PLATELET PRODUCTION, STRUCTURE, AND FUNCTION
Platelets, like other blood cells, arise from precursors in the bone marrow. Platelet maturation and production from marrow precursors, known as megakaryocytes, is highly regulated by the platelet growth factor, thrombopoietin. Platelet production occurs through the specialized process of endomitosis, by which DNA replicates without cell division, resulting in the polyploid nuclei characteristic of megakaryocytes. Proplatelet extensions form from megakaryocytic cytoplasm; once granule and cytoplasmic organization is complete, platelets are released from the ends of the proplatelets. After leaving the bone marrow, approximately one-third of the platelet mass is sequestered in the spleen, while the remainder circulate in the blood with a life span of 7 to 10 days. Thrombopoiesis is balanced by platelet senescence and consumption to maintain a normal blood platelet count (150,000–400,000/mm3) via plasma thrombopoietin. Platelets have an average diameter of 2.0 to 5.0 µm and typical mean volume of 6 to 10 fL. The platelet external surface consists of a lipid bilayer containing a variety of structural glycoproteins. The anuclear cytoplasm contains abundant alpha granules that contain biologically active proteins including fibrinogen, von Willebrand factor, factor V, and other adhesive molecules. Dense granules are a less abundant but important component of the cytoplasm, and they store and secrete calcium, serotonin, adenosine diphosphate (ADP), and adenosine triphosphate (ATP).
Circulating platelets have a critical role in hemostasis, through adhesion to sites of vascular injury, amplification of the platelet response, secretion of mediators of hemostasis, and aggregation via fibrinogen binding. After vascular injury, these processes lead to formation of a platelet plug, which constitutes primary hemostasis. Platelets also play a central role in activation of the coagulation cascade, or secondary hemostasis (Fig. 435-1), by providing a phospholipid (cofactor) surface on which several key coagulation reactions take place. For example, the activated platelet surface accelerates the conversion of prothrombin to thrombin by several hundred-thousandfold. Additionally, platelets have many important roles beyond hemostasis, including angiogenesis, innate immunity, and inflammation.
The role of platelets in hemostasis. A: With vessel injury, circulating unactivated platelets adhere to the exposed subendothelial matrix via specific platelet receptors (circles and rods) and undergo activation, shape change, and exposure of other activated receptors (crosses). B: With progressive adhesion of platelets to each other and the subendothelium, the platelet plug is formed. C: At the activated platelet surface (crescentic forms), phosphatidylserine (a coagulation cofactor) is preferentially exposed at the outer membrane leaflet and potentiates the activation of factor X to factor Xa and of factor II (prothrombin) to factor IIa (thrombin) via cofactor factor VIII (or FVIIIa in the activated form) and zymogen factor IX (or FIXa in activated form). Thrombin is the critical enzyme that allows formation of the fibrin clot. Factor VIII normally circulates in the plasma bound to von Willebrand factor. Roman numerals signify coagulation proteins, functioning as either zymogens (circles), enzymes (excised circles), or cofactors (ovals). (Reproduced with permission from Yee DL: Platelets as modifiers of clinical phenotype in hemophilia, Scientific World Journal. 2006 Jun 14;6:661-668.)
Platelet disorders can be generally categorized as either quantitative defects, affecting platelet number (thrombocytopenia or thrombocytosis), or qualitative defects, impacting platelet structure or hemostatic function.
There are 3 general mechanisms of thrombocytopenia: (1) increased destruction, which may be either immune or nonimmune (consumptive or mechanical); (2) decreased production; or (3) sequestration and pooling. A comprehensive list of potential causes of thrombocytopenia is provided in Table 435-1. The etiology of thrombocytopenia can often be determined by a detailed history and physical examination, a complete blood count (CBC), and a review of the peripheral blood smear. It is also important to ascertain whether the thrombocytopenia is acquired or congenital. A bone marrow examination can often provide diagnostic information if the physical exam and peripheral blood smear are inconclusive. An approach to the child with thrombocytopenia is shown in Fig. 435-2.
TABLE 435-1POTENTIAL CAUSES OF THROMBOCYTOPENIA BY AGE ||Download (.pdf) TABLE 435-1POTENTIAL CAUSES OF THROMBOCYTOPENIA BY AGE
|All Age Groups ||Neonates/Infants |
|Immune-Mediated ||Immune-Mediated |
Drug-induced immune thrombocytopenia
Autoimmune lymphoproliferative syndrome (ALPS)
Systemic lupus erythematosus and other systemic autoimmune conditions
Vascular malformations including Kasabach-Merritt syndrome
Disseminated intravascular coagulation
Thrombotic thrombocytopenic purpura
Hemolytic uremic syndrome
Viral infections including Epstein-Barr virus, human immunodeficiency virus, parvovirus
Sepsis, Rocky Mountain spotted fever
Bone Marrow Infiltration/Failure
Bone marrow failure syndromes including aplastic anemia, Fanconi anemia, RUNX1 mutations, and others
Congenital thrombocytopenia, late onset
von Willebrand disease type 2B
Platelet-type von Willebrand disease
Congenital heart disease
|Neonatal alloimmune thrombocytopenia (NAIT) |
|Neonatal autoimmune thrombocytopenia (maternally derived antibodies) |
|TORCH infection (toxoplasmosis, rubella, cytomegalovirus, herpes simplex) |
|Necrotizing enterocolitis |
|Sepsis with and without disseminated intravascular coagulation |
|Pregnancy and Perinatal Factors |
|Preeclampsia/pregnancy-induced hypertension |
|Maternal drug use |
|Neonatal cold injury |
|Congenital Thrombocytopenia Syndromes |
|Congenital amegakaryocytic thrombocytopenia |
|Thrombocytopenia-absent radii syndrome |
|Wiskott-Aldrich syndrome and X-linked thrombocytopenia |
|Congenital thrombocytopenia (eg, MYH9-related disorders, Bernard-Soulier syndrome) |
|DiGeorge syndrome |
|Bone Marrow Infiltration |
|Transient myelopoiesis associated with trisomy 21 |
|Infant leukemia |
|Storage diseases |
A diagnostic approach to the patient with thrombocytopenia. While this outlines a general approach, it should be emphasized that exceptions as well as overlap in symptoms and findings can occur, often necessitating additional considerations. ALPS, autoimmune lymphoproliferative disorder; BSS, Bernard-Soulier syndrome; CAMT, congenital amegakaryocytic anemia; CBC, complete blood count; DIC, disseminated intravascular coagulation; EBV, Epstein-Barr virus; HIV, human immunodeficiency virus; ITP, immune thrombocytopenia; MYH9-RD, MYH9-related disorders; SLE, systemic lupus erythematosus; TAR, thrombocytopenia-absent radius syndrome; TTP, thrombotic thrombocytopenic purpura; WAS, Wiskott-Aldrich syndrome; XLT, X-linked thrombocytopenia.
