diseases are inherited or acquired. This brief review is intended to introduce the reader to this fascinating group of diseases.

Transcription

1 CE UPATE HEMATOLOGY IV Platelet isorders Ann Marie Braman, MS, MT(ASCP), and Kenneth A. Schwartz, M This review relates concepts of platelet function and laboratory evaluation to clinical problems. isorders of thrombocytopenia secondary to abnormal platelet production, destruction, and sequestration and malignant and benign causes of thrombocytosis are summarized. In addition, inherited and acquired disorders of platelet function are reviewed. ^ latelets are a key component in the \ J hemostatic system. Current nomenclature categorizes platelet disorders based on normal, increased, or decreased platelet counts; normal or abnormal platelet function; and whether the diseases are inherited or acquired. This brief review is intended to introduce the reader to this fascinating group of diseases. k 1\ From the epartment of Medicine, Michigan State University, East Lansing, MI ormal Platelet Function 1 The primary function of platelets > is their role in hemostasis. Briefly, under normal physiological conditions, platelets will adhere to and begin to spread over the surface of subendothelial cells exposed by damage to the vascular endothelium.1 Adhesion is dependent on the platelet membrane glycoprotein lb complex. The von Willebrand factor (vwf) is required for both adhesion and spreading. The spreading phenomenon involves a platelet shape change from discoid to spheroid, with the extension of pseudopodia.2 uring this time, the platelets will also begin to release the contents of their dense and alpha granules including adenosine diphosphate (AP), serotonin, vwf, and fibrinogen. The combined effects of platelet shape change and release prepare the initial layer of adherent platelets for interaction with circulating inactivated platelets and the start of platelet aggregation. Aggregation, ownloaded from unlike adhesion, requires fibrinogen binding to the platelet membrane glycoprotein Ilb/IIIa complex.3 The subsequent interaction of the aggregated platelet mass and coagulation factors leads to the formation of a stable hemostatic plug. ^ ^ ^ iagnosis and Evaluation of 1 Bleeding isorders L ^ r Normal hemostasis works through the intricate interdependence of platelets, coagulation factors, and the vascular system. Hence, the diagnosis and/ or evaluation of patients with potential bleeding disorders may require investigation of several different clotting systems. Bleeding disorders in which there are platelet abnormalities may exist as independent entities, in conjunction with coagulation factor and/or vascular defects, or as secondary manifestations of numerous other diseases. Careful examination of the patient's history, physical condition, and laboratory results are all essential for proper diagnosis and management. istory and Clinical Symptoms ^ J A detailed patient history should 1 include questions to ascertain the nature and frequency of any past bleeding episodes as well as familial information, current medications (prescription and over-the-counter- preparations), and in- Laboratory Medicine ecember

2 formation regarding any past and/or coexisting medical conditions. The classic clinical symptoms that suggest a platelet disorder include hemorrhages that are superficial (as opposed to the deep bleeding more commonly associated with coagulation factor defects), petechial hemorrhages, and bleedings that stop after the application of pressure and do not spontaneously restart several hours or days later. L aboratory Evaluation The screening tools most readily available for the evaluation of platelet function are the platelet count, bleeding time, and observation of clot retraction. More rigorous testing, such as aggregation studies, determination of platelet factor 3 (PF 3) levels, and methods for the detection of antiplatelet antibodies, should be carried out when indicated by preliminary test results and/or the patient history and clinical symptoms.4-5 The normal range for platelet counts in healthy adults is 150 to 440xl03L.