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ET defined as a platelet count of more than 450,000 mm³ is frequently encountered in clinical practice.
Most patients have a normal life expectancy while 5% to 10% may experience transformation into post-thrombocythemic myeloid metaplasia or acute leukemia.one
6-7000 new cases diagnosed annually in the United States.
Incidence 1.2 to 3.0 per hundred thousand population.
Median age at diagnosis 58 years.
It also develops in adults younger than 40 years in 20% of cases.
Women are more often affected than men.
Median survival 19.8 years.
The life expectancy of patients with ET does not significantly differ from that of the general population as it is overall an indolent disease.
The 15 year survival has been reported to be 80% and the 10 year risk of transformation to AML or myelofibrosis is less than one percent: treating every patient with ET is therefore not recommended.
The most important determinant of survival among patients with essential thrombocythemia is age, with a median survival ranging from 8.1 years among patients older than 70 years of age, to 34.7 years among patients younger than 40 years of age.
Leukocytosis and history of thrombosis are key factors for survival.
Prevalence of essential thrombocytosis is 38-57 per 100,000.
Incidence estimated to be 1.2-3.0 per 100,000 population per year.
Predilection for females-up to 67%.
Fewer than 25% of patients have nonrandom chromosome abnormalities.
Medium platelet count at diagnosis 876,000/ cubicmm³.
The hallmark of ET is an elevated thrombocyte count in the peripheral blood, accompanied by excessive megakaryopoiesis in the bone marrow.
Splenomegaly, that is palpable, present in about 17% of patients at onset.
Leukocytosis is documented in about 26% of patients and diagnosis of an abnormal karyotype is detected in 9%.
A history of thrombosis at or before diagnosis is present in about 21% of cases.
Older age, previous thrombosis, the presence of a JAK2 mutation, immobilization, surgery, pregnancy, and the use of contraceptive pills increase the risk of development of venous fibrosis.
Clinical features include headache, visual disturbances, lightheadedness, dysesthesia, and less frequently erythromelalgia.
In a Mayo clinic study ff 1076 patients median follow about 10 years was associated with a 43% death rate, leukemic transformation of 4%, myelofibrosis in 13%, new thrombosis in 21%, median survival rate of 18 years (Tefferi A).
In the above study the median survival was 26.7 years among patients with a low risk of thrombosis, a significantly shorter survival rate than that of age and sex match controls for the general population.
A feature of ET is the occurrence of venous thrombosis in unusual locations, such as the splancnic system, including the hepatic, portal, splenic, and mesenteric venous systems.
The diagnosis is established after four major criteria, were the first three major criteria and one minor criteria, have been met.
Major criteria- 1-platelet count greater than 450000, 2 – bone marrow biopsy showing proliferation mainly of megakaryocytes line with increased numbers of enlarged, mature megakaryocytes that have hyper lobulated nuclei, no significant increase or left shift in neutrophil granulopoiesis or erythropoiesis and very rarely minor increase in reticular fibers, 3 -not meeting the criteria for bcr-abl positive CML, PV, pmf, myelodysplastic syndromes or other myeloid neoplasms, and 4-presence of JAK2, CALR, or MPL mutation.
MPL myeloproliferative virus oncogene
Minor criteria-presence of a clonal marker or absence of evidence of reactive thrombocytosis.
Has no distinguishing clinical features from other myeloproliferative diseases and is a diagnosis of exclusion.
Patients have greater than 20% risk for thrombotic complications.
Polycythemia rubra vera and myelofibrosis can present as isolated thrombocytosis in a significant percentage of patients.
Bone marrow examination is necessary for proper diagnosis.
Characterized by abnormal megakaryocytic morphology.
Characterized by a persistent nonreactive thrombocytosis and increased risk of arterial and venous vascular events.
Higher platelet counts did not correlate well with thrombotic risk, but the association between thrombocytosis and bleeding is more clearly supported.
Hemorrhage events or less common than thrombotic events in ET.
Bleeding in ET is due usually to acquired von Willebrand syndrome and a selective loss of von Willebrand mutimers.
Bone marrow shows megakaryocytic proliferation and varying degrees of reticulin fibrosis.
Acquired von Willebrand disease is common among patients with essential thrombocytosis (11 to 17%).
