RESUMEN
Rheumatoid arthritis (RA) is a chronic autoimmune disease characterised by symmetric inflammatory polyarthritis. However, RA limited to a single joint is extremely rare. Here, we report a middle-aged woman who presented with insidious right elbow arthritis. She had no other peripheral joint pain, tenderness or swelling. She had high-positive anti-cyclic citrullinated peptide antibodies. An MRI of the right elbow showed capsular distension, joint effusion and bone marrow oedema. Synovial biopsy revealed hyperplasia with lymphoplasmacytic infiltrate consistent with RA. Therapy with methotrexate 7.5 mg orally weekly was effective to control her inflammatory arthritis. This case highlights the relevance of synovial tissue analysis for patients presenting with chronic inflammatory monarthritis when the cause is not clinically evident, and the importance of considering RA even in the absence of polyarticular involvement. Delayed diagnosis and treatment of inflammatory monarthritis can lead to joint destruction and disability.