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LEARNING OBJECTIVES
After studying this chapter you should be able to:
Distinguish among the different causes of thrombocytopenia.
Understand the pathogenesis of immune thrombocytopenic purpura and the principles of treatment.
Understand the pathogenesis of thrombotic thrombocytopenic purpura and the principles of treatment.
Define mechanisms that lead to qualitative platelet defects.
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Platelets assume an important role in a wide variety of disease states. Morbidity related to platelet dysfunction is generally due to bleeding, but occasionally thrombosis is dominant. As summarized in Table 14-1, bleeding can occur if the platelet count is too low or if there is a qualitative defect in platelet function. In patients with platelet disorders, bleeding is usually superficial and localized to skin and mucous membranes, a phenomenon known as purpura. Petechiae are small, 2-to-5 mm, red or purple macules (spots) that appear most often in the distal lower extremities (Fig. 14-1A) but also on the conjunctiva and palate. Small petechiae often merge to form larger lesions. Very large, purpuric lesions are called ecchymoses (Fig. 14-1B). In contrast to telangiectasia, petechiae and ecchymoses do not blanch under pressure. Mucosal purpura is generally associated with severe thrombocytopenia and may be a harbinger of complications such as gastrointestinal bleeding or even brain hemorrhage. In contrast to the superficial bleeding seen in patients with thrombocytopenia and qualitative platelet disorders, defects in the soluble coagulation factors, such as hemophilia (Table 14-1), typically present with deep bleeding, such as hemarthroses.
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Platelet defects:
Superficial bleeding
Petechiae, ecchymoses
Coagulation factor defects:
Deep bleeding
Hemarthrosis, etc
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In a clear and informative parallel to anemia, thrombocytopenia can be due either to decreased platelet production or enhanced platelet destruction. The life span of normal platelets in the circulation is about 7 to 9 days. Therefore, if the bone marrow stops producing platelets, it takes nearly a week before severe thrombocytopenia develops. In contrast, an acute immune or consumptive process that suddenly and drastically curtails platelet survival can produce severe thrombocytopenia within hours.
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Damage or suppression of pluripotent hematopoietic stem cells may result in not only thrombocytopenia but also anemia and leukopenia (pancytopenia) accompanied by marrow aplasia. Aplastic anemia and other causes of pancytopenia are covered in Chapter 4. Transient thrombocytopenia due to marrow aplasia ...