once, however, the first infant's platelet count at birth may be a predictor of severe thrombocytopenia in subsequent pregnancies and may justify further obstetric management.^323,332,334 On the other hand, discordances in degree of thrombocytopenia between dichorionic twins in ITP indicate that fetal factors also are important.³³⁵ In conclusion, there is as yet no definitive method to predict fetal platelet count in pregnant ITP patients, and the method of delivery should be determined by obstetrical evaluation. During vaginal delivery, the target maternal platelet count should be 50 × 10⁹/L or higher. If cesarean section or epidural anesthesia is required, the platelet count should be maintained over 70 to 80 × 10⁹/L.^147,148,319 Glucocorticoids, IVIG and platelet transfusions may help to keep platelet counts in a safe range in these patients. Blood products should be available for possible severe bleeding during labor, although it is quite rare even in ITP patients with platelet counts lower than 20 × 10⁹/L.

Severe neonatal thrombocytopenia (platelet counts <20 × 10⁹/L) occurs in 3 to 5 percent of ITP pregnancies and moderate neonatal thrombocytopenia (platelet counts <50 × 10⁹/L) in 9 percent.³³¹ Severe bleeding occurs in less than 1 percent of the babies. If the newborn is thrombocytopenic, the platelet count should be measured daily for 1 week. IVIG is preferred in neonates with severe thrombocytopenia. Platelet transfusions and glucocorticoids are added if bleeding is life-threatening.

If thrombocytopenia associated with SLE and APS has been complicated with prior pregnancy loss and thromboembolism, pregnant patients should receive antithrombotic prophylaxis with low molecular heparin and/or aspirin if possible. Although there is no defined threshold platelet level for these patients, platelet counts over 50 × 10⁹/L are considered safe for both anticoagulant and antiplatelet therapy.³¹⁹

MICROANGIOPATHIC DISORDERS IN PREGNANCY: PREECLAMPSIA–ECLAMPSIA, HELLP, THROMBOTIC THROMBOCYTOPENIC PURPURA-HEMOLYTIC UREMIC SYNDROME, AND ACUTE FATTY LIVER OF PREGNANCY

Preeclampsia

This condition is a systemic disorder characterized by new onset hypertension after 20 weeks of gestation, and primarily occurs near term. Although proteinuria occurs in the majority of these cases, the American College of Obstetricians and Gynecologists (ACOG) 2012 classification accepts the presence of one of the following in the absence of proteinuria: thrombocytopenia (less than 100 × 10⁹/L), abnormal liver function tests, renal insufficiency, pulmonary edema, or cerebral and visual symptoms. Eclampsia is defined by the occurrence of epileptic seizures in a preeclamptic woman during the peripartum period.^336–338 Preeclampsia complicates 5 to 8 percent of all pregnancies, and is a major contributor to maternal and fetal morbidity and mortality (Chap. 7).^336,339 Thrombocytopenia is seen in approximately 50 percent of women with preeclampsia, with the severity of thrombocytopenia correlating with the severity of the preeclampsia.³⁴⁰

Attempts to define the pathogenesis of preeclampsia have engendered numerous theories.³⁴¹ One clear aspect of the pathogenesis is the requirement for a placenta, given that the condition can be produced in abdominal pregnancies and molar pregnancies.³⁴² The disease appears to be initiated by defective invasion of the uterine spiral arteries by placental cytotrophoblasts. During normal implantation, these cells convert from epithelial to endothelial morphology, a process called pseudovasculogenesis.^343,344 In preeclampsia, this process is defective, resulting in diminished maternal blood flow to the placenta and placental hypoxia. Through unknown mechanisms, the production of membrane and soluble forms of the vascular endothelial growth factor (VEGF) receptor fms-like tyrosine kinase-1 (Flt1) is increased,³⁴⁵ with resultant increases of soluble Flt1 (sFlt1) in the amniotic fluid³⁴⁶ and maternal circulation.³⁴⁷ sFlt1 is the product of an alternately spliced form of the Flt1 messenger RNA that lacks the transmembrane and cytoplasmic domains present in the full-length receptor. A large volume of evidence implicates sFlt1 as playing a key role in the pathogenesis of preeclampsia. By binding to VEGF and the related placental growth factor, sFlt1 blocks their favorable effects on vascular endothelium. Its expression in rats produces a syndrome akin to preeclampsia: hypertension and proteinuria associated with glomerular endotheliosis (occlusion of glomerular capillaries by swollen endothelial cells). Endoglin is another angiogenic receptor expressed on endothelial cells and placental syncytiotrophoblasts, functioning as a coreceptor for the potent angiogenic factor transforming growth factor-β.³⁴⁸ Expression of its messenger RNA is increased in preeclamptic placenta.³⁴⁸ The levels of the soluble extracellular domain, produced by proteolysis, are elevated in the blood of preeclamptic patients. In pregnant rats, soluble endoglin works synergistically with sFlt1 to produce vascular damage and a HELLP-like syndrome.³⁴⁸ These findings strongly suggest that a tonic level of VEGF-like angiogenic factors is required to maintain the normal function of vascular endothelial cells and that this process is dysregulated during preeclampsia/eclampsia.

