A rare and misleading condition: isolated skeletal involvement of Erdheim-Chester Disease

We are describing an unusual presentation of Erdheim-Chester Disease (ECD), a non -Langerhans histiocytosis provoked by abnormal aggregation of foamy histiocytes in several organs and systems. Our patient, already with osteoporosis, presented severe bilateral ankle pain with functional impairment, clinically mimicking “Complex Regional Pain Syndrome” (CRPS); the radiological images and subsequently the bone biopsy were fundamental for achieving the proper diagnosis. The diagnosis was challenging given the isolated bone localization which is uncommonly seen in ECD.