Idiopathic hypereosinophilic syndrome—diagnosis and treatment

Sustained elevation of eosinophils above 5 × 10⁹ /l in peripheral blood (PB) should prompt further investigation. Clonal eosinophilia accounts for the much smaller proportion of eosinophilias (< 10%), but exclusion of such a neoplasia is prognostically and therapeutically relevant. Molecular genetic analysis from PB, cytogenetics from bone marrow, and bone marrow histology are primarily used to exclude clonal eosinophilia. Far more common is reactive eosinophilia, the cause of which may be drugs, allergies, solid tumors, lymphomas, worm infections, autoimmune diseases, or idiopathic hypereosinophilic syndrome (HES). Because of the diverse organ infiltration patterns in eosinophilia, a specific search for possible organ involvement (including heart, lung, gastrointestinal tract, kidney, skin, etc.) should be performed, depending on the patient’s symptoms. The diagnosis of HES is made when organ infiltration with consecutive dysfunction is diagnosed in persistent eosinophilia after exclusion of other causes. Therapeutically, oral corticosteroids (OSC) are used in HES. This can also be helpful in the differential diagnosis, as patients with clonal eosinophilia are usually not expected to achieve remission with OCS. When OCS requirements are high, other immunosuppressants (e.g., methotrexate [MTX], cyclophosphamide) and the interleukin (IL)-5 antagonist mepolizumab are used. In clonal eosinophilia, tyrosine kinase inhibitors are the first-line therapy, depending on the underlying genetic alteration.