«Newborns typically have slightly lowerplatelet counts in the range of 85 to 420xl0 3 L. 7 Patient results significantly above or below this range indicate thrombocytosis or thrombocytopenia, respectively, but the relationship between platelet count and risk of thrombotic or hemorrhagic complications is clear-cut only when platelet function is normal. Bleeding time tests evaluate the function of platelets and are also influenced by the availability of vwf. When performed properly, prolongation of the template bleeding time in the presence of adequate numbers of platelets indicates defective platelet function.8 The specific nature of a functional defect can usually be determined by examination of patterns from platelet aggregation studies. A variety of methods are available for the detection of antiplatelet antibodies, a few of which are also capable of quantitation.' lassification of Platelet isorders Platelet disorders that result in impaired hemostasis are most commonly classified by the type of functional abnormality present and the underlying causative mechanism. Quantitative platelet disorders in which the number, rather than the function, of platelets is the primary problem can readily be divided into C two categories thrombocytopenia and thrombocytosis. Q uantitative Platelet isorders Thrombocytopenia Thrombocytopenia is characterized primarily by an abnormally low platelet count. This category includes a wide variety of both congenital and acquired platelet disorders that can be further subdivided based on the causative mechanism decreased or defective production, abnormal sequestration, enhanced destruction, or excessive loss of platelets.10 isorders involving platelet production or destruction account for the majority of these cases and will be discussed in detail. ecreased or efective Production Thrombocytopenias due to decreased or defective production of platelets are most commonly acquired disorders. The underlying mechanisms that can produce this thrombocytopenia are summarized in Table I and include aplastic anemia, megakaryocyte hypoplasia, bone marrow infiltration, and ineffective thrombopoiesis. Numerous substances have been implicated as causative agents of generalized marrow aplasia and several of these appear to preferentially influence platelet production, including alcohol, estrogens, and thiazide diuretics.1113 Although megakaryocytes may be slightly less sensitive than other blood cell line precursors to the effects of ionizing radiation and myelosuppressive drugs, thrombocytopenia will be induced by large doses. Infiltration of the marrow by carcinoma and other malignant conditions will adversely influence the production of all blood cell lines, with thrombocytopenia occasionally being the most prominent peripheral blood abnormality. efective megakaryocyte maturation may play a role in the thrombocytopenia associated with viral infections. Nutritional deficiencies can also influence platelet production and shorten platelet life span as is observed with severe folic acid or iron deficiencies.14 Cyclic thrombocytopenia occurs in normal menstruating women, and occasionally this may be of sufficient severity to result in thrombocytopenic purpura. Congenital forms of thrombocytopenia due to production problems include Fanconi's syndrome, amegakaryocytic thrombocytopenia with congenital mal- ownloaded from Laboratory Medicine ecember 1989 formations, and proposed deficiencies of humoral substances required for normal megakaryocyte development. Fanconi's syndrome (constitutional aplastic anemia) is one of the best-characterized disorders in this group. Although this aplasia of the marrow eventually results in pancytopenia, thrombocytopenia frequently occurs before granulocytopenia or anemia, and platelets appear to be less responsive to treatment than granulocytes or erythrocytes.15 Hereditary forms of thrombocytopenia have been described in which there are production defects, and many of these also appear to involve qualitative platelet abnormalities. These disorders are generally quite rare and are not well characterized (Table II). Enhanced estruction Thrombocytopenia due to the enhanced destruction of platelets occurs in a variety of circumstances. Many of these conditions have a suspected or confirmed underlying immune mechanism, and these will be discussed as a group. Nonimmunological mechanisms include platelet consumption disorders and situations in which there is direct destruction of platelets by physical forces or toxic substances. Consumption isorders Thrombocytopenia due to platelet consumption may occur in association with numerous conditions, including sepsis, neoplasms, massive hemolysis, and obstetric complications. The predominant consumption disorder present is disseminated intravascular coagulation (IC), although primary fibrinolysis may be seen in selected cases. The initial event occurring in IC is activation of the coagulation mechanism with possible formation Table I: Acquired Thrombocytopenias ue To ecreased or efective Production Aplastic anemia Myelosuppressive drugs Ionizing radiation Marrow infiltration rugs that act specifically on platelet production Viral infections Nutritional deficiencies (iron, folic acid) Renal failure Paraoxysmal nocturnal hemoglobinuria Cyclic thrombocytopenia Hyperbaric exposure Megakaryocytic aplasia

