Purpose of review: In 2012, idiopathic hypereosinophilic syndrome (HES) is still the prevalent diagnosis in patients with persistent eosinophilia, in which a primary or secondary cause of eosinophilia has not been identified. HES is considered a provisional diagnosis until a primary or secondary cause of hypereosinophilia is established. The discovery of imatinib-sensitive fusion proteins in a subset of patients with hypereosinophilia has changed the way we approach the diagnosis and treatment of eosinophilic myeloid neoplasms [eosinophilic myeloproliferative neoplasms (MPNs)]. Despite the recent diagnostic developments, diagnosis of hypereosinophilic MPN is only made in 10–20% of patients with persistent primary hypereosinophilia.
Recent findings: In 2008 the World Health Organization (WHO) established a semi-molecular classification of hypereosinophilic MPNs. The discovery of PDGFRA, PDGFRB, FGFR1, JAK-2, and FLT3 fusion proteins in patients with eosinophilic MPNs provide opportunities for targeted therapy. Patients with hypereosinophilic MPNs associated with PDGFRA and PDGFRB fusion genes are responsive to imatinib.
Summary: Ongoing research continues to expand our understanding of the pathophysiology of persistent primary hypereosinophilia and clarify the boundaries between some of these disorders. A key challenge is to identify new targets for therapy and limit the number of patients who are classified as having HES.