Hypereosinophilic syndromes - An enigmatic group of disorders with an intriguing clinical spectrum and challenging treatment.

Hypereosinophilic syndromes (HES) comprises a group of rare disorders characterized by blood hypereosinophilia (>1.5 × 109/l) accompanied by eosinophil-associated organ damage. The 2016 World Health Organization classification recognizes a category of myeloid/lymphoid neoplasms with prominent eosinophilia (M/Leo) and well-characterized gene rearrangements of PDGFRA/B, FGFR1 or JAK2. PDGFRA/B-rearranged patients usually manifest as imatinib-sensitive myeloproliferative neoplasms (MPNs). FGFR1- and JAK2- rearranged cases may manifest as MPNs or aggressive lymphomas/leukemias and historically have had a dismal prognosis, although clinical trials with targeted treatment are promising. A negative screen for M/Leo in a patient with myeloid features should prompt consideration of a diagnosis of chronic eosinophilic leukemia-not otherwise specified. If these are excluded and a secondary cause is not identified, a diagnosis of idiopathic HES and/or other rare variants of HES should be considered. This review, through an illustrative case, summarizes current knowledge on HES pointing at new directions in diagnosis and treatment.