Reactive Histiocytic Proliferation in Rheumatoid Arthritis Mimicking Erdheim-Chester Disease.

Erdheim-Chester disease (ECD) is a rare non-Langerhans cell histiocytosis that can mimic inflammatory and neoplastic conditions, complicating diagnosis. We present a case in which ECD was initially suspected due to histiocytic proliferation but was ultimately diagnosed as rheumatoid arthritis (RA) with reactive histiocytosis. An adult female with a 50-year smoking history presented with polyarticular joint pain, dyspnoea, and peripheral oedema. Chest imaging revealed a left pleural effusion, confirmed as an exudate (protein 39 g/L) with negative cytology. Fluorodeoxyglucose positron emission tomography-computed tomography (FDG PET-CT) showed a large joint uptake, suggestive of inflammatory arthropathy and bilateral pleural effusions without focal pulmonary lesions. Thoracoscopy and pleural biopsy revealed histiocytic proliferation, raising suspicion for ECD. Clinical examination and serology (rheumatoid factor >200 Units U/mL and anti-cyclic citrullinated peptide >340 U/mL) confirmed RA. Genetic testing of the biopsy showed no mutations, suggesting reactive histiocytosis. Methotrexate treatment for RA led to symptomatic improvement, and repeat PET-CT was stable. Haematology follow-up ruled out malignancy, discharging the patient from the hospital. This case highlights the diagnostic challenge of distinguishing ECD from RA when histiocytic proliferation and joint involvement coexist. Genetic testing and multidisciplinary evaluation are crucial for accurate diagnosis and management.