Often has an indolent course.
50-60% of patients carry the JAK2 V617 F mutation.
5% carry mutated thrombopoietin receptor MPL.
Patients commonly have mutations in the calreticulum (CALR) gene-20%.
CALR mutations occur almost exclusively in patients with thrombocytosis.
90% of patients with ET will have one of these three mutations and no mutations or detected in 10% of patients.
The calreticulin gene encodes for a chaperone protein that plays arole in cellular proliferation, differentiation, and a apoptosis.
10-20% of patients have none of the three driver mutations and are triple negative.
The incidence of thrombosis in ET is about 6.6% per year.
Leukocytes contribute to platelet activation in ET and are a stronger predictor of thrombosis than the platelet count: the risk of thrombosis is not correlated with the platelet count, per se.
Patients with JAK2 mutated ET tend to be older, have higher white blood cell counts and hemoglobin levels and have lower platelet counts, but are more likely to develop thrombosis.
Men are commonly affected by mutations, and the risk of thrombosis is lower in this sub group.
While a bone marrow examination is required for conclusive diagnosis, the presence of a drive a mutation, combined with appropriate clinical features strongly suggested diagnosis of essential thrombocythemia.
Up to 76% of patients experience arterial thrombosis, about 15% have venous thrombosis, about 63% experience hemorrhage, and about 69% encounter functional symptoms.
Next generation sequencing demonstrated 15% of patients harbor one or more non-driver mutations in SRSF2, U2AF1, SF3B1, or TP 53: these patients have lower rates of myelofibrosis free and leukemia free survival.
Platelet thromboxane production is increased in these patients, as noted by increased excretion of a urinary metabolite Thromboxane A2.
A major goal of treatment is thrombosis prevention.
The main COX-1 mediated product of platelets which causes platelet aggregation, vascular proliferation and vasoconstriction.
Cyclooxygenase-one and cyclooxygenase-2 are isoenzymes that catalyze the conversion arachidonic acid to prostaglandin H2, the initial step in prostanoid synthesis that leads to the formation of thromboxane A2, PGI2, anf PGE2.
Under physiological conditions platelet thromboxane production is driven primarily by Cox-1, and fewer than 10% of normal platelets have Cox-2.
Cyclooxygenase-2 is expressed in young platelets and the megakaryocytes.
Estimated risk for thrombotic episodes is 6.6% per patient year and the risk increases to 15% per year for patients older than 60 years.
Goals of treatment is to keep the platelet count less than 600,000, although the target platelet count for patients with high risk disease is less than 400,000 per cubic millimeter.
Much more common in women.
The International Prognostic Scoring System for essential thrombocytosis helps predict survival.
Four risk levels of ET is noted: very low, low, intermediate, and high.
In the above system patients receive points for the following 1 -leukocyte count equal to or greater than 11 times 10 to the 9th/L, 2-age of 60 years or older, and 3 – history of thrombosis.
The above model is not used to estimate survival for low-risk patients.
Intermediate risk is associated with the median survival of approximately 25 years whereas high-risk carries a median overall survival of 15 years.
Extreme thrombocytosis has not been clearly defined, but generally completely count higher than 1000×10 to the ninth per liter is used arbitrarily lead to define it.
Approximately 22% of patients who have ET present with extreme thrombocytosis, which is associated with increased risk of bleeding.
Bleeding affects approximately 5 to 10% of patients with ET.
High-risk patients involve those over the age of 60 years, or have a history of thrombosis should be treated with hydroxyurea.
Hydroxyurea is associated with reduced incidence of thrombotic events and improved overall survival in older patients with ET.
In intermediate and high-risk patients cytoreductive therapy with hydroxyurea reveals superior antithrombotic effect compared the observation alone or anagrelide therapy.
Anagrelide reduces the platelet count and is approved for essential thrombocytosis but is in frequently use because of conflicting results from control trials.
Anagrelide is non-inferior to hydroxyurea in preventing vascular events.
In a large retrospective study the risk of developing AML and myelofibrosis with myeloid metaplasia, which were 2% and 4%, respectively, was not significantly influenced by single agent chemotherapy, including hydroxyurea.
Incidence of acute myocardial infarction in such patients is 9.4% (Rossi).