Myeloproliferative Disorders
The myeloproliferative disorders include chronic myelogenous leukemia, polycythemia vera, myelofibrosis and essential thrombo-cythemia (Table 7). Because all of these entities may present with leukocytosis, differentiation can be difficult and usually requires special laboratory studies and bone marrow examinations.

Polycythemia vera usually presents with excessive numbers of erythroid cells, but increased white blood cell and platelet counts may also be evident. Symptoms resulting from hypervolemia and hyperviscosity, such as headache, dizziness, visual disturbances and paresthesias, are sometimes present. Less frequently, patients with polycythemia veradevelop myocardial infarction, stroke, venous thrombosis and congestive heart failure. Overall survival is long (10 to 20 years).

Myelofibrosis is a bone marrow disorder in which fibroblasts replace normal elements of the marrow. Patients with myelofibrosis are usually 50 years or older and have a median survival of less than 10 years. As bone marrow fibrosis develops, patients can present with leukocytosis, although decreased white blood cell, red blood cell and platelet counts are more common. Patients are asymptomatic early in the course of the disease and are usually diagnosed incidentally based on changes in blood cell counts. Symptomatic patients have fatigue, shortness of breath, weight loss, bleeding or abdominal discomfort related to splenomegaly. Acute leukemia can develop over time and, when it occurs, progresses rapidly.

Leukocytosis is also found in patients with essential thrombocythemia (primary thrombocythemia). Although elevated platelet counts occur in all myeloproliferative disorders, essential thrombocythemia is distinguished by the singular prominence of platelets. Markers of other disorders, such as the Philadelphia chromosome and bone marrow fibrosis, are absent. It is important to exclude secondary thrombocytosis caused by nonmarrow disorders (e.g., iron deficiency or bleeding). Most patients with essential thrombocythemia are asymptomatic and require little, if any, therapy, although some patients develop thrombosis or hemorrhage secondary to increased numbers of dysfunctional platelets.

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