Recognition of heterogeneity in hypereosinophilic syndrome (HES) is important as identification of the exact pathologic mechanisms becomes relevant to the design of treatment strategies and the overall management of individual patients. This importance is best illustrated by seminal discoveries of the unique therapeutic efficacy of imatinib mesylate in a subset of patients with HES66-72 and by the identification of the FIP1L1-PDGFRa TK as the molecular basis in a subpopulation of patients and as a therapeutic target of imatinib6973 and potentially other small-molecule TK inhibitors.74

Although hypereosinophilia may have any number of causes, the clinical symptoms and organ damage appear to be related to the pathologic consequences of the chronically elevated eosinophils.75 76 Because the benign and malignant disorders are not yet always distinguishable, treatment of patients with hypere-osinophilia is aimed primarily at suppressing eosinophil counts (particularly in symptomatic patients); the ultimate goal is to prevent or limit organ damage.

A therapeutically relevant classification of HES plus chronic eosinophilic leukemia (CEL) is emerging, still largely under development and dictated by the exclusion criteria spelled out in the most recent WHO clas-sification76 and by other criteria. Eradication of symptoms and prevention of eosinophil-triggered organ damage remain the primary overall concerns, but in a subcategory of patients, the choice of treatment is now based on the likely underlying molecular mecha-nism(s). Recognition of therapeutically important features of HES suggests a need for closely integrated diagnostic workup with therapeutic decision making.

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