3 Table II: Heredity Forms of Thrombocytopenia and Patterns of Inheritance isease Mode of Inheritance Comments Allport's syndrome Autosomal dominant Bernard-Soulier syndrome Chediak-Higashi syndrome Ehlers-anlos syndrome Variable inheritance patterns Glanzmann's thrombasthenia Hermansky-Pudlak syndrome May-Hegglin anomaly Macrothrombocytopenia, nerve deafness, and renal disease efective platelet adhesion due to decreased GPIb Generalized cellular dysfunction with increased fusion of cytoplasmic granules, neutropenia, and decreased levels of platelet AP and serotonin Connective-tissue disorder with vascular bleeding diathesis, defective platelet aggregation efective platelet aggregation due to decreased GPIIa and GPIb Oculocutaneous albanism, platelet storage pool deficiencies Leukopenia with ohle bodies, giant and bizarre platelets due to megakaryocyte maturation and/or fragmentation abnormality Absent radius, decreased megakaryocytes, platelet storage pool deficiencies Eczema, defects in cellular and humoral immunity, platelet storage pool deficiencies Autosomal dominant TAR syndrome Variable inheritance patterns Wiskott-Aldrich syndrome X-linked recessive of circulating thrombi that may cause obstruction of the microcirculation of organs. However, the most common clinical problems involve hemorrhagic complications due to the consumption of platelets and coagulation factors. The thrombocytopenia found in the Kasabach-Merritt syndrome (thrombocytopenia with congenital giant cavernous hemangioma) is primarily the result of intravascular coagulation and platelet consumption localized within hemangiomas, but in some cases this occurs on a systemic level.16 Thrombotic thrombocytopenic purpura (TTP) is a disorder of unknown etiology that is characterized by thrombocytopenia, renal failure, hemolytic anemia, shistocytes on blood smear, and neurological abnormalities. Although the laboratory results may be similar to those found in IC, the severity of hemolysis and red blood cell fragmentation is much greater in TTP. The hemolytic-uremic syndrome (HUS), particularly in adults, is very similar to TTP except that the neurological problems are less pronounced and renal failure is much more common in HUS. irect estruction Thrombocytopenia due to destruction of platelets by physical forces may occur in conjunction with extensive burns. More commonly, direct destruction of platelets is the result of circulating substances that act as platelet toxins. Ristocetin, protamine sulfate, and heparin are capable of causing thrombocytopenia by this type of mechanism Although the red cell destruction seen in erythroblastosis fetalis is antibody induced, the associated thrombocytopenia may be the result of platelet destruction due to interaction with red cell breakdown products and/or exposure to ultraviolet light. Snake bites or infections may result in thrombocytopenia. Although venom or viral toxins may directly destroy platelets, consumption and immune-related mechanisms have also been detected. Immune-Related Mechanisms Antiplatelet antibodies are associated with premature platelet destruction in several different clinically defined thrombocytopenias. Patients with idiopathic (immune) thrombocytopenic purpura (ITP) usually have an increase in platelet associated IgG (PAIgG), normal or increased numbers of marrow megakaryocytes, and an absence of associated clinical conditions such as connective tissue diseases, associated malignancies, or medications known to cause thrombocytopenia. Patients with ITP usually ownloaded from demonstrate and increase in platelet count following therapy with high-dose steroids. An increase in platelet-associated immunoglobulins may also be detected in thrombocytopenic patients with associated malignancy (both B-cell lymphoproliferative and solid tumors), connective-tissue disorders such as systemic lupus, and in