Eosinophilia is defined as an increase in the eosinophil count in the peripheral blood, specifically characterized by an absolute eosinophil count (AEC) greater than 500 eosinophils per microliter of blood. This condition can be indicative of various underlying disorders. Idiopathic hypereosinophilic syndrome (I-HES) is a rare disorder characterized by persistent eosinophilia without an identifiable secondary cause and evidence of eosinophil-mediated organ damage. Early diagnosis and intervention are crucial to prevent irreversible organ damage and improve patient outcomes.

A 41-year-old female with a history of hypertension presented to the emergency department with generalized malaise, subjective fever, headache, and neck stiffness over the past few days. Initial laboratory testing revealed leukocytosis with eosinophilia predominance with an absolute eosinophil count of 4,100 cells/μL and an elevated sedimentation rate and C-reactive protein. The autoimmune work-up revealed ANA and ANCA negative, with low suspicion for giant cell arteritis given her symptoms. Imaging was ordered to evaluate her neurological complaints as well as to find any underlying malignancy, which consisted of computed tomography (CT) of head, chest, abdomen and pelvis as well as magnetic resonance imaging (MRI) of the head – all of which were unremarkable. Additionally, infectious disease was consulted, resulting in an unremarkable work-up involving urinalysis, echocardiogram, lumbar puncture, stool ova/parasite/culture, and blood culture, which revealed that her hematologic derangement was not infectious in nature.Notably, cerebrospinal fluid cytology revealed benign, virtually acellular fluid with no eosinophils, ruling out eosinophilic meningitis or CNS involvement

Hematology was thus consulted with a work-up including a peripheral blood smear (PBS), serum tryptase, vitamin B12 levels, protein and immunoelectrophoresis, flow cytometry, and a bone marrow biopsy with FISH evaluation of BCR-ABL, PDGFRA, PDGFRB, and FGFR1. The results of the hematologic work-up was unremarkable, with PBS demonstrating leukocytosis with eosinophilia, and bone marrow biopsy significant for myeloid-predominant marrow with increased eosinophilic forms. By exclusion, patient was diagnosed with I-HES. The patient was discharged on prednisone, which led to a significant improvement in her symptoms and a reduction in her eosinophil count. The patient was further followed by Allergy and Immunology in the outpatient setting, stating the degree of eosinophilia atypical for an atopic process.

As described earlier, eosinophilia is characterized by an increased AEC which has been noted in multiple conditions. They include neoplasm, allergy/atopy/asthma, autoimmune disease, connective tissue disease, and parasitic infections. This work-up typically includes a complete blood count with differential, rule out of malignancy and parasitic infections, including stool examination of ova and parasites, as well as evaluation for allergic and autoimmune conditions. After a thorough work-up is unfruitful – other causes of eosinophilia are considered. I-HES is a rare disorder characterized by persistent eosinophilia (AEC >1.5 x 10^9/L) without an identifiable secondary. The clinical presentation of I-HES is highly variable, ranging from asymptomatic eosinophilia to severe, life-threatening organ dysfunction depending on organ involvement. First-line treatment for I-HES typically involves corticosteroids, which are effective in reducing eosinophil counts and controlling symptoms. It is essential to have a thorough work-up to rule out all possible etiologies of eosinophilia and keep I-HES on the differential if the work-up is unrevealing.

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2025
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