Immune thrombocytopenia (ITP) is an acquired disorder that has a multifactorial etiology including generation of antiplatelet autoantibodies and subsequent reticuloendothelial clearance, direct T cell cytotoxicity, and abnormal platelet production in the bone marrow.
Immune thrombocytopenia in children occurs most frequently between the ages of 2 and 10 years. The typical presentation is an otherwise well-appearing child with platelet-type bleeding, including petechiae, purpura, and bruising. Moderate bleeding that may warrant treatment can include oral bleeding, epistaxis, wet purpura, and menorrhagia. Life-threatening bleeding, including gastrointestinal or genitourinary tract and central nervous system (CNS) bleeding, occurs very rarely. The child is typically previously healthy, with the exception of a recent viral infection, immunization, or other immune trigger. Immune thrombocytopenia is a diagnosis of exclusion. There are multiple disorders that should be ruled out with clinical history, blood count, smear review, and other laboratory testing as indicated. A benign history (absence of systemic symptoms including bone pain and weight loss) and lack of hepatosplenomegaly or lymphadenopathy on physical examination are important considerations in differentiating ITP from conditions such as leukemia or lymphoma. Immune thrombocytopenia at diagnosis is classified as either primary or secondary ITP. Primary ITP, or ITP not triggered by another disorder, occurs in up to 75% of children. Secondary ITP is that triggered by another disorder, including autoimmune diseases, immunodeficiency syndromes, thyroid disease, infection, or pregnancy.
The cornerstone of the laboratory evaluation of suspected ITP is the CBC and peripheral blood smear review. The CBC should be normal except for thrombocytopenia, and platelet size is generally variable to large. Patients with ITP may be anemic due to significant bleeding; however, alternative diagnoses such as leukemia, aplastic anemia, Evans syndrome, and thrombotic thrombocytopenic purpura should be considered if multiple cytopenias are identified. Reticulocytosis should prompt consideration of concomitant hemolysis. Review of the peripheral blood smear reveals normal leukocyte and red blood cell morphology, and platelets that should appear normally granulated and variable in size with the presence of scattered large/giant platelets. Direct antiglobulin test (DAT; formerly known as direct Coombs test) and serum immunoglobulins are recommended in all newly diagnosed ITP patients, as these markers may be associated with an underlying tendency toward autoimmunity. Antinuclear antibody (ANA) testing can be obtained in patients with high suspicion for autoimmune disease.
Antiplatelet antibody testing has not been shown to have adequate sensitivity and specificity for use in the diagnosis or management of ITP. For this reason, the American Society of Hematology and International Working Group have both recommend against routine platelet antibody testing for the diagnosis of ITP. Bone marrow examination is recommended only in circumstances where the diagnosis is not clear due to the presence of atypical features. Current recommendations do not include routine bone marrow studies prior to initiating steroid treatment or in the case of a patient who fails intravenous immunoglobulin (IVIG) therapy. Thrombopoietin levels are not recommended, as they are often not increased despite increased peripheral destruction. This observation led to the development of new therapies with recombinant thrombopoietin agonists.
In approximately 75% of children, ITP is a self-limited condition with spontaneous resolution regardless of interventions and treatments during the course of the disease. After remission, the chance of having another episode of ITP is small (<5%), and following recovery, children are not at increased risk for other blood disorders or cancers. The most feared outcome, intracranial hemorrhage, is fortunately rare, occurring in less than 1% of affected children. The International Working Group Consensus Report recommends the following categories to stratify ITP cases according to duration of the disease in order to facilitate guidance and treatment decisions:
Newly diagnosed: 0–3 months from diagnosis
Persistent: 3–12 months from diagnosis
Chronic: > 12 months from diagnosis
This categorization was changed from the former terminology (acute, < 6 months, and chronic, > 6 months, from diagnosis) due to the large number of children entering spontaneous remission beyond 12 months from diagnosis.
Given the typically mild clinical symptoms and expectation of resolution in childhood ITP, most experts recommend observation without treatment regardless of the platelet count in children with no symptoms or with only mild cutaneous findings. However, for children with “wet” bleeding symptoms, including wet purpura, epistaxis, or menorrhagia, treatment is recommended to elevate the platelet count to a hemostatic range, facilitating cessation of bleeding and decreasing the risk of intracranial hemorrhage (ICH) and other forms of life-threatening bleeding. Treatment to raise the platelet count and lessen bleeding symptoms includes corticosteroids, IVIG, and anti-D immunoglobulin. These front-line therapies function at least in part by interfering with immune destruction of antibody-coated platelets.
Steroid dosing and duration recommendations are varied, but oral prednisone, dexamethasone, and intravenous methylprednisolone are the most commonly used agents. Commonly, a course of 1 to 4 mg/kg/d of oral prednisone for 2 to 4 weeks (including a taper) is effective. Platelets usually rise about 3 to 5 days after steroid initiation, but it can take up to 2 weeks for an effect to be seen. Side effects of steroids include weight gain, irritability, hypertension, and hyperglycemia.