The connection between preeclampsia and thrombocytopenia is not clear, although many cases have evidence of activation of blood coagulation detected by elevated levels of fibrin-degradation products and thrombin–antithrombin complexes.³²² Low levels of the VWF-cleaving metalloprotease ADAMTS13 (a disintegrin and metalloprotease with a thrombospondin type 1 motif member 13) have also been described,³⁴⁹ as have elevated levels of VWF, including the hyperadhesive ultralarge forms.³⁵⁰

HELLP Syndrome

This syndrome occurs in the peripartum period and is defined by the presence of microangiopathic hemolytic anemia, elevated liver enzymes, and low platelets. In approximately 70 to 80 percent of patients, HELLP occurs in the setting of preeclampsia.³⁵¹ Microangiopathic hemolysis results from shearing of the erythrocytes as they pass through arterioles occluded by platelet–fibrin deposits. Adhesion and aggregation of platelets on damaged and activated endothelium presumably accounts for the low platelet count (Chap. 114). HELLP shares a number of features with TTP, including the presence of microangiopathic hemolysis and thrombocytopenia. Involvement of the central nervous system is a more prominent feature of TTP, whereas HELLP more commonly displays severe liver function abnormalities (Chaps. 7, 49, and 124).³⁵² Because the two syndromes can be confused with one other, one study attempted to distinguish the two by measuring the activity of ADAMTS13, which usually is absent or severely deficient in TTP.³⁴⁹ The study found that essentially all 17 patients in a cohort with the HELLP syndrome had mild to moderate reductions in the activity of ADAMTS13 in the plasma, and none was severely deficient.

Acute Fatty Liver of Pregnancy

This abnormality is a very severe, but fortunately very rare (1 in 20,000 to 100,000 pregnancies) condition that occurs during the third trimester of pregnancy or early postpartum period. Acute fatty liver of pregnancy (AFLP) is characterized by microvesicular fatty infiltration of liver resulting in hepatic failure and encephalopathy. The “Swansea Criteria” used for the diagnosis of AFLP include encephalopathy, vomiting, abdominal pain, polydipsia/polyuria, elevated transaminases, elevated ammonia, elevated uric acid, elevated bilirubin, leukocytosis, coagulopathy, renal impairment, hypoglycemia, ascites or bright liver on ultrasound evaluation, and microvesicular steatosis on liver biopsy. Six or more of these criteria should be present in a patient who has no obvious reason for hepatic failure. Both maternal and fetal mortality rates are high, ranging from 7 to 18 percent and 9 to 23 percent, respectively.³⁵³

Delivery of the fetus is the most effective treatment for preeclampsia, HELLP syndrome and AFLP. The platelet count nadir and the peak of serum lactate dehydrogenase may occur postpartum, during the first postpartum day in most patients, but as late as 5 to 7 days in some. For patients with severe thrombocytopenia and microangiopathic hemolytic anemia, plasma exchange may be indicated if the fetus cannot be delivered or if improvement does not follow delivery. This treatment is empirically based on the similarity of the clinical picture to that of TTP. Postpartum day 3 often is considered the limit for supportive therapy in anticipation of a spontaneous recovery.³⁴⁹ If thrombocytopenia and hemolysis (as assessed by serum lactate dehydrogenase levels) continue to worsen beyond this time, intervention with plasma exchange is appropriate for the presumed diagnosis of TTP-HUS (Chap. 133). At this point, TTP-HUS cannot be distinguished from atypical preeclampsia/HELLP syndrome, for which plasma exchange treatment may be beneficial.³⁵⁴ Earlier intervention with plasma exchange is indicated for more severe clinical problems, such as neurologic abnormalities or acute, anuric renal failure. In patients with AFLP, however, liver insufficiency, encephalopathy and coagulopathy may not improve despite immediate delivery and intensive supportive care. These patients may require liver transplantation.³⁵³

The platelet count in the fetus reaches normal adult levels (>150 × 10⁹/L) after the first trimester, and is maintained throughout gestation. However, thrombocytopenia is more common in preterm infants, of several potential etiologies. Severe thrombocytopenia (<50 × 10⁹/L) is an important finding in neonates, and should be carefully managed because of high bleeding risk (Chap. 6).³⁵⁵

