Hypereosinophilic syndrome (HES) constitutes a rare and heterogeneous group of disorders, defined as persistent and marked blood eosinophilia and/or tissue eosinophilia associated with a wide range of clinical manifestations reflecting eosinophil-induced tissue/organ damage.

The hypereosinophilic syndrome (HES) is a disease characterized by a persistently elevated eosinophil count (≥ 1500 eosinophils/mm³) in the blood for at least six months without any recognizable cause, with involvement of either the heart, nervous system, or bone marrow. HES is a diagnosis of exclusion, after clonal eosinophilia (such as leukemia) and reactive eosinophilia (in response to infection, autoimmune disease, atopy, hypoadrenalism, tropical eosinophilia, or cancer) have been ruled out.