Intravenous immunoglobulin inhibits reticuloendothelial clearance of platelets via blockage of Fc receptors. A single dose of 0.8 to 1.0 g/kg commonly leads to an increased platelet count within 24 to 48 hours; side effects include fever and chills during the infusion, headache, and rarely, aseptic meningitis. Similar to IVIG, anti-D immunoglobulin binds and blocks Fc receptors, specifically via binding with antibody-coated red blood cells and is therefore only used in patients who are Rh positive. Typical dosing is 50 to 75 μg/kg, and like IVIG, the response time is typically rapid. A decline in hemoglobin concentration by 0.5 to 2.0 g/dL or more can be expected due to induced red blood cell hemolysis, and rare cases of severe intravascular hemolysis and renal failure have been described in predisposed patients.
The choice of therapy depends on a variety of factors, including the side effect profile of each agent and the indication for treatment, as well as patient-related factors. Platelet transfusion is generally avoided due to the expected rapid antibody-mediated clearance of transfused platelets and theoretical risk of increased antibody development, but can be used in the management of serious or life-threatening bleeding. Antifibrinolytic agents (aminocaproic acid and tranexamic acid) can be helpful adjuncts to therapy for mucosal bleeding symptoms. Medications that affect platelet number or function (aspirin, NSAIDs) should be avoided. Activity restrictions may be required depending on the platelet count and bleeding symptoms.
In the case of severe or life-threatening hemorrhage, an aggressive approach with a combination of therapies is warranted. Intravenous immunoglobulin and high-dose IV steroids should be given, with consideration of platelet transfusion/drip to facilitate hemostasis acutely. If the patient has signs and symptoms worrisome for ICH, expeditious imaging should be obtained with surgical and neurosurgical consultations as needed. Urgent/emergent splenectomy can be life-saving in the setting of uncontrolled bleeding or neurologic compromise.
Management of patients with chronic ITP is based on evaluation of the patient’s bleeding symptoms, platelet count, overall quality of life, and ability to perform daily activities. Because approximately one-third of patients will have spontaneous remission of their chronic ITP even years after diagnosis, management often consists of observation alone, with pharmacologic intervention reserved for severe thrombocytopenia or bleeding episodes, or before anticipated hemostatic challenges (invasive procedures or high-risk physical activities). However, some children with chronic ITP have more severe thrombocytopenia or significant and frequent bleeding episodes or quality-of-life impairments that necessitate more definitive and durable treatment. Options include rituximab, thrombopoietin receptor agonists, alternative immunosuppressive regimens, and splenectomy. Splenectomy is successful in approximately 70% of children, but concerns over the risk of thrombosis and long-term risk of severe infection, as well as favorable side effect profiles of nonsurgical options, limit its use.
The monoclonal CD20 antibody rituximab has been reported to elicit a complete platelet response for up to a year in 30% of patients with chronic ITP. Infusion reactions, risk of hypogammaglobulinemia and/or infections and very rare risk of progressive multifocal leukomalacia can occur. The thrombopoietin agonists romiplostim and eltrombopag are efficacious in approximately 70% of children and are increasingly used due to their favorable side effect profiles and potential for long-term use. These agents work by increasing platelet production, to compensate for peripheral loss and increase circulating platelet count. Short-term side effects are generally mild and improve with duration of use. Serious side effects are rare and include risk of thrombosis and potential for bone marrow fibrosis. Eltrombopag is an oral agent taken once daily. The dose is weight based with a maximum dose of 75 mg. Patients of East Asian descent are started at lower doses and monitored closely, as they have been shown to have greater drug exposure. Liver function abnormalities are possible and generally reversible on drug discontinuation. Romiplostim is administered subcutaneously once weekly. The manufacturer recommends weight-based dosing and escalating to a maximum of 10 μg/kg.
Evans syndrome is characterized by multiple immune cytopenias. Classic Evans syndrome is the combination of immune thrombocytopenia and DAT-positive hemolytic anemia; however, neutropenia secondary to antineutrophil antibodies may also occur. In up to one-half of patients, Evans syndrome is secondary to other underlying conditions, including systemic lupus erythematosus, autoimmune lymphoproliferative disorder (ALPS), common variable immunodeficiency, and other immune dysregulations/deficiencies. It can also occur after hematopoietic stem cell transplantation. It is important to have a high suspicion for these underlying disorders because treatment of secondary Evans syndrome is optimal management of the underlying condition.
Typically, Evans syndrome is treated acutely with corticosteroids, which are tapered very gradually based on the patient’s hematologic response. Rituximab is a second-line treatment option in the case of steroid dependence/refractoriness. Intravenous immunoglobulin is not as effective in Evans syndrome as it is in ITP, and thus should not be used as a first- or second-line agent in the treatment of Evans syndrome. Splenectomy may be required for refractory cases but is generally avoided in cases secondary to ALPS.
The most common drugs causing thrombocytopenia are heparin, quinine/quinidine, sulfa derivatives, vancomycin, penicillin, rifampin, carbamazepine, ceftriaxone, ibuprofen, mirtazapine, oxaliplatin, suramin, and glycoprotein (GP) IIb-IIIa inhibitors. In almost all cases, thrombocytopenia results from drug-dependent antiplatelet antibodies that bind to specific platelet antigens. Drug withdrawal leads to a prompt rise in the platelet count. Heparin-induced thrombocytopenia (HIT) is relatively uncommon in children. It is a specific immune-mediated condition in which antibodies bind to complexes of heparin and platelet factor 4 (PF4) released from platelet alpha granules, resulting in platelet aggregation. Thrombocytopenia develops 5 to 14 days after first heparin exposure, and venous or arterial thrombosis can occur as a result of platelet activation and thrombin generation. Heparin-induced thrombocytopenia is a clinical-pathologic syndrome and is diagnosed based on the presence of 1 or more HIT-associated clinical symptoms and the detection of heparin-PF4-IgG immune complexes. A precipitous drop in platelet count to fewer than 100,000/mm3, or a reduction in platelet count greater than 50% in an individual exposed to heparin, should raise suspicion for this diagnosis. A robust clinical scoring system can help determine whether confirmatory HIT testing should be performed. Laboratory testing, often with long turn-around times, should only be pursued for patients with a high clinical suspicion. If the decision is made to send laboratory testing, the platelet [14C] serotonin release assay should be sent, as it is the gold standard test for the diagnosis of HIT. However, this test is technically challenging and not widely available, limiting usefulness when making treatment decisions. Enzyme-linked immunosorbent assay (ELISA) to detect heparin/PF4 complexes is more readily available, but the result should be carefully interpreted within the clinical context as false positives can occur. Discontinuation of all heparin and use of an alternative anticoagulant is standard treatment for HIT and should be initiated when testing is performed.