Fetal–neonatal alloimmune thrombocytopenia (NAIT) is a leading cause of severe thrombocytopenia and life-threatening bleeding in neonates. NAIT is caused by the transplacental transfer of maternal alloantibodies against fetal platelet antigens inherited from the father. NAIT resembles neonatal alloimmune hemolytic anemia (Rh hemolytic disease of the newborn) in many aspects. In both diseases, maternal alloantibodies against fetal blood cell antigens cross the placenta and destroy antigen-positive fetal cells, resulting in significant fetal/neonatal morbidity and mortality. However, unlike neonatal alloimmune hemolytic anemia, which tends to spare the first-born child, the first child is affected in 40 to 60 percent of NAIT cases.³¹⁸ Transplacental transfer of antiplatelet antibodies can also occur in babies born from mothers with ITP. Nevertheless, maternal ITP rarely causes serious thrombocytopenia or bleeding in the fetus, whereas thrombocytopenia tends to be more severe and the rate of intracranial hemorrhage is higher (10 to 20 percent) in NAIT.¹⁰⁶ In contrast to maternal ITP, in NAIT the maternal platelet count is normal, a key differential diagnostic finding.

PREVALENCE AND PATHOGENESIS

The estimated frequency of NAIT varies from 1 in 500 to 1 in 2000 livebirths.^356,357 Maternal alloantibodies against human platelet alloantigens (HPAs) are responsible for platelet destruction in NAIT. In populations of European ancestry, the most frequently implicated antigens are HPA-1a or Pl^A1 (78 percent of cases) and HPA-5b or Br^a (19 percent of cases).³⁵⁸ These antigens are rare in Asian populations. HPA-4a (80 percent of cases) and HPA-3a (15 percent of cases) are responsible for platelet destruction in the majority of Asian NAIT cases. Besides targeting the HPA system, anti–HLA-2 antibodies have been reported, but whether they are responsible for NAIT is not clear.^356,359,360

The frequency of NAIT in populations of European ancestry is lower than would be expected given that the prevalence of HPA-1a negativity is 2.5 percent. Only 10 percent of HPA-1a–negative mothers exposed to HPA-1a–positive platelets during pregnancy become immunized. HPA alloimmunization is strongly correlated with the presence of specific class II HLA antigens, with increased risk demonstrated in HPA-1a–negative mothers expressing HLA-B8, HLA-DR3, and HLA-DR52a antigens.^318,361,362 The presence of the HLA-DRB3^*0101 allele in HPA-1a–negative women increases the NAIT risk as much as 140-fold.³⁶²

NAIT tends to be clinically more severe in cases with alloantibodies against HPA-1a.¹⁰⁶ HPA-1 (Pl^A) antigens are expressed on platelet integrin β₃. Anti–HPA-1a antibodies possibly impair platelet aggregation, which may explain the severity of bleeding symptoms.³⁶³

CLINICAL FEATURES

IgG alloantibodies can cross the placenta as early as week 14 of pregnancy, and placental passage increases with gestational age.³⁵⁶ These antibodies bind to fetal platelets and lead to their destruction. In severe cases, intracranial hemorrhage and hydrocephalus may develop and cause fetal death or severe neurologic sequelae. The diagnosis can be difficult in the first affected fetus in a family. Ultrasonography is usually not helpful unless it detects bleeding or hydrocephalus. Unexplained fetal deaths in the maternal history or fetal hydrocephalus or bleeding in previous pregnancies may alert the physician to the possibility of NAIT. Usually the diagnosis of NAIT is possible after birth. NAIT should be suspected in a thrombocytopenic neonate with extensive purpura or visceral hemorrhage but no evidence of sepsis, skeletal anomalies, or other systemic diseases that may cause thrombocytopenia, including maternal ITP. Affected babies may have no signs or symptoms (13 to 59 percent of cases), or they may have signs of bleeding (18 to 65 percent of cases) or intracranial hemorrhage (22 to 23 percent of cases).³⁶⁴ In a case series of 88 infants with NAIT resulting from anti–HPA-1a antibodies, 90 percent had purpura, 66 percent had hematomas, 30 percent had gastrointestinal bleeding, and 14 percent had intracerebral hemorrhage. Bleeding may be delayed, as the platelet count usually falls further during the first several days of life. Death or neurologic impairment occurs in up to 25 percent of infants. Platelet counts recover to normal in 1 to 2 weeks.³⁶⁵

The diagnosis of NAIT usually can be confirmed by tests for circulating maternal alloantibodies against fetal antigens (usually by MAIPA) or modified antigen capture enzyme-linked ımmunosorbent assay (MACE) or by platelet typing of the parents and neonate by either genotyping or ELISA. These tests may fail to yield the diagnosis because private HPA antigens may be responsible for NAIT.^318,323,358

