When Blood Cell Counts Matter: Hypereosinophilic Syndrome as a Rare Cause of Ischemic Strokes.

Hypereosinophilic syndrome (HES) is a rare condition characterized by elevated eosinophil counts (>1.5 x 10 on two consecutive measurements), which are of myeloid clonal in origin or are driven by excess cytokines. One subtype of HES exhibits the Fip1-like 1-platelet-derived growth factor receptor alpha (FIP1L1-PDGFRA) fusion gene, a gain-of-function mutation resulting in a hyperactive tyrosine kinase.