Thrombocytosis



Elevation of the platelet count above 500,000/ u.1 may be physiologic, secondary to bleeding, trauma, or infection, or can result from a primary bone marrow disease (thrombocythemia or other myeloproliferative disease). Transitory thrombo­cytosis follows stress or exercise and represents mobilization of platelets from the spleen or lung under the influence of epinephrine (Table 55-5).
Secondary, or reactive, thrombocytosis results from increased platelet production in response to hemorrhage, hemolysis, infection (such as tuber­culosis), inflammatory disease, or malignancy. The platelets are normal in function and the plate­let count may reach 10e/|xl. A similar secondary thrombocytosis follows splenectomy and lasts for several weeks. In general, secondary thrombocy­tosis does not lead to hemorrhagic or thrombotic complications. Therapy to lower the platelet count or inhibit platelet function is not necessary. Treatment of the underlying disease usually re­sults in a return of the platelet count to normal levels.

Primary thrombocytosis, or thrombocythemia, represents increased platelet production indepen­dent of normal regulatory control, except that the platelet count can increase further after hemor­rhage. Platelet counts may exceed 1 to 2 x 106/ u.1. The platelets are large and bizarre in ap­pearance, may appear in clumps on the blood smear, and are dysfunctional. The number of ep­inephrine receptors is decreased, and platelet ag­gregation responses to epinephrine are abnormal.

Other metabolic abnormalities are also present. Both the platelet mass and megakaryocyte mass are markedly increased, and megakaryocyte mor­phology may be abnormal. Clusters of megakar­yocytes are frequently present in the marrow and may be found in the spleen and lung as well. The clinical course of primary thrombocytosis mani­fests both hemorrhage and thrombosis, both com­plications occurring in the same patient and with­out relation to the actual platelet count. Thrombocythemia usually occurs in myeloproli­ferative diseases such as chronic myelocytic leu­kemia, myelofibrosis, and polycythemia but may occur alone (essential thrombocythemia). Because of the high rate of hemostatic complications in these conditions, control of the platelet count with hydroxyurea or busulfan is generally em­ployed. Antiplatelet therapy with low dose as­pirin is often used, as the risk of gastric bleeding with higher doses of aspirin is increased. Mucous membrane bleeding and hemorrhage or throm­bosis with surgery are common clinical compli­cations. Even when the platelet count-is con­trolled by chemotherapy, platelet dysfunction persists. Plateletpheresis for very rapid control of platelet counts can be used in emergency settings, but it only transiently reduces platelet number. In erythromelalgia, aspirin dramatically relieves the painful erythema of the extremities related to in­travascular platelet aggregation. This condition is a premyeloproliferative disorder with thrombo­cytosis.

In thrombocythemias, the increased platelet mass results in several abnormalities in serum values because of release of platelet constituents during blood clotting: serum potassium, lactic de­hydrogenase, acid phosphatase, and zinc levels are elevated, while plasma levels are normal.