[Essential thrombocytosis].

Essential thrombocythemia (ET) belongs to the group of clonal myeloproliferative disorders such as polycythemia vera (PV), chronic myelogenous leukemia (CML), and idiopathic myelofibrosis (MF). This rare disorder, characterized by an important thrombocytosis, includes a mucocutaneous hemorrhagic diathesis and thromboembolic events. Neurologic manifestations are frequent in ET and are due to obstruction of the cerebral microvasculature. Both thrombocytosis and platelet dysfunction can be responsible for the thrombo-hemorrhagic phenomena in ET. First symptoms of ET in our patient was thrombosis of the vertebral artery with a secondary embolic event in the thalamus region although the platelet count was below 600.10(9)/l, the classic diagnostic limit for ET. These data strongly suggest that qualitative platelet abnormalities rather than thrombocytosis are the main cause for thrombo-embolic events in ET.