NEONATAL ALLOIMMUNE THROMBOCYTOPENIA
Neonatal alloimmune thrombocytopenia (NAIT) is a common cause of severe, isolated thrombocytopenia in the newborn, with an estimated occurrence of 1 in 1000 to 10,000 live births. Otherwise healthy infants (including firstborns) can present with petechiae, purpura, mucosal bleeding, or severe bleeding manifestations, including intracranial hemorrhage in utero or in the immediate neonatal period.
Neonatal alloimmune thrombocytopenia results from incompatibility between the maternally and paternally derived fetal platelet antigens (analogous to hemolytic disease of the newborn). Maternal IgG antibodies directed at incompatible human platelet alloantigens (HPA) cross the placenta and lead to peripheral platelet destruction in the fetus/neonate. The most common antigens involved in the United States are HPA-1a and HPA-5b (~95% of cases). Neonatal alloimmune thrombocytopenia should be high on the differential for otherwise well newborns with thrombocytopenia (Table 435-2). Other considerations include congenital infection, disseminated intravascular coagulation (DIC), congenital heart disease, other congenital disorders, and maternal autoimmune thrombocytopenia. Important differences between NAIT and maternal autoimmune thrombocytopenia can be found in Table 435-2.
TABLE 435-2FEATURES OF MATERNAL AUTOIMMUNE THROMBOCYTOPENIA VERSUS NEONATAL ALLOIMMUNE THROMBOCYTOPENIA ||Download (.pdf) TABLE 435-2FEATURES OF MATERNAL AUTOIMMUNE THROMBOCYTOPENIA VERSUS NEONATAL ALLOIMMUNE THROMBOCYTOPENIA
| ||Maternal Autoimmune Thrombocytopenia ||Neonatal Alloimmune Thrombocytopenia |
|Incidence ||1:5000 ||1:1000 |
|Mother’s platelet count ||Reduced ||Normal |
|Antibody specificity ||Nonspecific antigen on maternal and neonatal platelets ||Usually HPA-1a or HPA-5b on neonatal platelets |
|Effect on platelet function ||None ||Significant |
|Hemorrhage ||Usually mild ||Often severe |
|Treatment ||Intravenous immunoglobulin for bleeding or severe thrombocytopenia ||Transfusion of maternal platelets, if available; random donor platelets and intravenous immunoglobulin if maternal platelets unavailable |
The diagnosis of NAIT should be considered in any well-appearing term infant who presents with severe thrombocytopenia in the first 24 to 28 hours of life but can also be considered in ill-appearing infants with thrombocytopenia. Although the diagnosis is confirmed by specific alloantibody detection using ELISA testing, these tests often do not yield timely results to assist with treatment decisions. Treatment should thus proceed based on clinical presentation and exam/laboratory findings.
Specific platelet surface molecule genotyping of the mother and father can identify the antigen incompatibility so that parents may be informed for management of future pregnancies.
The major source of morbidity and mortality in NAIT is ICH, which occurs in 10% to 20% of cases. Up to 75% of these hemorrhages occur prenatally between 20 weeks of gestation and term. All infants with a platelet count of fewer than 50,000/mm3 should have cranial ultrasonography as soon as possible after delivery to assess for ICH, which usually occurs when platelet counts are below 20,000/mm3. While NAIT is self-limited, resolving within 2 to 6 weeks, treatment aimed at rapidly raising the platelet count and decreasing the risk of ICH in the immediate postnatal period is critical. Healthy term infants without risk factors for hemorrhage should be transfused if the platelet count is below 30,000/mm3 or if there are any signs of bleeding. Neonates with significant illness or evidence of ICH should have a higher threshold for platelet transfusions (< 50,000–100,000/mm3). Platelet count thresholds should be maintained during the first 72 to 96 hours of life because the ICH risk is highest during this timeframe. Afterwards, the decision to transfuse platelets should be based on the clinical scenario. Platelet transfusion with maternally derived platelets is preferred; however, lack of availability of maternal platelets should not delay treatment. Random donor platelets have been shown to adequately raise the platelet count and are suitable for urgent use, but the survival of incompatible platelets may be short, which requires frequent reassessments and transfusions. Intravenous immunoglobulin can increase the platelet count, but because response can take up to 24 to 72 hours, it should be used in conjunction with transfusion therapy. Additionally, IVIG is often effective in prolonging the survival of transfused platelets. In cases of severe bleeding, when the above measures fail, exchange transfusion has been successful.
Mothers of infants with NAIT must be counseled on the risk during subsequent pregnancies, which is highest for those women who have had a previous child with ICH from NAIT. Management strategies for cases of NAIT recognized in utero include administration of IVIG and/or corticosteroids to the mother and/or fetal blood sampling with weekly in utero platelet transfusions. Because of the high risk associated with weekly fetal blood sampling, initial management should begin with maternal therapy. In addition, many advocate cesarean-section delivery of all subsequent pregnancies, although this remains controversial as a vaginal delivery may be less traumatic. Maternal platelets should be obtained prior to delivery in all known cases of NAIT so that immediate transfusion with compatible platelets can occur.
