Idiopathic hypereosinophilic syndrome

Idiopathic hypereosinophilic syndrome (IHES), also known as idiopathic HES, is a rare hematologic disorder characterized by persistent, marked eosinophilia (an absolute eosinophil count exceeding 1,500 cells per microliter of blood for at least six months) with evidence of eosinophil-mediated organ damage, in the absence of an identifiable underlying cause such as parasitic infection, allergy, or a clonal hematologic neoplasm. The term "idiopathic" distinguishes this form from myeloproliferative variants (such as those associated with FIP1L1-PDGFRA fusion) and lymphocytic variants driven by aberrant T-cell populations. The disease can affect virtually any organ system, but the most commonly involved are the cardiovascular system (endomyocardial fibrosis, restrictive cardiomyopathy, intracardiac thrombi), the skin (urticaria, angioedema, eczematous dermatitis, pruritus), the lungs (cough, infiltrates, fibrosis), the gastrointestinal tract (abdominal pain, diarrhea), and the nervous system (peripheral neuropathy, central nervous system dysfunction). Cardiac involvement is the leading cause of morbidity and mortality. Patients may also experience fatigue, fever, myalgias, and weight loss. Organ damage results from the toxic effects of eosinophil granule proteins released into tissues. Treatment is aimed at reducing eosinophil counts and preventing or reversing organ damage. First-line therapy typically involves systemic corticosteroids, which are effective in many patients. For steroid-refractory or steroid-dependent cases, second-line agents include hydroxyurea, interferon-alpha, and immunomodulatory drugs. The tyrosine kinase inhibitor imatinib is highly effective in the myeloproliferative variant but is generally not effective in truly idiopathic cases. More recently, the anti-interleukin-5 monoclonal antibody mepolizumab has been approved for use in HES and has shown benefit in reducing eosinophil counts and allowing corticosteroid tapering. Regular monitoring of cardiac function and other potentially affected organs is essential for long-term management.

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