patients with drug-induced thrombocytopenia (quinidine thrombocytopenia). Platelet antibodies are related to clinical thrombocytopenias in patients who develop thrombocytopenia following transfusion (posttransfusion purpura). Most commonly, these patients are negative for the P1A1 platelet antigen, but develop a strong anti-pla1 antibody that destroys their own PlA1-negative platelets.19 The precise mechanism of the P1A1 antibody destroying antigen- negative platelets remains unknown. Neonatal isoimmune thrombocytopenia occurs in newborns whose mothers produce an antiplatelet antibody in response to a fetal antigen inherited from the father and absent in the mother, analogous to erythroblastosis fetalis.20 Similarly, mothers with ITP may also produce an antibody that may cross the placenta and produce thrombocytopenia in the neonate.21 Platelet antibodies commonly are produced in patients receiving multiple platelet transfusions. Some patients may fail to increase theirplatelet count following transfusions because the transfused platelets are destroyed by the antibodies. Such patients are said to be "refractory" to random donor platelet transfusions and need to have immune-compatible platelet donors selected via HLA compatibility testing or a platelet crossmatch assay. Abnormal Sequestration Under normal physiological conditions, approximately one third of the body's total platelet mass is sequestered within the spleen. A transient thrombocytopenia may be seen in association with hypothermic conditions as a result of increased platelet sequestration, but this is usually clinically insignificant. Hypersplenism can lead to an increase sequestration of all blood cell lines, although the resulting thrombocytopenia is rarely severe. Excessive Loss Thrombocytopenia due to the excessive loss of platelets may occur as the result of extensive hemorrhage or extracorporeal Laboratory Medicine ecember

4 perfusion. Transfusion of stored whole blood does not provide sufficient numbers of viable platelets to replace those which are lost during a hemorrhage. The decrease of platelets observed during extracorporeal perfusion is primarily attributed to the formation of microaggregates that are removed by the perfusion apparatus filtration system.22 In both of these situations the bone marrow is unable to produce platelets quickly enough to compensate for the acute reduction in the level of circulating platelets. T hrombocytosis Thrombocytosis is generally defined as a platelet count above 400,000 per /il. For the sake of clarity, primary and secondary thrombocytosis will be used to differentiate between those cases that involve primary myeloproliferative disorders and those that are related to other secondary disease states. Primary Thrombocytosis Primary thrombocytosis can be seen in conjunction with a number of different myeloproliferative disorders, including polycythemia vera, chronic myelogenous leukemia, and myeloid metaplasia. Primary thrombocythemia refers to a specific myeloproliferative disorder in which platelet counts generally exceed 1,000,000 per /il with extreme megakaryocytic hyperplasia of the bone marrow.23 Examination of peripheral blood smears show a broad range in platelet size and shape, including giant platelets and large aggregates. Although overlap with other myeloproliferative conditions is common, primary thrombocytosis can usually be differentiated from other myeloproliferative diseases by the lack of increased red cell mass (polycythemia vera), a positive Philadelphia chromosome (chronic myelogenous leukemia), and massive splenomegaly (myeloid metaplasia). Patients with primary thrombocytosis may experience thrombotic and/or bleeding complications. Hemorrhagic complications are more common and may result from defects in platelet function, consumption of coagulation factors, and/or the ulceration of infarcts. Primary thrombocythemia, as well as primary thrombocytosis associated with other myeloproliferative disorders, may arise from a clonal disorder in a multipotential stem cell. Secondary Thrombocytosis The most common conditions that can result in secondary thrombocytosis are listed in Table III. The mechanisms that influence the overproduction of platelets include overcompensation for previously decreased platelet levels, presence of a platelet-stimulating factor in the plasma associated with an increased sedimentation rate and increased in-acute-phase reactants, anemia, iron