An Unusual Case of Erdheim Chester Disease (ECD) with Knee Pain: A Case Report.

Background: Erdheim Chester disease (ECD) is a rare, non-Langerhans cell histiocytosis of unknown etiology that occurs in multiple organs. The clinical characteristics of ECD are unknown, making it difficult to diagnose. Case presentation: A 61-year-old woman presented with left knee pain and contracture. She had recent medical problems such as recurrent urinary tract infection, pericardial effusion, and pleural effusion. Simple radiography and magnetic resonance imaging of the knee revealed an osteosclerotic lesion. Under suspicion of malignancy, other radiologic modalities were performed, but there were no significant results showing malignancy. A bone biopsy of the knee lesion led to a final diagnosis of ECD. The patient was treated with systemic steroids and was ultimately tried on PEG-interferon. Conclusion: This report describes an unusual presentation of ECD involving the skeletal system and multiple extraskeletal organs. Owing to its non-specific nature, ECD was notably difficult to diagnose. Therefore, if a patient has knee pain and other multiorgan presentations without malignancy, clinicians should suspect ECD.