hypereosinophilia

Hypereosinophilia (HE) is a sustained elevation of eosinophils in the peripheral blood, typically defined as an absolute eosinophil count exceeding 1.5 x 10^9/L (1500/µL). When eosinophilia leads to end-organ damage, it is often referred to as hypereosinophilic syndrome (HES).

HE can be classified as primary (clonal or myeloproliferative), secondary (reactive) to another condition, or idiopathic when no cause is found. In some cases, genetic rearrangements such as FIP1L1-PDGFRA define a myeloproliferative variant that may respond to targeted therapy with imatinib.

Common secondary causes include parasitic infections, allergic diseases, drug reactions, autoimmune disorders, and malignancies. Primary HE arises from clonal eosinophil proliferation due to specific mutations. Idiopathic HE refers to persistent eosinophilia without an identifiable cause after evaluation.

Clinical features reflect eosinophilic tissue infiltration and organ involvement. Cardiac disease is the most serious complication, potentially causing restrictive cardiomyopathy and endomyocardial fibrosis. Other affected systems include skin, lungs, digestive tract, and nervous system.

Diagnosis involves repeated complete blood counts with differential, peripheral smear, and exclusion of secondary causes. Testing may include serologies, imaging, bone marrow examination, and targeted molecular studies for rearrangements such as FIP1L1-PDGFRA and other mutations. Cardiac evaluation is often indicated when organ involvement is suspected.

Treatment aims to control eosinophilia and prevent organ damage. Corticosteroids are first-line for many cases. In PDGFRA- or PDGFRB-rearranged disease, imatinib is effective. Anti-IL-5 therapies (e.g., mepolizumab) and other immunosuppressants may be used. Anticoagulation is considered if thrombosis is present.

Prognosis varies with the cause and extent of organ involvement. Cardiac involvement and progression to organ damage worsen outcomes. Ongoing monitoring of eosinophil counts and organ function is recommended.