Thrombocytosis

Thrombocytosis describes a platelet count elevated above 450 × 10⁹/litre, which can be (1) primary—including essential thrombocythaemia, chronic myeloid leukaemia, polycythaemia vera and myelodysplastic syndromes; or (2) secondary—including iron deficiency, infection, blood loss, malignancy.

Normal megakaryocytopoiesis

Platelets are released from megakaryocytes, whose development is principally regulated by thrombopoietin. This is chiefly produced in the liver and binds to its receptor (c-Mpl) to cause activation via the JAK-STAT signalling pathway at different levels of the platelet production pathway, ranging from the proliferation and survival of haematopoietic stem cell/progenitor cells to megakaryocyte maturation. Thrombopoietin production is increased by a wide variety of stimuli, which explains the many causes of secondary thrombocytosis.

Essential thrombocythaemia

Aetiology—the JAK2 V617F missense mutation typical of polycythaemia vera (see Chapter 22.3.8) is found in about 50% of cases.

Clinical features—many patients (usually in late middle age) are asymptomatic at diagnosis, but common manifestations include (1) thrombotic episodes—(a) venous thromboses, including of the hepatic veins; (b) arterial thromboses, including stroke, erythromelalgia (redness and burning pain in the extremities), and (occasionally) frank arterial thrombosis with gangrene; (2) bleeding episodes; (3) splenic enlargement—splenectomy, which is not recommended but may be performed in ignorance of the diagnosis, causes extreme elevation of the platelet count with a greatly increased risk of thrombosis and/or bleeding.

Diagnosis—this requires all of the following four criteria: (1) platelet count >450 × 10⁹/litre, (2) megakaryocyte proliferation with large and mature morphology, and no or little granulocyte or erythroid proliferation, (3) not meeting criteria for other specified haematological disorders, and (4) presence of JAK2(V617F) or similar mutation, or no evidence of reactive thrombocytosis.

Treatment—this requires risk stratification based on the age of the patient and any prior history of thrombosis, with treatment being reserved for those at a high risk of developing complications and not introduced simply on the basis of platelet counts alone unless there is extreme thrombocytosis (>1500 × 10⁹/litre). Therapies include: (1) low dose aspirin—should be considered in patients with platelet counts <1000 × 10⁹/litre and without evidence of acquired von Willebrand disease; (2) cytoreduction—hydroxyurea effectively reduces platelet counts and thrombotic episodes in high-risk patients; interferon-α, anagrelide, and other agents are also used.

Prognosis—most patients will survive more than 10 years from diagnosis. Most deaths result from thrombotic complications.