Essential thrombocytosis is an uncommon myeloproliferative disorder of unknown cause in which marked proliferation of the megakaryocytes in the bone marrow leads to elevation of the platelet count. As with polycythemia vera, the finding of a high frequency of mutations of JAK2 and others in these patients promises to advance the understanding of this disorder.
Signs and symptoms
The median age at presentation is 50–60 years, and there is a slightly increased incidence in women. The disorder is often suspected when an elevated platelet count is found. Less frequently, the first sign is thrombosis, which is the most common clinical problem. The risk of thrombosis rises with age. Venous thromboses may occur in unusual sites such as the mesenteric, hepatic, or portal vein.
Some patients experience erythromelalgia, painful burning of the hands accompanied by erythema; this symptom is reliably relieved by aspirin. Bleeding, typically mucosal, is less common and is related to a concomitant qualitative platelet defect. Splenomegaly is present in at least 25% of patients.
An elevated platelet count is the hallmark of this disorder, and may be over 2,000,000/mcL (2000 × 109/L). The white blood cell count is often mildly elevated, usually not above 30,000/mcL (30 × 109/L), but with some immature myeloid forms. The hematocrit is normal. The peripheral blood smear reveals large platelets, but giant degranulated forms seen in myelofibrosis are not observed. Red blood cell morphology is normal.
The bone marrow shows increased numbers of megakaryocytes but no other morphologic abnormalities. The peripheral blood should be tested for the bcr/abl fusion gene (Philadelphia chromosome) since it can differentiate CML, where it is present, from essential thrombocytosis, where it is absent.
Essential thrombocytosis must be distinguished from secondary causes of an elevated platelet count. In reactive thrombocytosis, the platelet count seldom exceeds 1,000,000/mcL (1000 × 109/L). Inflammatory disorders such as rheumatoid arthritis and ulcerative colitis cause significant elevations of the platelet count, as may chronic infection. The thrombocytosis of iron deficiency is observed only when anemia is significant. The platelet count is temporarily elevated after splenectomy. JAK2 mutations are found in over 50% of cases. MPL and CALR mutations frequently occur in patients with JAK2-negative essential thrombocytosis.
Regarding other myeloproliferative disorders, the lack of erythrocytosis distinguishes it from polycythemia vera. Unlike myelofibrosis, red blood cell morphology is normal, nucleated red blood cells are absent, and giant degranulated platelets are not seen. In CML, the Philadelphia chromosome (or bcr/abl by molecular testing) establishes the diagnosis.
Patients are considered at high risk for thrombosis if they are older than 60 years, have a leukocyte count of 11,000/ mcL (11 × 109/L) or higher, or have a previous history of thrombosis. They also have a higher risk for bleeding. The risk of thrombosis can be reduced by control of the platelet count, which should be kept under 500,000/mcL (500 × 109/L).
The treatment of choice is oral hydroxyurea in a dose of 500–1000 mg/day. In rare cases in which hydroxyurea is not well tolerated because of anemia, low doses of anagrelide, 1–2 mg/day orally, may be added. Higher doses of anagrelide can be complicated by headache, peripheral edema, and heart failure.
Pegylated interferon alfa-2 can induce significant hematologic responses and can potentially target the malignant clone in CALR-mutant cases. Strict control of coexistent cardiovascular risk factors is mandatory for all patients.
Vasomotor symptoms such as erythromelalgia and paresthesias respond rapidly to aspirin, and its long-term low-dose use (81 mg/day orally) may reduce the risk of thrombotic complications in low-risk patients. In the unusual event of severe bleeding, the platelet count can be lowered rapidly with plateletpheresis. In cases of marked thrombocytosis (greater than or equal to 1,000,000/mcL (1000 × 109/L) or of any evidence of bleeding, acquired von Willebrand syndrome must be excluded before starting low-dose aspirin.