|Classification and external resources|
Essential thrombocytosis (ET, also known as Essential thrombocythemia) or Primary thrombocytosis (PT) is a rare chronic blood disorder characterized by the overproduction of platelets by megakaryocytes in the bone marrow in the absence of an alternative cause. In some cases this disorder may be progressive, and rarely may evolve into acute myeloid leukemia or myelofibrosis. It is one of four myeloproliferative disorders.
Essential thrombocythemia is diagnosed at a rate of about 2 to 3 per 100,000 individuals annually. The disease usually affects middle aged to elderly individuals, with an average age at diagnosis of 50–60 years, although it can affect children and young adults as well.
The pathologic basis for this disease is unknown. However, essential thrombocythemia resembles polycythemia vera in that cells of the megakaryocytic series are more sensitive to growth factors. Platelets derived from the abnormal megakaryocytes do not function properly, which contributes to the clinical features of bleeding and thrombosis. A mutation in the JAK2 kinase (V617F) has been found  to be associated with essential thrombocythemia in around 40–50% of cases. About 3–4% of such cases go on to develop acute leukemia. JAK2 is a member of the Janus kinase family. This mutation may be helpful in making a diagnosis or as a target for future therapy.
The major symptoms are bleeding and thrombosis. Other symptoms include epistaxis (nosebleeds) and bleeding from gums and gastrointestinal tract. One characteristic symptom is throbbing and burning of the hands and feet due to the occlusion of small arterioles by platelets (erythromelalgia). An enlarged spleen (splenomegaly) may be found on examination.
The diagnosis of essential thrombocythemia requires the presence of a persistent thrombocytosis of greater than 600 × 103/µL in the absence of an alternative cause.
The following revised diagnostic criteria for essential thrombocythemia were proposed in 2005. The diagnosis requires the presence of both A criteria together with B3 to B6, or of criterion A1 together with B1 to B6.
- A1. Platelet count > 600 × 103/µL for at least 2 months
- A2. Acquired V617F JAK2 mutation present
- B1. No cause for a reactive thrombocytosis
- normal inflammatory indices
- B2. No evidence of iron deficiency
- stainable iron in the bone marrow or normal red cell mean corpuscular volume
- B3. No evidence of polycythemia vera
- hematocrit < midpoint of normal range or normal red cell mass in presence of normal iron stores
- B4. No evidence of chronic myeloid leukemia
- B5. No evidence of myelofibrosis
- no collagen fibrosis and ≤ grade 2 reticulin fibrosis (using 0–4 scale)
- B6. No evidence of a myelodysplastic syndrome
- no significant dysplasia
- no cytogenetic abnormalities suggestive of myelodysplasia
Not all patients will require treatment at presentation. In those who are at increased risk of thrombosis or bleeding (older age, prior history of bleeding or thrombosis, or very high platelet count), reduction of the platelet count to the normal range can be achieved using hydroxyurea (also known as hydroxycarbamide), interferon-α or anagrelide. Low-dose aspirin is widely used to reduce the risk of thrombosis, but there may be an increased risk of bleeding if aspirin is initiated while the platelet count is very high.
The PT1 study compared hydroxyurea in combination with aspirin to anagrelide in combination with aspirin as initial therapy for essential thrombocytosis. Hydroxyurea was superior, with lower risk of arterial thrombosis, lower risk of severe bleeding and lower risk of transformation to myelofibrosis (although the rate of venous thrombosis was higher with hydroxycarbamide than with anagrelide).
In rare cases where patients have life-threatening complications, the platelet count can be reduced rapidly using platelet apheresis (a procedure that removes platelets from the blood directly).
Essential thrombocytosis is sometimes described as a slowly progressive disorder with long asymptomatic periods punctuated by thrombotic or hemorrhagic events.
However, well-documented medical regimens can reduce and control the number of platelets, which reduces the risk of these thrombotic or haemorrhagic events. The lifespan of a well controlled ET person is well within the expected range for a person of similar age but without ET.
Hydroxyurea and anagrelide are contraindicated during pregnancy and nursing. There is current debate as to the safety of interferon during pregnancy and nursing. Essential thrombocytosis can be linked with increased risk of spontaeous abortion or miscarriage in the first trimester of pregnancy. Throughout pregnancy, close monitoring of the mother for thrombosis and placenta is recommended to ensure blood clots are caught. Post partum, often daily injections of low dose low molecular weight heparin (e.g. enoxaparin) are prescribed for several weeks as this is a period where the mother is at higher risk of developing a blood clot.
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- Hoffman: Hematology: Basic Principles and Practice, 4th ed., 2005 Churchill Livingstone, Chapter 71.
- Kralovics R, Passamonti F, Buser AS, Teo SS, et al. (2005). "A gain-of-function mutation of JAK2 in myeloproliferative disorders". N Engl J Med 352 (17): 1779–90. doi:10.1056/NEJMoa051113. PMID 15858187.
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- Campbell PJ, Green AR (2005). "Management of polycythemia vera and essential thrombocythemia". Hematology Am Soc Hematol Educ Program 2005: 201–8. doi:10.1182/asheducation-2005.1.201. PMID 16304381.
- Harrison CN, Campbell PJ, Buck G, et al. (2005). "Hydroxyurea compared with anagrelide in high-risk essential thrombocythemia". N. Engl. J. Med. 353 (1): 33–45. doi:10.1056/NEJMoa043800. PMID 16000354.
- MPN Research Foundation - Advancing Research, Empowering Patients
- MacMillan Cancer Support Essential Thrombocytosis page
- CMPD Education Foundation