MATERNAL AUTOIMMUNE NEONATAL THROMBOCYTOPENIA
Infants born to mothers with a history of ITP or systemic lupus erythematosus (SLE) may develop thrombocytopenia at birth due to transfer of maternal antiplatelet IgG across the placenta into the fetal circulation. The degree of maternal thrombocytopenia is unrelated to the infant’s platelet count. The risk of serious bleeding is significantly less than that associated with neonatal alloimmune thrombocytopenia (the ICH frequency is similar to ITP at <1%). In utero assessment of platelet count is not indicated, but a cord blood platelet count should be performed at birth and repeated periodically, as platelet counts can decline with a nadir around 4 days of age. The majority of hemorrhagic events occur within 24 to 48 hours after birth, so infants should be monitored closely during this period and consideration given to appropriate imaging studies.
If significant hemorrhage or severe thrombocytopenia (< 20,000/mm3) is encountered in the infant, IVIG is the treatment of choice. Platelet transfusions may be given in the case of significant hemorrhage but may not be effective secondary to autoantibody reaction with transfused donor platelets. Thrombocytopenia typically lasts for 1 to 2 months, reflecting the survival of maternal IgG in the infant. Subsequent births are also at risk.
INCREASED PLATELET DESTRUCTION/CONSUMPTION
Disseminated Intravascular Coagulation
Disseminated intravascular coagulation, a consumptive coagulopathy resulting from infection, trauma, malignancy, or a variety of other causes, leads to bleeding and microthrombosis. Clues suggestive of DIC include elevations in the prothrombin time/activated partial thromboplastin time and in D-dimers, low fibrinogen levels, thrombocytopenia, and the presence of an underlying condition known to be associated with DIC. Resolution of DIC is dependent upon successful treatment of the underlying condition. Often coagulation abnormalities can result in significant bleeding or thrombosis, and thus blood product support and/or anticoagulation are required to minimize bleeding risk. Blood product replacement should not be given to correct coagulation tests, but rather to treat patients with clinically significant bleeding symptoms.
Thrombotic Thrombocytopenic Purpura
Thrombotic thrombocytopenic purpura (TTP) is a very rare but potentially life-threatening disorder that results from inadequate cleavage of von Willebrand factor (vWF) multimers due to either congenital absence of or acquired antibody to the specific vWF–cleaving metalloprotease ADAMTS13. The resulting large vWF multimers promote formation of platelet microthrombi in the microvasculature, leading to end-organ damage and development of microangiopathic hemolytic anemia. The classic diagnostic pentad for TTP comprises microangiopathic hemolytic anemia, thrombocytopenia, neurologic findings, renal manifestations, and fever; however, not all features need be present to make the diagnosis. Laboratory findings include anemia, thrombocytopenia often less than 20,000/mm3, reticulocytosis, the presence of schistocytes and erythrocyte fragments on the peripheral blood smear, and an elevated lactic dehydrogenase (LDH) from a combination of hemolysis and tissue damage/ischemia. Measurement of low (<10%) or absent ADAMTS13 activity is confirmatory of the disease; to further distinguish congenital versus acquired types, plasma inhibitory and/or noninhibitory anti-ADAMTS13 antibodies can be measured. ADAMTS13 and antibody levels are performed at specialty hematology laboratories that can require up to a 7-day turnaround, and therefore, awaiting these results should not delay diagnosis and treatment because mortality without treatment approaches 80%. Recommended therapy for congenital TTP involves plasma infusions, whereas the more common acquired TTP is treated urgently with plasma exchange, often supplemented with corticosteroids and/or rituximab. Hemolytic uremic syndrome is thought to exist on a continuum with TTP and thus may feature thrombocytopenia, but it exhibits more extensive renal involvement and often does not require plasma exchange.
Platelet destruction can also occur within vascular lesions (typically kaposiform hemangioendotheliomas or tufted angiomas) of the skin, liver, or spleen, and some patients may even have evidence of a consumptive coagulopathy (Kasabach-Merritt syndrome). Management of the thrombocytopenia and coagulopathy is dependent upon management of the lesion.
DECREASED PLATELET PRODUCTION
Congenital thrombocytopenias represent a group of disorders in which thrombocytopenia can be present at birth with variable severity. As outlined below, these conditions are often associated with other physical findings or disorders.
The MYH9-related disorders represent a group of autosomal dominant macrothrombocytopenias characterized by mutations in the MYH9 gene, which encodes the nonmuscle myosin heavy chain II. The syndrome includes previously classified disorders May-Hegglin anomaly, Sebastian syndrome, Fechtner syndrome, and Epstein syndrome. They are characterized by macrothrombocytopenia and mucocutaneous bleeding in early life, with the development of hearing loss, glomerulonephritis, and cataracts with aging, depending on the syndrome. Although the platelet count may be variable from patient to patient, a peripheral blood smear showing large or giant platelets and/or neutrophils with Döhle body inclusions is highly suggestive of MYH9. Mutation analysis of the MHY9 gene can confirm the diagnosis. Treatment may include antifibrinolytic agents for minor bleeding and platelet transfusions for significant bleeding events and as prophylaxis for surgery. Other causes of macrothrombocytopenia are outlined in Figure 435-2.
Wiskott-Aldrich Syndrome and X-Linked Thrombocytopenia
Wiskott-Aldrich syndrome (WAS) and the less severe X-linked thrombocytopenia (XLT) syndrome are X-linked disorders associated with microthrombocytopenia caused by mutations in the WAS gene located at Xp11.22-11.23. X-linked thrombocytopenia presents with isolated thrombocytopenia, whereas WAS presents with thrombocytopenia and a severe immunodeficiency leading to recurrent infections, allergies and eczema, autoimmunity, lymphoid malignancy, and lymphoproliferative disorders. Bleeding manifestations during infancy may bring patients to medical attention and continue throughout life, with 30% of patients encountering at least 1 severe bleeding episode. Wiskott-Aldrich syndrome and XLT should be suspected in any male patient with thrombocytopenia and small platelets on the peripheral blood smear. Other laboratory abnormalities in WAS include a decrease in the number and function of T lymphocytes, low IgM levels, and high IgA and IgE levels. If WAS is suspected, consultation with an immunologist is warranted.
