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Langerhans cell histiocytosis (LCH) is a group of myeloid neoplastic disorders characterized by infiltration of Langerhans cells, which can accumulate in tissues and cause multisystem manifestations. Erdheim-Chester disease (ECD) is a rare non-LCH histiocytosis, characterized by multisystem infiltration of lipid-laden foamy macrophages. Both ECD and LCH can lead to severe systemic disease, but reports of their overlap remain rare. We describe a female patient with ECD-LCH overlap syndrome presenting predominantly with pericardial effusion. She was admitted to the Department of Respiratory and Critical Care Medicine, Xiangya Hospital, Central South University, on November 10, 2020, due to dyspnea and bilateral lower-extremity edema, with a disease course of 13 months. Computed tomography revealed multi-system involvement, and genetic testing identified a BRAF^V600E mutation. Immunohistochemical staining analysis eventually confirmed ECD-LCH overlap syndrome. Symptomatic treatment was initiated, and follow-up showed stable clinical symptoms. To our knowledge, this is the first reported case worldwide of adult ECD-LCH overlap syndrome with pericardial involvement.