Incidental Diagnosis of Erdheim–Chester Disease in a 42‐Year‐Old Man With Persistent Elevated <scp>ESR</scp> and Peri‐Ocular Xanthomas: A Case Report

Erdheim-Chester disease (ECD) is a rare non-Langerhans cell histiocytosis characterized by multisystem involvement and frequent diagnostic delay due to nonspecific clinical manifestations. We report a 42-year-old asymptomatic man with persistently elevated erythrocyte sedimentation rate (ESR) detected during routine laboratory evaluation. Physical examination revealed bilateral peri-ocular xanthomas. Multimodal imaging demonstrated bilateral perirenal soft-tissue infiltration with a characteristic "hairy kidney" appearance on computed tomography, symmetric skeletal involvement on bone scintigraphy, and metabolically active lesions on ¹⁸F-FDG PET/CT. Histopathological examination of a peri-ocular xanthoma biopsy revealed CD68-positive, CD1a-negative foamy histiocytes, confirming the diagnosis of Erdheim-Chester disease. Given the patient's oligosymptomatic presentation and absence of critical organ involvement, a watch-and-wait strategy with close surveillance was adopted. This case highlights the diagnostic value of subtle clinical findings, such as isolated inflammatory marker elevation and peri-ocular xanthomas, and emphasizes the importance of accessible tissue biopsy combined with characteristic imaging features for early diagnosis of ECD.