The diagnosis of WAS can be confirmed by flow-cytometric detection of absent WAS protein or by genetic testing. Direct mutational analysis can identify mutations; however, up to 10% of patients will have no identifiable mutation. Bleeding symptoms associated with WAS and XLT can be managed with platelet transfusions. They may also respond to immune-directed therapies that are used in ITP, although high-dose or prolonged steroid exposure should be avoided due to the risk of infection. Splenectomy has been successful at correcting thrombocytopenia, but the risks of severe infection often outweigh the benefits. Treatment should also include prophylactic antibiotics against Pneumocystis jirovecii pneumonia. Early stem cell transplantation is considered standard of care for any patient with a matched sibling or a 10/10 HLA-matched unrelated donor, as this is the only proven curative treatment. Ongoing gene therapy trials may provide new options for patients without a suitable donor available.
Thrombocytopenia-Absent Radius Syndrome
Thrombocytopenia-absent radius syndrome (TAR) is an autosomal recessive disorder characterized by severe congenital thrombocytopenia (< 50,000/mm3) and bilateral absence of radii. The presence of normal thumbs helps distinguish this from other disorders that cause thrombocytopenia and radial abnormalities such as Fanconi anemia. The inheritance of this disorder is complex, but involves mutations in the RNA binding motif protein 8A (RBM8A) gene on chromosome 1q21.1. Most cases are caused by compound heterozygous inheritance of a rare null mutation on 1 allele and a low frequency noncoding polymorphism on the other allele. Additional features include facial capillary hemangiomas, developmental delay, and skeletal, cardiac, and renal anomalies. There is a high frequency of cow milk allergy associated with TAR, and it is recommended that children avoid cow’s milk formula for the first year of life. Although more severe thrombocytopenia can be triggered by infection and diet, the platelet counts tend to improve with age, and TAR does not progress to bone marrow aplasia. Any neonate with bilateral absent radii should be evaluated with serial platelet counts. Bone marrow evaluation will demonstrate decreased to absent megakaryocytes. Platelet transfusions for bleeding episodes are the mainstay of treatment, but with aging, treatment is rarely indicated.
Congenital Amegakaryocytic Thrombocytopenia
Congenital amegakaryocytic thrombocytopenia (CAMT), characterized by severe thrombocytopenia in the newborn period, is a rare autosomal recessive disorder of platelet production affecting the MPL gene. Mutations in this gene result in altered expression or function of the thrombopoietin receptor, rendering it unresponsive to thrombopoietin and resulting in severe thrombocytopenia. Unlike patients with TAR, skeletal findings are absent and thrombocytopenia does not improve with age. Patients often progress to pancytopenia and/or severe aplastic anemia within 5 to 10 years. Thrombocytopenia diagnosed in infancy, along with reduced or absent megakaryocytes in the bone marrow, is suggestive of the disease; definitive diagnosis is obtained with confirmation of mutations in the MPL gene. Initial management is aimed at the prevention and control of hemorrhage via platelet transfusions. Early stem cell transplantation should be considered, particularly if there is a matched sibling donor, because of the high rate of bone marrow failure.
Bone Marrow Failure and Infiltration
Bone marrow failure and infiltration should be considered in any patient with multiple cytopenias. The differential diagnosis is extensive and includes acquired aplastic anemia, Fanconi anemia, parvovirus (or other viral suppression), leukemia, and osteopetrosis. Each of these entities is discussed more extensively in their respective chapters.
Splenic Sequestration and Pooling
Increased platelet sequestration occurs in the setting of hypersplenism and is often accompanied by other cytopenias. Causes of hypersplenism include portal hypertension, portal vein thrombosis, storage diseases, hematological disorders such as sickle cell anemia, and infiltrative diseases of the liver and spleen. Virtually all patients have marked splenomegaly.
Pseudothrombocytopenia is a cause of spurious thrombocytopenia in an otherwise healthy individual with no clinical findings. It results from in vitro ethylenediaminetetraacetic acid (EDTA)-dependent platelet aggregation and clumping. Upon repeating testing using an alternative anticoagulant such as citrate, the thrombocytopenia resolves, confirming the presence of pseudothrombocytopenia.
Thrombocytosis (defined as a platelet count exceeding 500 × 103/mm3) occurs secondary to another condition in up to 15% of hospitalized children and is known as reactive thrombocytosis (Table 435-3). A characteristic common to many of the potentially associated disorders is that they often lead to stimulation of megakaryopoiesis, either through increased interleukin 6 (IL-6) release or via direct increase in thrombopoietin production. A careful history and physical examination can frequently help identify an underlying diagnosis; acute phase reactants may also be increased. As a secondary process, reactive thrombocytosis tends to be transient, with normalization of the platelet count coinciding with resolution of the primary condition. High platelet counts (even surpassing 1000 × 103/mm3) can be encountered under these circumstances and are usually not associated with increased risk of thrombosis, unless they occur following splenectomy in conjunction with other prothrombotic risk factors. Treatment should be directed at the underlying disorder, but thromboprophylaxis or other intervention may warrant consideration if the patient has coexisting risk factors such as those mentioned above.