MANAGEMENT

Postnatal

In the clinical setting, the confirmation of a diagnosis of NAIT by platelet genotyping, MAIPA, or MACE will require days; thus, an infant born with severe thrombocytopenia with no obvious cause such as sepsis should be regarded as having NAIT. The alternatives in the management of affected neonates are IVIG, glucocorticoids (alone or combined with IVIG), and platelet transfusions. IVIG and/or glucocorticoid therapy may increase platelet counts rapidly, although a substantial increase of platelet counts usually occurs after 24 to 72 hours.³⁵⁸ In cases with severe bleeding, platelets should be transfused. Transfused platelets should be ABO and (Rh)D compatible and HPA-1a–negative if possible.³⁶⁶ If such platelet suspensions are not available, transfusion of washed and irradiated maternal platelets to the affected fetus is another alternative.¹⁰⁶ Repeated platelet transfusions may be required.³¹⁸ All affected infants should be screened with ultrasound for intracranial hemorrhage.³⁶⁷

Prenatal

Pregnant women who had a previous thrombocytopenic infant attributable to NAIT should be carefully monitored in a center with experience with NAIT, because thrombocytopenia will be more severe in a second affected child. Current therapeutic alternatives for antenatal management of NAIT are unsatisfactory. Fetal platelet typing is important, but available tests usually require invasive procedures such as amniocentesis or fetal blood sampling. Cell-free fetal DNA obtained from maternal blood has been studied for fetal platelet genotyping.³⁶⁸ However, these tests need validation. The treatment options in high-risk NAIT are weekly IVIG administration to the mother, with or without glucocorticoids, serial in utero platelet transfusions, in utero IVIG administration, and early delivery (after 32 weeks of gestation). Maternal IVIG administration at a dose of 1 g/kg per week with or without glucocorticoids may increase fetal platelet counts,³⁶⁹ although not all studies support this conclusion.^356,363 IVIG can be administered directly to severely thrombocytopenic fetuses, although this also may fail to raise fetal platelet counts.³⁷⁰ In patients who do not respond to IVIG and glucocorticoid administration, serial transfusion of matched platelets should be considered. Matched platelet transfusions will only transiently increase the fetal platelet count because the transfused platelets also are targeted by the offending antibodies.³¹⁸ Serial platelet transfusions may increase the cumulative risk of hemorrhage and procedure-related hemorrhage and fetal loss.³⁶³ In severely thrombocytopenic fetuses, early delivery by cesarean section may reduce the risk of intracranial hemorrhage.³⁶³ New therapeutic strategies are under investigation, including vaccines and competitive molecules that competitively bind anti–HPA-1a antibodies.³⁶⁶

SPLENOMEGALY AND HYPERSPLENISM

Splenomegaly may lead to thrombocytopenia by inducing a reversible pooling of up to 90 percent of total body platelets.^371,372 This process can be thought of as an exaggeration of normal splenic pooling, in which approximately one-third of the platelet mass is contained within the spleen at any one time (Chap. 55). The survival of platelets within the spleen can be normal or moderately reduced. Thus, the total blood platelet pool in a patient with splenomegaly could be normal even when the counts measured in venous blood are only 20 percent of normal. Platelet production is usually normal in patients with splenomegaly, as estimated by dividing the total body platelet mass by the platelet life span.³⁷¹ This finding provides further evidence that platelet production is more closely tied to total platelet mass than to circulating platelet count.

The most common disorder causing thrombocytopenia because of splenic pooling is chronic liver disease with portal hypertension and congestive splenomegaly. In patients with cirrhosis and portal hypertension, moderate thrombocytopenia is the rule. However, in such cases the thrombocytopenia often results from both splenic pooling and reduced hepatic production of TPO.

Thrombocytopenia associated with splenomegaly is often of no clinical importance and generally does not require therapy. Signs and symptoms are related to the primary disorder, and bleeding manifestations result primarily from coagulation abnormalities caused by the underlying liver disease. This finding is consistent with the relatively moderate degree of thrombocytopenia, the near-normal total body content of platelets,³⁷¹ and the ability to mobilize platelets from the spleen to replenish losses.³⁷³ When splenectomy is performed for another reason, however, the platelet count predictably returns to normal or thrombocytosis may even occur.³⁷¹ Platelet counts may also return to normal in patients following surgical correction of portal hypertension by portosystemic shunting.³⁷⁴ Platelet transfusions usually are not needed for splenomegaly-associated thrombocytopenia and rarely produce significant increases in the platelet count because as much as 90 percent of the transfused platelets will be sequestered in the spleen.

