Essential thrombocythaemia (ET) is defined as persistent elevation of the peripheral blood platelet count (thrombocytosis) as a result of increased marrow platelet production in the absence of a systemic cause for thrombocytosis (see Box 26.2).
Aetiology and pathophysiology
It is similar to polycythaemia rubra vera (PRV), and distinction between the two conditions is not exact. ET occurs more frequently in younger adults than PRV. JAK2 is mutated in 50% of cases.
- Thrombosis, both arterial (peripheral vessels with gangrene of toes, cerebral, coronary and mesenteric arteries) and venous (e.g. Budd–Chiari syndrome, deep vein thrombosis, superficial thrombophlebitis).
- Headaches, visual disturbance.
- At least 30% of patients are asymptomatic and detected as an incidental finding.
- Excessive haemorrhage may occur spontaneously or after trauma or surgery.
- Pruritus and sweating are uncommon.
- Splenomegaly in about 30% of patients; in others, the spleen is atrophied because of infarction.
- Platelet count is persistently raised and often >1000 × 109/L, raised red cell and/or white cell count is present in about 30%.
- Blood film shows platelet anisocytosis with circulating megakaryocyte fragments (Fig. 28.1). Autoinfarction of the spleen may cause changes in red cells (target cells, Howell–Jolly bodies).
- The JAK2 mutation is present in about 50% of cases. In a few, this is homozygous and these tend to be more severe.
- Serum uric acid is often raised, serum lactic dehydrogenase is normal unless marrow fibrosis is present.
- Bone marrow is hypercellular with increased numbers of megakaryocytes, often in aggregates.