TABLE 435-3CAUSES OF THROMBOCYTOSIS ||Download (.pdf) TABLE 435-3CAUSES OF THROMBOCYTOSIS
|Familial thrombocytosis |
|Myeloproliferative disorders |
|Essential thrombocythemia |
|Polycythemia vera |
|Chronic myelogenous leukemia |
|Secondary (Reactive) |
|Infectious/inflammatory state |
|Tissue damage (trauma/surgery/burns) |
|Iron deficiency |
|Kawasaki disease |
|Renal disease |
|Hemolytic anemia |
|Autoimmune disease |
|Blood loss |
|Vinca alkaloids |
|Other causes |
|Low birth weight |
|Metabolic diseases |
|Rebound thrombocytosis |
In contrast to reactive thrombocytosis, primary thrombocytosis, caused by a clonal hematopoietic cell abnormality, is extremely rare in children (1 per 10 million). Diagnosis depends on the exclusion of other underlying conditions and thus requires extensive testing, typically in consultation with a hematologist. Organomegaly is a common finding, as are platelet morphologic abnormalities. Identifying affected individuals is important because of increased thrombotic and hemorrhagic risks that may accompany the condition. Treatment options include long-term antiplatelet and cytoreductive therapy. About half of the cases of primary thrombocytosis are thought to be familial; reports link the pathophysiology in such cases to aberrant thrombopoietin biology. Thrombohemorrhagic risk appears to be lower in such cases.
QUALITATIVE PLATELET DISORDERS
Adequate platelet function requires ligand-receptor interactions, molecular pathways relevant to signal transduction, metabolism, protein synthesis and modification, granule secretion, cytoskeletal rearrangement, and procoagulant activity. Derangements of any of these processes can lead to altered platelet structure and/or function. Most of the currently recognized platelet dysfunction disorders involve decreased platelet function and are thus associated with bleeding symptoms; these disorders, both acquired and inherited, are the focus of this section. Clinicians should be aware that platelet hyperfunction is an increasingly recognized feature of various conditions (such as in heparin-induced thrombocytopenia) and should be taken into consideration, especially in patients who have other increased thrombosis risks.
METHODS OF ASSESSING QUALITATIVE PLATELET ABNORMALITIES
Several laboratory methods are available to the clinician for evaluating a patient with suspected platelet dysfunction, but the results of such tests should be interpreted cautiously in consultation with a pediatric hematologist.
Aggregometry testing assesses platelet aggregation in response to various platelet agonists (eg, adenosine diphosphate, collagen, epinephrine, ristocetin) using either optical or impedance testing. Optical aggregometry is performed on platelet-rich plasma (PRP) and is thus the more cumbersome option. It is considered the standard method for assessing platelet function, but its technical requirements have limited its widespread use. In this test, the PRP is placed between a light source and a measuring photocell. Addition of the agonists to the PRP results in platelet responses (shape change/aggregation) depending on normal platelet function, the presence of platelet inhibitors, and the concentration of the agonist. The transmission of light is detected and recorded as a function of time. Impedance aggregometry measures the increase in electrical impedance as a current is applied to an agonist-stimulated whole blood specimen placed between 2 wire electrodes. Platelets aggregate on the instrument’s electrodes, changing the impedance with time. This test is faster and simpler to perform.
The bleeding time has fallen out of widespread clinical use due to concerns about its poor sensitivity, specificity, and reproducibility. In many settings it has been replaced by the PFA-100 test, which some regard as an ex vivo bleeding time measurement. However, despite its acceptable reproducibility, the PFA-100 can be insensitive to mild or moderate platelet function defects and should only be used as a screening tool for platelet dysfunction.
Electron microscopy allows ultrastructural analysis of platelet morphology and can be especially useful for evaluation of storage pool disorders. Flow cytometry is used to identify deficiencies of platelet surface glycoproteins (eg, Bernard-Soulier syndrome and Glanzmann thrombasthenia). The method can also be used to diagnose storage pool disorders, monitor antiplatelet therapy, and quantify platelet activation. Thromboelastography, using either the thermoelectric generator or rotational thromboelastometry (ROTEM) systems, assesses platelet function and coagulation by testing parameters of clot formation in whole blood. It is most often used to predict thromboembolic events in surgical patients, but the role for its general use in hematology practice is less defined.
ACQUIRED QUALITATIVE PLATELET DISORDERS
Acquired platelet function disorders can be caused by underlying infection or disease, drugs, autoimmunity, or trauma. The clinical impact of these factors on bleeding tendency is often uncertain, can vary between patients, and may only be appreciated in the presence of other risk factors for bleeding (eg, thrombocytopenia, coagulation disorders, use of antithrombotic therapy).
A careful medication history (including other exposures, such as herbs) should be obtained in patients with unexplained bleeding or suspected platelet dysfunction. Aspirin (acetylsalicylic acid) irreversibly blocks the formation of thromboxane A2 by platelet cyclooxygenase, resulting in an inhibitory effect on platelet aggregation for the lifespan of the platelet (7–10 days). Other nonsteroidal anti-inflammatory drugs such as ibuprofen, naproxen, and indomethacin bind and inhibit cyclooxygenase, but do so reversibly, thus impacting platelet function for a shorter duration. β-Lactam antibiotics induce platelet dysfunction by interfering with the interaction between platelet agonists and their receptors. Many other agents have been reported to induce platelet dysfunction, but their clinical relevance for a given patient may be difficult to assess.
Other systemic disorders, including uremia and liver disease, can contribute to platelet dysfunction, often via multiple mechanisms. The causes of bleeding in uremia are multifactorial, leading to decreased platelet aggregation and adhesion as a result of intrinsic dysfunction of GP IIb-IIIa. This membrane protein normally functions in platelet aggregation and adhesion by binding with fibrinogen and/or vWF. Additionally, an increased concentration of nitric oxide (NO) and anemia can add to the bleeding diathesis in uremia. Nitric oxide, an inhibitor of platelet aggregation, is often increased in the plasma of patients with uremia, due in part to accumulation of the NO precursor guanidinosuccinic acid. The elevated NO levels lead to decreased thromboxane A2 and ADP levels and impaired platelet aggregation. Anemia is proposed to affect platelet function via attenuation of interactions between platelets and the vessel wall. Maintaining the hemoglobin above 10 g/dL has been shown to reduce bleeding diatheses in renal disease. Renal replacement therapy, cryoprecipitate, 1-deamino-8-D-arginine vasopressin (DDAVP; increases vWF release from endothelial cells), and conjugated estrogens (decrease NO levels) have all been reported to mitigate bleeding to varying degrees.