HYPERSPLENISM

Hypersplenism is distinguished from uncomplicated splenomegaly in that pooling is accompanied by increased destruction of platelets, leukocytes, and erythrocytes in association with increased marrow precursors of the deficient lines and correction of the cytopenia by splenectomy.^375–378 The clinical manifestations, laboratory findings, and specific treatment are aimed at the underlying disease (Chap. 55).³⁷⁹

Imaging studies, such as computed tomographic scans, can be useful for defining the size of the spleen and identifying intrasplenic and extrasplenic disease. Magnetic resonance imaging defines the blood flow pattern, which is especially useful for detecting portal or splenic vein thromboses. Cell survival studies using radiolabeled platelets or red blood cells can be helpful for identifying hypersequestration when weighing the need for splenectomy. Most patients with splenomegaly require therapy for the underlying disease rather than for thrombocytopenia.

Several definitions are used for massive transfusion including transfusion of one blood volume or more than 10 units of packed red blood cells (RBCs) in 24 hours and transfusion of more than 4 units of packed RBC over 1 hour.³⁸⁰ Massive transfusion is required in patients with uncontrolled and heavy bleeding. One study of patients requiring massive transfusion demonstrated that mild thrombocytopenia (47 to 100 × 10⁹/L) occurred in all patients after transfusion of 15 red cell units, and more severe thrombocytopenia (25 to 61 × 10⁹/L) developed after 20 red cell units.^381,382 Several factors contribute to thrombocytopenia in massive transfusion, including direct loss of platelets in the exsanguinated blood, dilution of platelets by the transfused RBCs, DIC triggered by the disease responsible for the blood loss or that develops after trauma, and hypothermia (Chap. 140). Massively transfused patients should be treated with fresh-frozen plasma to replace coagulation factors, and with platelets.³⁸³ The precise ratio of platelets to red cells has not been determined, but studies show that massively transfused trauma patients demonstrated improved survival with increased transfusion of platelet concentrates.^384,385

Transient thrombocytopenia occurs during hypothermia, in both animals and humans, when the body temperature falls below 25°C.³⁸⁶ The degree of thrombocytopenia correlates with the degree of the body temperature drop. Thus, thrombocytopenia is less severe in cardiac surgery patients supported by normothermic systemic perfusion (35°C to 37°C) than in those supported by moderately hypothermic systemic perfusion (25°C to 29°C).³⁸⁷ In this case, the drop in platelet count likely results from splenic and hepatic pooling³⁸⁸ and from cold activation and clearance of platelets. Cold induces clustering of the GPIb complex and rearrangement of its carbohydrate chains, which then serve as ligands for the macrophage integrin α_Mβ₂, which mediates their clearance in hepatic macrophages.^389,390 In hypothermic dogs, radiolabeled platelets are sequestered in the spleen, liver, and other organs; the platelets return to the circulation when normal body temperature is restored.^386,391 The clinical relevance of these observations is illustrated by reports of patients, often elderly, who are hypothermic after periods of unconsciousness in inadequately heated rooms. In one report, a 69-year-old woman had 13 admissions over an 8-year period with repeated hypothermia, her temperature ranging from 31°C to 34°C during the hospitalizations. On each admission she was thrombocytopenic (platelet count 7 to 39 × 10⁹/L). With no therapy other than rewarming, platelet counts returned to normal in 4 to 10 days.³⁹² However, a review of 75 patients admitted with hypothermia (body temperatures 26°C to 35°C) demonstrated that only three patients were thrombocytopenic.³⁹²

Kasabach-Merritt syndrome is defined as profound thrombocytopenia related to platelet trapping within a vascular tumor, either a Kaposi-like hemangioendothelioma or a tufted angioma.^393–396 The syndrome presents predominantly during infancy, but several adult cases have been reported.³⁹⁷ These vascular tumors should be differentiated from vascular malformations such as classic benign hemangiomas. Benign hemangiomas usually are superficial, multiple, not associated with severe thrombocytopenia or DIC (Chap. 130), and usually disappear during childhood. On the other hand, Kaposi-like hemangioendothelioma and tufted angioma are low-grade malignant vascular tumors associated with high morbidity and mortality.

Vascular tumors usually are solitary, can reach 20 cm in diameter, and can be superficial or invade internal organs and the retroperitoneum.^398–400 Superficial tumors can be recognized by the local red to purple discoloration of the skin. The histologic types more frequently associated with Kasabach-Merritt syndrome are Kaposi-like hemangioendothelioma and tufted angiomas or angioblastomas.^{393,394,401,402} Kaposi-like hemangioendothelioma is a locally aggressive, low-grade malignant tumor characterized by infiltrating sheets or lobules of poorly formed vascular channels and aberrant lymphatic vessels. These tumors are composed predominantly of plump, round, oval, and/or spindled endothelial cells with hemosiderin deposits.³⁹³ A tufted angioma is a lesion characterized by the presence of vascular tufts and aggregates of round dilated capillaries, lymphangiomatosis, microthrombi, and hemosiderin deposits.^{393,394,403,404} Electron microscopic examination shows abnormal endothelial cells with prominent cytoplasmic projections and wide intercellular gaps, fibrin deposition, and platelet aggregates within the vessels.³⁹⁴ The histology of the tumor is useful for differentiating the vascular tumors associated with Kasabach-Merritt syndrome from benign capillary hemangiomas.⁴⁰⁵

