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Perth Haematology :: Dr Steven Ward

Essential Thrombocytosis (ET)

 

ET is sometime also called Essential Thrombocythaemia.

ET is due to the primary overproduction of marrow platelets by their precursor cell, the megakaryocyte.

 

An elevated platelet count can be:

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reactive - secondary to some stimulus leading to reactive thrombocytosis. The common reasons are recent surgery, infection, inflammation, trauma (this type of rise in platelets can last up to 6-8 weeks)

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Primary - ET - no known cause.

 

 

 

Revised WHO Diagnostic Criteria for ET

Diagnosis requires all 4 criteria

 

1.      Sustained platelet count >= 450 x 109/L

2.      BM biopsy showing proliferation mainly of the megakaryocytic lineage with increased numbers of enlarged, mature forms; no significant increase or left-shift of neutrophil granulopoisis or erythropoiesis.

3.      Not meeting WHO criteria for PRV, MF, CML, MDS or other myeloid neoplasia.

4.      Demonstration of JAK2 2617V>F or other clonal marker, or in the absence of a clonal marker no evidence for reactive thrombocytosis.

 

Features of ET

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Related to excessive platelet function: thrombosis (clotting) due to the increased number of platelet in the circulation. Cerebral and coronary infarcts (stroke and heart attack) are more common, as is DVT.

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Related to inadequate platelet function: bleeding. Paradoxically the bleeding risk increases with ET. This is due to each platelet being dysfunctional and unable to activate the clotting mechanisms at the site of blood vessel breaches. It can also be thought of as the marrow being unable to supply each platelet with sufficient "glue" to stop bleeding when the count is 2-5 x higher than normal simply due to sheer volume.

 

Treatment

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Treatment depends on patient age, platelet count, past thrombosis or bleeding and risk of complications

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At moderately elevated platelet counts no therapy may be needed.

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Aspirin is useful in helping prevent thrombotic complications, but increases bleeding risk. however the consequences of thrombosis are usually more severe.

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Reduction of the platelet count to normal. Several agents can be used. The right agent depends on the situation:

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Interferon-alpha: useful in reducing platelet count; has to be injected sub-cutaneously; side effects of lethargy and flu-like symptoms

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Anagrelide: a specific platelet inhibitor; oral tablets; concerns regarding cardiac events recently. Not PBS funded in Australia, expensive

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Hydroxyurea:  effective therapy; few side effects (mainly GI upset); concerns regarding possible increase in rate of transformation to acute leukaemia have been disproven.