[Is it CNS vasculitis? Essential thrombocythaemia with a normal platelet count]

A 58-year old woman was admitted with transient ischaemic attacks. Cerebral MRI was compatible with vasculitis. Blood chemistry was normal, but after 9 months she developed thombocytosis above 600 x 10(9)/L. A bone marrow biopsy showed myeloproliferative changes. Further investigations revealed over-expression of the polycythaemia rubra vera-1 gene and she carried the Janus Kinase-2 mutation, V617F. This case emphasizes that a myeloproliferative disorder should be considered in patients with unexplained ischemic CNS manifestations, even if blood counts are normal.