Thrombocytopenia in Kasabach-Merritt syndrome usually is severe and associated with DIC.⁴⁰⁶ Contributing factors include “platelet trapping” by abnormally proliferating endothelium within the hemangioma^407,408 and platelet consumption associated with DIC. Platelet trapping has been demonstrated by immunohistochemical staining of the tumors with anti-CD61 antibodies (a marker of platelets and megakaryocytes)⁴⁰⁹ and by nuclear studies using ⁵¹Cr-labeled platelets⁴¹⁰ and ¹¹¹In platelet scintigraphy to monitor response to therapy.^411,412 How platelets become trapped is not clear. Initial physical entrapment of the platelets within twisted abnormal vessels may favor their adhesion to abnormal endothelium, which can lead to platelet activation and aggregation followed by activation of the coagulation cascade, fibrin deposition, and formation of microthrombi. Excessive flow and shear rates generated by arteriovenous shunting within the tumor further increase the level of platelet activation. Continuous thrombus formation leads to platelet consumption and activation of the fibrinolytic cascade. Severe thrombocytopenia and DIC result.

The mainstay of treatment is eradication of the tumor. Several specific therapeutic modalities have been proposed, but none has been established as consistently effective.⁴¹³ Among the therapies are high-dose glucocorticoids,⁴¹³ IFN-α,^413,414 vincristine,⁴¹⁵ cyclophosphamide,⁴¹⁶ combination chemotherapy,⁴¹⁷ and radiation.^418–420 For severe cases, interventions such as arterial embolization,^421,422 surgical resection,^423,424 and pneumatic compression can be attempted.

The mortality rate for advanced Kasabach-Merritt syndrome is approximately 12 percent; the rate is higher when associated with retroperitoneal or intraabdominal tumors. Patients die of complications resulting from DIC, low platelet count, and infections secondary to immunosuppression.

Cyclic thrombocytopenia (CTP) is a very rare acquired disorder characterized by a periodic decrease in the platelet count, sometimes followed by rebound thrombocytosis without therapy (>500 × 10⁹/L).⁴²⁵ Fluctuating levels of endogenous TPO, inversely related to the platelet count, was reported in one case.⁴²⁶ Each thrombocytopenic cycle typically spans a period of 3 to 6 weeks, and women are more often affected than men. The platelet counts may fluctuate across a wide range. In reported cases, the median nadir and peak platelet counts were 10 × 10⁹/L (range: 1 to 90 × 10⁹/L) and 330 × 10⁹/L (range: 72 to 2300 × 10⁹/L), respectively.⁴²⁷ Rebound thrombocytosis is an important and distinctive feature of CTP. Although some cases are reported as associated with myeloproliferative neoplasms, most CTP cases are idiopathic.^428,429 The pathophysiology is unclear and a number of potential mechanisms have been proposed, including autoimmune platelet destruction, megakaryocytic hypoplasia/aplasia, infections, and hormonal disturbances. Although most premenopausal female CTP patients studied have had low platelet counts during their menstrual periods, hysterectomy with bilateral salpingo-oophorectomy has not been shown to affect the course of the platelet fluctuations.⁴²⁷

The clinical presentation of CTP is similar to that of ITP. The bleeding tendency ranges from asymptomatic, to easy bruising, gingival bleeding, recurrent epistaxis, menorrhagia, and hematuria, to more serious bleeding, including gastrointestinal or central nervous system hemorrhage.⁴²⁷ CTP is rarely considered in the differential diagnosis of thrombocytopenia, so patients are usually diagnosed and treated as having ITP. CTP is a rare disorder, but the diagnosis should be considered in patients with “ITP” who have not responded to therapies such as glucocorticoids, splenectomy, and IVIG, and who have rebound thrombocytosis. Responses have been reported to respond to hormone therapy and cyclosporine. In female patients, oral contraceptives may be useful to prolong the menstrual cycle and cover low-platelet-count days. Antifibrinolytic drugs such as aminocaproic acid or tranexamic acid may also be useful to decrease bleeding symptoms.