deficiency, and release of platelets from normal storage sites.24 While secondary thrombocytosis is generally an asymptomatic condition, some patients may experience thrombotic complications due to spontaneous platelet clumping or increased platelet coagulant activity. Unlike primary thrombocytosis, abnormal bleeding problems are rare with secondary thrombocytosis. Q ualitative Platelet isorders Congenital platelet defects in which there are qualitative abnormalities can be classified based on the specific aspect of platelet function that is abnormal adhesion, aggregation, or secretion. The majority of acquired qualitative platelet disorders do not readily lend themselves to this type of classification and will be discussed separately. The most widely used tool for the diagnosis and/or differentiation of these disorders is the study of platelet aggregation patterns. efects of Adhesion The best characterized congenital disease involving defective platelet adhesion is the Bernard-Soulier syndrome, also referred to as the giant platelet syndrome. The mode of inheritance of this disorder is autosomal recessive, and the hemorrhagic manifestations may be very severe. Bernard-Soulier platelets have reduced levels of membrane glycoprotein lb (GP lb), which is involved in the binding of vwf and adhesion.25 Aggregation studies show normal results with collagen and AP but not with ristocetin. Although this pattern is identical to that seen in patients with a deficiency of vwf, the addition of exogenous vwf to Bernard-Soulier platelets does not correct the lack of aggregation with ristocetin as it does in those with von Willebrand's disease. ownloaded from Laboratory Medicine ecember 1989 Table III: Conditions Associated With Secondary Thrombocytosis Chronic inflammatory disorders Acute or chronic hemorrhage Hemolytic anemia Iron deficiency Malignant diseases Osteoporesis After surgery Recovery from acute inflammatory disease thrombocytopenia Response to drugs or exercise efects of Primary Aggregation Glanzmann's thrombasthenia is an autosomal recessive disorder characterized by defective platelet aggregation. This disorder is quite rare and the bleeding manifestations vary greatly among patients with seemingly similar degrees of platelet abnormalities. Reduced quantities of membrane GP complex l i b / Ilia have been detected in association with this disorder and laboratory studies show no aggregation in response to collagen, AP, epinephrine, or thrombin. 26 Unlike Bernard-Soulier platelets, thrombasthenia platelets will aggregate with ristocetin. efects of Secretion Congenital disorders in which there are abnormalities of platelet secretion can be divided into two groups those in which the platelets contain decreased levels of a secretable substance, or storage pool deficiencies (SPs), and those which have defects in the physical process of secretion itself, or primary secretory defects. Bleeding episodes in these patients are usually minor. Platelet aggregation studies usually demonstrate abnormal aggregation with collagen and an absence of a second wave in response to AP. The SP classification is a heterogeneous group of disorders in which one or more substances normally present in platelet granules are decreased or absent. In general, these platelets appear to be of normal size, but cases have been reported where particularly small or large platelets were noted. eficiencies of substances stored within the dense granules (AP, serotonin, calcium, and/or pyrophosphate, specifically) are more common than alpha granule deficiencies (beta-

5 thromboglobulin, platelet factor 4, and/ or platelet-derived growth factor deficiencies).27 Platelets lacking dense granules usually appear morphologically normal, while those with alpha granule deficiencies have an overall gray appearance. Primary defects in platelet secretion have been described in which selected enzymes necessary for secretion are diminished or absent. These disorders appear clinically similar to SP, and often the only difference detected is a normal platelet AP/ATP ratio as opposed to the increase noted in most cases of SP. A cquired Qualitative efects Idiopathic Thrombocytopenia Purpura The increased destruction of platelets that occurs in idiopathic thrombocytopenia purpura (ITP) is often the result of antiplatelet antibodies as has been discussed earlier. Platelet functional abnormalities, including aggregation defects and reduced levels