Essential thrombocythaemia is a myeloproliferative neoplasm (MPN) characterized by sustained over-production of platelets and resultant thrombocytosis. The incidence is 1-3/100,000/year. The disease arises as a result of mutation in a multipotent or pluripotent haemopoietic stem cell. A familial predisposition exists .
An incidental diagnosis when a blood count is performed for some unrelated condition is the most frequent presentation. Those patients who are symptomatic usually present with cerebrovascular symptoms (e.g. transient ischaemic attacks, transient hearing or visual loss, recurrent headaches), peripheral ischaemia, erythromelalgia, arterial or venous thrombosis or haemorrhage. The spleen may be palpable but usually it is not. The liver is less often enlarged. Haemorrhage is sometimes due to acquired von Willebrand disease in patients with a very high platelet count, as a result of removal of large multimers of von Willebrand factor from the circulation by platelets.
Evolution to myelofibrosis can occur. Development of myelodysplastic syndrome (MDS) or acute myeloid leukaemia (AML) occurs in a small minority of patients; cytogenetic evolution may be present at this time.
Haematological and pathological features
The blood count and film show thrombocytosis. The white cell count (WBC) is sometimes increased. The absolute basophil count is occasionally increased and there may be small numbers of circulating granulocyte precursors. Platelets typically include large forms and sometimes hypogranular forms (Figure 7.1). The haemoglobin concentration (Hb) is normal unless there has been haemorrhage. The neutrophil alkaline phosphatase score is often increased, particularly in patients with a JAK2 mutation .
Peripheral blood film of a patient with essential thrombocythaemia showing thrombocytosis and platelet anisocytosis. MGG, high power.
Bone marrow aspiration (Figure 7.2) and trephine biopsy (Figures 7.3 and 7.4) show normal or mildly increased cellularity. Megakaryocytes are increased and on average are larger than normal with large well lobulated nuclei. Emperipolesis is increased. There may be granulocytic hyperplasia. Erythropoiesis is usually normal. On trephine biopsy sections, the megakaryocytes are often clustered. Reticulin may be mildly increased.
Bone marrow aspirate film of a patient with essential thrombocythaemia showing increased numbers of large megakaryocytes. MGG, low power.
Bone marrow trephine biopsy section from a patient with essential thrombocythaemia showing normal cellularity and increased numbers of mainly large megakaryocytes present in loose clusters. H&E, low power.
Bone marrow trephine biopsy section from a patient with essential thrombocythaemia showing a hypercellular marrow with increased numbers of mainly large megakaryocytes. H&E, low power.