Development of thrombocytopenia after quinine was first described by Vipan in 1865, and since then a large number of drugs have been found to cause thrombocytopenia. Drugs should be considered as potentially causative in any thrombocytopenic patient on medication, taking herbal remedies, or using iodinated radiocontrast solutions.⁴³⁰ Drug-induced thrombocytopenia generally affects only a small percentage of patients taking a particular drug, and is usually not severe, although it can be fatal. Genetic and environmental factors both influence susceptibility to drugs. Discontinuation of the causative drug(s) is the main treatment strategy; glucocorticoids may help in some patients. Drugs may cause thrombocytopenia by different mechanisms. Dose-dependent myelosuppression and immune destruction of the platelets are two well-known causes. One of the most severe and life-threatening forms of drug-induced thrombocytopenia is HIT, an immune-mediated disorder caused by antibodies that recognize a neoepitope in platelet factor 4 that is exposed when platelet factor 4 binds heparin. The result is activation of platelets and the coagulation cascade and, ultimately, venous and arterial thrombosis. HIT affects up to 5 percent of patients exposed to therapeutic doses of unfractionated heparin (Chap. 133). This section discusses drugs, other than heparins, that cause isolated thrombocytopenia by immune platelet destruction; Chap. 34 discusses drug-induced aplastic anemia with thrombocytopenia.

ETIOLOGY

Reviews of drug-induced thrombocytopenia often contain such extensive lists of implicated drugs, many of which are commonly used, that they are not helpful for decisions regarding which therapy to interrupt first. To address the issue of which drugs most likely cause thrombocytopenia, a systematic review of all published case reports defined levels of evidence to document the causal relation between the drug and thrombocytopenia.⁴³¹ This review distinguished drugs with definite or probable causal relationships from those for which the evidence was weaker.⁴³¹ Table 117–7 lists the drugs for which there is definite evidence of a causal role in producing thrombocytopenia (which includes recurrent thrombocytopenia with rechallenge in the same patient) and drugs for which the causal relation to thrombocytopenia has been validated by at least two reports with probable evidence (thus meeting all of the criteria for definite evidence except for the lack of rechallenge). Quinidine is by far the most commonly cited drug. Other commonly cited drugs are similar to drugs documented in a case-control study.⁴³² A remarkable observation from the systematic review was how many case reports did not provide sufficient clinical information to allow a determination of even a probable causal relation.³²⁰

PATHOGENESIS

Thrombocytopenia is usually assumed to result from immune platelet destruction by drug-dependent antibodies.⁴³⁰ Most of these antibodies bind the platelets only in the presence of the offending drugs. Drugs may trigger different immune mechanisms, as depicted in Table 117–8.

Drugs may bind covalently to membrane proteins, and may induce hapten-dependent antibodies in patients receiving penicillin and cephalosporin. In quinine-induced thrombocytopenia, antibodies bind to membrane proteins only in the presence of soluble drug. In patients receiving tirofiban or eptifibatide, the drug binds to integrin α_IIbβ₃, creating a conformation-dependent neoepitope and inducing antibody production. Gold salts and procainamide, however, may induce true autoantibodies, with those induced by gold being unique in targeting platelet GPV.⁴³³ These antibodies can bind and destroy platelets in the absence of the drug. In HIT, heparin–platelet factor 4 complexes induce autoantibodies.

Initial experimental observations suggested that drug–antibody complexes bind to platelets via the platelet Fcγ receptor. This mechanism is confirmed for HIT (see below in this section), but for other drugs, the drug-dependent antibodies appear to bind to platelets via their Fab regions.⁴³⁴

The target antigens are the major platelet surface GPs (GPIb-IX-V and integrin α_IIbβ₃). Different drugs may provoke drug-dependent antibodies that preferentially react with one of these GPs, or drug-dependent antibodies from a single patient may react with multiple epitopes on both GPs. For example, a study of sera from 15 patients with quinine-induced thrombocytopenia demonstrated that, in the presence of quinine, the antibodies bound to two distinct domains on GPIb-IX, one on GPIbα, and one on GPIX. Some patients had only one of the antibodies; some had both.⁴³⁵ The same domains on GPIb-IX also appear to be the antigenic targets for quinidine- and ranitidine-dependent antiplatelet antibodies.^435,436 Definition of the specific epitope involved in patient reactions with drug-dependent antibodies may not only elucidate the mechanism of drug-induced thrombocytopenia but also identify polymorphisms in GPIb-IX that cause sensitivity in producing drug-dependent antiplatelet antibodies. Sulfonamides, quinidine, and quinine are frequent causes of drug-induced thrombocytopenia. Studies of sera from 15 patients with thrombocytopenia caused by sulfamethoxazole or sulfisoxazole demonstrated that the antigenic epitope was part of integrin α_IIbβ₃.⁴³⁷ Some antibodies from patients with quinidine- and quinine-dependent antiplatelet antibodies also react with integrin α_IIbβ₃.⁴³⁸

In addition to specificity for discrete epitopes on platelet surface GPs, drug-dependent antibodies are highly specific for the structure of the drug. For example, no cross-reactivity occurs between quinidine and quinine-dependent antibodies or between sulfamethoxazole and sulfisoxazole-dependent antibodies, even though both pairs of drugs have similar structures. Therefore, the neoantigens produced by drug binding to platelets create discrete epitopes that are sensitive to minor changes in drug structure.