of platelet factor 3 (PF 3), have also been reported in patients with ITP.28 The biochemical basis of these defects and their influence on hemorrhagic complications have not yet been clearly established. rug-induced isorders A variety of drugs have been observed to influence platelet function through a number of different mechanisms. Aspirin ingestion directly affects platelet function by irreversibly inhibiting cyclooxygenase, a key enzyme in the production of thromboxane A2, and laboratory tests reveal an aggregation pattern similar to that observed with SPs. Penicillin, in high doses, has also been shown to impair platelet aggregation.29 extran and other plasma expanders appear to interfere with both adhesion and PF 3 activity. A summary of the drugs that have been most frequently implicated in platelet functional abnormalities is given in Table IV. Although the functional defects resulting from pharmacological agents do not generally lead to bleeding disorders in healthy persons, their use must be carefully evaluated in those who may have compromised hemostasis due to other factors. onclusion This review integrates the laboratory evaluation of platelet disorders with clinical diagnoses. Linking clini- C Table IV: rugs Most Frequently Implicated in Platelet Functional Abnormalities Aspirin Nonsteroidal antiinflammatory agents ipyridamole Aminophyline, caffeine, theophylline Penicillins extran Beta-adrenergic blockers (propranolol) Calcium channel-blocking agents General anesthetics (halothane) Phenothiazines cal and laboratory evaluations enhances understanding from a physiologic perspective and leads to further investigation of the mechanisms of platelet abnormalities References 1. 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Lab Med 1989;20: Schwartz K: Platelet antibody: Review of detection methods. Am J Hematol 1988;29: Karnad A, Poskitt T: The automated complete blood cell count: Use of the red cell volume distribution width and mean platelet volume in evaluating anemia and thrombocytopenia. Arch Intern Med 1985;145: Post R, es Forges J: Thrombocytopenia and alcoholism. Ann Intern Med 1968;68: Kutti J, Weinfeld A: The frequency of thrombocytopenia in patients with heart disease treated with oral diuretics. Acta Med Scand 1968; 183: Cooper B, Bigelow F: Thrombocytopenia associated with the administration of diethylstilbestrol in man. Ann Intern Med 1960;52: Smith M, Smith, Fletcher M: Haemorrhage associated with thrombocytopenia in megaloblastic anemia. Br Med J 1962;1: ownloaded from Aster R: Thrombocytopenia due to diminished or defective platelet production, in Williams W, Beutler E, Erslev A, et al (eds): Hematology. New York, McGraw-Hill International Book Co, 1983, pp Shin W: Hemangiomas of infancy complicated by thrombocytopenia. Am J Surg 1968;116: Gangarosa E, Johnson T, Ramos H: Ristocetininduced thrombocytopenia: Site and mechanism of action. Arch Intern Med 1960;105: Godal H: Thrombocytopenia and heparin. Thrombosis Haemostasis 1980;43: Zeigler Z, Murphy S, Gardner F: Post-transfusion purpura: A heterogenous syndrome. Blood 1975;45: Kaplan C, affos F, Forestier F, et al: Management of alloimmune thrombocytopenia: Antenatal diagnosis and in utero transfusion of maternal platelets. Blood 1988;72: Scott J, Rote N, Cruikshank : Antiplatelet antibodies and platelet counts in pregnancies complicated by autoimmune thrombocytopenic purpura. AmJ Obstet Gynecol 1983;145: Solis R, Kennedy P, Beall A, et al: Cardiopulmonary bypass: Microembolism and platelet aggregation. Circulation 1975;52: Harker L, Finch C: Thrombokinetics in man. J Clin Invest 1969;48: Shreiner, Levin J: The effects of hemorrhage, hypoxia, and a preparation of erythropoietin on thrombopoiesis. J Clin Lab Med 1976;88: Jamieson G, Okumura T, Fishback B, et al: Platelet membrane glycoproteins in thrombasthenia, Bernard-Soulier syndrome and storage pool disease. J Lab Clin Med 1979;93: Nieuwenhuis H, Akkerman J, SixmaJ: Patients with a prolonged bleeding time and normal aggregation tests may have a storage pool deficiency: Studies on one hundred six patients. Blood 1987;70: Heyns A, Fraser J, Retief F: Platelet aggregation in chronic idiopathic thrombocytopenic purpura. J Clin Pathol 1978;31: Weiss J: Antiplatelet therapy. N Engl J Med 1978;298:1344, Cazenave J, Guccione M, Packham, et al: Effects of cephalothin and penicillin G on platelet function in vitro. BrJ Haematol 1977;35: Laboratory Medicine ecember