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Introduction: Hypereosinophilia syndrome (HES) is defined as 1500 eosinophils/mL or above with evidence of end-organ damage due to the eosinophils at least 4 weeks apart. Case description: A 3-year-old male was admitted for newly found anterior mediastinal mass and left pleural effusion. About 6 weeks prior to admission, he was seen in his primary care office for difficulty breathing. Despite the appropriate treatment for his symptoms, the patient continued to have significant respiratory symptoms and a chest x-ray showed a large left pleural effusion. He was immediately transferred to a tertiary care center and a CT chest showed large left pleural effusion, possible abscess, small pericardial effusion, and concern for an anterior mediastinal mass due to mediastinal shift. He underwent a thoracentesis, which showed eosinophilia within the pleural fluid. While his peripheral eosinophils had fluctuated at the beginning of his hospitalization, the patient’s absolute eosinophil count suddenly increased from 600 to 25,048 and remained elevated above 20,000. He developed recurrent eosinophilic pericardial effusions requiring multiple pericardiocentesis. Despite high dose steroids and inpatient use of mepolizumab, his hypereosinophilia persisted. After two inconclusive fine-needle aspirations of his mediastinal mass, the patient’s family agreed to a thoracotomy with removal of the mediastinal mass. Pathology eventually revealed myeloid sarcoma, a variant of acute myeloid leukemia. Discussion: Steroid resistant HES should raise concern for underlying malignancy especially when eosinophils are above 20,000. Educational objective: After reviewing this case, participants should be able to include myeloid malignancy in their differentials when evaluating a patient with hypereosinophilia.


Allergy and Immunology


Presented at American College of Allergy, Asthma, and Immunology (ACAAI) Annual Scientific Meeting; Louisville, KY; November: 10-14th, 2022.

Malignancy masquerading as steroid resistant hypereosinophilic syndrome