The implications of this mechanism for platelet destruction are apparent. A patient with prior sensitivity to the drug has preformed antibodies that immediately react with the altered platelets upon repeat drug exposure, as demonstrated. An exception to this situation is the immediate acute thrombocytopenia that may occur with initial administration of antithrombotic agents that bind platelet integrin α_IIbβ₃,^42,439 especially abciximab. Abciximab is a humanized monoclonal antibody fragment that lacks the Fc domain, so thrombocytopenia is not caused by phagocytosis of the platelets by macrophages. Patients experiencing thrombocytopenia after receiving integrin α_IIbβ₃ inhibitors have been postulated to have preformed antibodies to epitopes exposed on the integrin by drug binding. These could be the same antibodies that cause in vitro EDTA-dependent platelet agglutination and pseudothrombocytopenia (see “Pseudo (Spurious) Thrombopenia” above).^33,440,441

DIAGNOSIS

The diagnosis of drug-induced thrombocytopenia can be made only by recovery from thrombocytopenia upon discontinuation of the drug and can be confirmed if thrombocytopenia recurs with rechallenge by the drug. Prompt recovery within 5 to 7 days is usual.⁴³¹ Gold-induced thrombocytopenia is an exception because gold salts are retained for long periods of time within the body and thrombocytopenia can persist for months, becoming indistinguishable from ITP.⁴⁴² Rechallenge with a suspected drug is dangerous, because severe thrombocytopenia can develop rapidly with even very small drug doses. However, when multiple drugs are potentially involved and all are important for management, it may be appropriate to reintroduce them individually, followed by several days of close observation. In general, the smallest possible dose of the drug should be administered. The administration should be performed under direct supervision of the patient, with platelets available for bleeding should it occur. If rechallenge leads to thrombocytopenia, the patient should be advised to wear a Medic Alert bracelet. For common drugs, especially those that can be purchased without a prescription, it may be safer to supervise a rechallenge and unequivocally document risk rather than risk future unintentional use.

Laboratory assays can detect drug-dependent antibodies, and positive results can support a clinical diagnosis. However, the laboratory role remains largely investigational because results are not promptly available when a clinical decision must be made about discontinuing a drug. Furthermore, no laboratory test has been validated that supports continuing a suspected drug with no adverse effects following a negative laboratory test.

Drug-dependent antibodies can be detected by flow cytometric techniques,⁴³⁷ MAIPA,⁴⁴³ and solid-phase red cell adherence assays.⁴⁴⁴ Strongly positive tests are apparent, but distinction of positive from negative tests is arbitrary and not yet clinically validated. Positive tests for heparin-dependent antibodies have been reported in patients without thrombocytopenia,^445–447 and patients with clinical evidence for drug-induced thrombocytopenia may have negative tests using multiple techniques.^437,448

CLINICAL AND LABORATORY FEATURES

In patients with newly discovered thrombocytopenia, all medications should be identified. Not only should the history explore use of prescription medications, use of nonprescription drugs should also be queried, including products containing acetaminophen,⁴³¹ and drinks that may contain quinine (“tonic water”).^449,450 Drug-induced thrombocytopenia is typically severe. Among the 247 case reports with evidence for a definite or probable causal relation of the drug to thrombocytopenia, 23 patients (9 percent) had major bleeding, including two patients who died of bleeding,⁴³¹ and 68 patients (28 percent) had overt but minor bleeding; 96 patients (39 percent) had only purpura or trivial bleeding, and the remainder had no bleeding.⁴³¹ The time from beginning the drug to the initial occurrence of thrombocytopenia varies from 1 day to 3 years, but the median time is 14 days. With rechallenge, acute thrombocytopenia may occur within minutes but almost always within 3 days.⁴³¹ Patients may have other signs and symptoms of drug sensitivity, such as nausea and vomiting, rash, fever, and abnormal liver function tests.⁴⁵¹ Laboratory data may demonstrate leukopenia, indicating that the drug-dependent antibodies target multiple cell types.⁴⁵¹ Patients who have systemic adverse reactions to drugs manifesting as TTP or HUS are described in Chap. 133.

TREATMENT

Withdrawal of the offending drug is the most important therapeutic measure. Prednisone is commonly given because the distinction of drug-induced thrombocytopenia from ITP is almost never clear initially; however, glucocorticoids do not appear to speed recovery.⁴⁵¹ In patients with major bleeding, emergency treatment should be the same as for ITP: platelet transfusions, high doses of parenteral methylprednisolone, and possibly IVIG.³²⁰