Erdheim–Chester disease: An elusive diagnosis in a 50‐year‐old Ethiopian man presenting with diffuse sclerotic bone lesion.

Key Clinical Message: Diagnosis of Erdheim–Chester disease (ECD) requires the clinician to be familiar with its various manifestations, classic radiologic and histologic features. This case highlights the significance of considering ECD in any patient presenting with bone pain and symmetric osteosclerosis of long bones of extremities to allow for early diagnosis and treatment. Erdheim–Chester disease (ECD) is a rare non‐Langerhans histiocytic disorder with diverse clinical manifestations, ranging from indolent, localized presentation to life‐threatening, multi‐systemic disease. Delayed or erroneous diagnosis is common. The presence of classic radiographic finding along with foamy histiocytes that is positive for CD68 but negative for CD1a on histologic examination establishes the diagnosis. We report a second case of ECD from Ethiopia. A 50‐year‐old Ethiopian man presented with a 13‐year history of bilateral lower leg bone pain, cold intolerance, somnolence, constipation, impotence, decreased libido, and secondary infertility. The diagnosis was suspected when skeletal X‐ray revealed bilateral symmetric sclerosis of metadiaphysis of femur, tibia, and humerus. The demonstration of foamy histiocytes that were positive for CD68 but negative for CD1a on histologic examination with immunohistochemical staining confirmed the diagnosis. Evaluation for the extent of the disease revealed coated aorta sign, hairy kidney sign, and cystic lesion with ground glass opacity of lung, primary hypothyroidism, and hypergonadotropic hypogonadism. ECD is rare histiocytic neoplasm with wide range of clinical features which often delay the diagnosis. Clinician should be mindful of the various presentations and the classic radiographic and histologic features of ECD. This case highlights the significance of entertaining ECD in any patient presenting with lower leg bone pain and symmetric osteosclerosis of long bones of lower extremities to allow for early diagnosis and treatment. [ABSTRACT FROM AUTHOR]