++
Chronic myeloproliferative disorder caused by a clonal
increase in platelets resulting in thromboembolic and hemorrhagic
complications.
++
Primary Thrombocytosis; Idiopathic Thrombocytosis;
Hemorrhagic Thrombocytosis.
++
Estimated at 1.4-2.5:100,000 per year. Overall, both
genders are approximately equally affected; however, there is a predilection
for females at younger ages. Although the majority of patients at
presentation are approximately 60 years old, approximately 20% of the
patients are younger than 40 years. Only rarely has essential thrombocytosis
(ET) been described in children.
++
Rare familial form with autosomal dominant
transmission has been described. However, in the vast majority of patients,
no genetic background is detectable.
++
Not fully elucidated yet. ET is caused by
proliferation of megakaryocytes, of which the exact pathogenesis remains to
be elucidated. However, evidence now indicates that a mutation of the
thrombopoietin gene (located on 3q26.3-q27) may play a major role.
Thrombopoietin is the key hormone in the regulation of megakaryocyte
differentiation and proliferation. The mechanisms by which ET leads to
thromboembolic and hemorrhagic complications remain unclear, although
different laboratory findings have been used to explain them (intrinsic
abnormality of the platelets resulting in hypoaggregation and
hyperaggregation of platelets, anomalies or decreased concentrations of
“von Willebrand factor,” or deficiencies of proteins C and S and/or
antithrombin III).
++
Based on the clinical findings of a sustained, not
otherwise explained platelet count greater than 600 × 109/liter, a hematocrit less than 40 (mild erythrocytosis is present in
approximately one third of patients, while others have a mild hypochromic,
microcytic anemia), and normal or increased leukocyte count. The mean
platelet volume usually is increased, but microcytosis has also been
described. Giant platelets can be found in the peripheral blood smear. A
bone marrow aspirate and biopsy are needed and show an increased cellularity
with megalokaryocytic predominance (often in clumps or sheets). Increased
red and white cell hyperplasia are common. The stainable iron in the bone
marrow, serum ferritin levels, and mean corpuscular erythrocyte volume are
normal. Bone marrow fibrosis is either absent or affects less than one third
of the biopsy area. No cytogenetic or morphologic evidence of a
myelodysplastic syndrome can be found. No cause for reactive thrombocytosis.
Essential thrombocythemia is diagnosed once reactive thrombocytosis has been
excluded.
++
Up to 50% of patients present with
microvascular occlusions (e.g., fingers and toes with risk of gangrene if
left untreated, priapism), whereas major vessel occlusions (e.g., coronary,
renal and femoral arteries, or hepatic-vein thrombosis [Budd-Chiari
Syndrome], portal vein, splenic, or femoral vein thrombosis) are less
common. Cerebrovascular infarction, transient ischemic attacks, and
myocardial infarction, although common in elderly patients, are less common
in young patients. Pulmonary embolism is a common finding and may result in
chronic pulmonary artery hypertension. Hemorrhage can be a presenting
feature, is generally not severe, and often is associated with platelet
counts greater than 1000 × 109/liter, with the gastrointestinal
tract (duodenum) the most common site ...