Liver disease can result in platelet dysfunction by a variety of mechanisms that affect aggregation. Platelet dysfunction can occur as a result accumulation of increased fibrin split products (which impair platelet surface glycoprotein function), degradation of platelet receptors, elevated NO levels, and/or a variety of intrinsic platelet abnormalities. Platelet transfusion and DDAVP are among the therapeutic options.
Cardiopulmonary bypass can be associated with a number of hemostatic abnormalities, including hyperfibrinolysis, heparin effect, thrombocytopenia, and platelet dysfunction. The latter, thought to be caused by platelet fragmentation and activation in the bypass circuit, can persist up to 24 hours after discontinuation of bypass. Treatments have included DDAVP and antifibrinolytic agents.
INHERITED QUALITATIVE PLATELET DISORDERS
Inherited platelet function disorders are uncommon and often difficult to diagnose. Those that have been more fully defined (such as Glanzmann thrombasthenia and Bernard-Soulier syndrome) involve critical components of the platelet machinery and typically demonstrate a more severe clinical phenotype (Figure 435-3). However, there is evidence to support molecular lesions involving other perhaps less critical (or more redundant) aspects of platelet function that are relatively more common and may contribute to an increased bleeding tendency. Assessment for qualitative platelet disorders should be considered early in the evaluation of patients with unexplained mucocutaneous bleeding and should be undertaken in consultation with a hematologist. Table 435-4 summarizes key aspects of the more effectively characterized qualitative platelet disorders. As some platelet disorders feature qualitative defects in platelet structure, these are also included. Treatment for the platelet function disorders includes antifibrinolytics or DDAVP for mild-to-moderate bleeding and platelet transfusions and/or recombinant factor VIIa for severe bleeding. Hematopoietic stem cell transplantation may be considered in severe cases. A complication of repeated platelet transfusions in Glanzmann thrombasthenia and Bernard-Soulier syndrome is development of alloantibodies directed against glycoproteins GP IIb-IIIa and GP Ib-IX-V, respectively, of the transfused platelets, and therefore care must be taken to avoid unnecessary transfusion.
TABLE 435-4INHERITED QUALITATIVE PLATELET DISORDERS ||Download (.pdf) TABLE 435-4INHERITED QUALITATIVE PLATELET DISORDERS
|Disorder ||Inheritance ||Chromosome and Gene ||Molecular Defect ||Laboratory Findings ||Associated Clinical Findings |
|Bernard-Soulier syndrome ||AR || |
17, 22, 3
|Decreased or dysfunctional GP Ib-IX-V || |
↓ Aggregation to multiple agonists, except ristocetin
Absent GP Ib-IX-V receptor on flow cytometry
|Heterozygous state associated with DiGeorge syndrome and Mediterranean thrombocytopenia |
|Glanzmann thrombasthenia ||AR || |
|Decreased or dysfunctional GP IIb-IIIa || |
↓ Aggregation to ristocetin
Absent GP IIb-IIIa receptor on flow cytometry
|Possible increased bone thickening and low fertility |
|Platelet-type von Willebrand disease ||AD || |
|Gain of function of GP Ib-IX-V for vWF || |
Heterogeneous platelet size
Absent high–molecular-weight vWF multimers
|Gray platelet syndrome ||Variable ||Unknown ||Impaired storage of α-granule proteins || |
Macrothrombocytopenia versus variable platelet counts
Variably decreased aggregation
↓ α granules
|Montreal platelet syndrome ||AD ||Unknown ||Calpain deficiency || |
↓ Aggregation to thrombin
Giant platelets with stimulation
|Quebec platelet disorder ||AD ||Unknown ||Increased u-PA in platelets with alpha-granule protein degradation || |
Variable platelet count
↓ Aggregation to epinephrine
↑ Urokinase-like plasminogen activator by ELISA and Western blot
|Hermansky-Pudlak syndrome ||AR ||Multiple ||Abnormal vesicular biogenesis || |
↓ Aggregation and secretion
↓ α granules
|Chédiak-Higashi syndrome ||AR || |
|Abnormal vesicular biogenesis || |
↓ Aggregation and secretion
↓ α granules
Giant, peroxidase-positive granules in leukocytes
|Paris-Trousseau syndrome ||AD || |
|Dysmegakaryopoiesis || |
Increased megakaryocytes and immature micromegakaryocytes in bone marrow
Giant, red α granules
Impaired α granule release
|Jacobsen syndrome |
|Scott syndrome ||AR ||Unknown ||Decreased phosphatidylserine exposure ||↓ Procoagulant activity || |
|Wiskott-Aldrich syndrome ||XL || |
|Defective cytoskeleton and signaling || |
↓ Aggregation and secretion
↓ α granules
Defective T cell function
|Platelet spherocytosis ||Unknown ||Unknown ||Decreased microtubules and microtubule coils || |
↓ Aggregation and secretion
|Sticky platelet syndrome ||AD ||Unknown ||Unknown ||↑ Aggregation to epinephrine and/or ADP ||Arterial thromboembolic events |
Schematic showing recognized inherited qualitative platelet disorders and their sites of dysfunction. A: Disorders of the surface membrane. B: Disorders of intracellular constituents of platelets. 5-HT, 5-hydroxytryptamine; ADP, adenosine diphosphate; δ-SPD, delta storage pool deficiency; PAF, platelet activating factor; TXA2, thromboxane A2; vWF, von Willebrand factor. (Reproduced with permission from Nurden AT. Qualitative disorders of platelets and megakaryocytes, J Thromb Haemost. 2005 Aug;3(8):1773-1782.)
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