Ultrasonographic findings in a patient with reactive arthritis induced by intravesical BCG therapy for bladder cancer

Intravesical bacillus Calmette–Guerin (BCG) therapy is an effective and safe immunotherapy for superficial bladder cancer. However, reactive arthritis (ReA) is reported as an adverse event in about 0.5–1% of patients receiving intravesical BCG therapy. ReA is known as a kind of spondyloarthritis, the main pathological finding of which is enthesitis. Here we report a patient with ReA induced by intravesical BCG therapy for bladder cancer, in whom the diagnosis of ReA was supported by ultrasonographic findings of enthesitis and synovitis. A 69-year-old male was diagnosed with carcinoma in situ of the bladder and treated with intravesical BCG therapy. After the third intravesical BCG injection, pain and swelling appeared in both wrists. Ultrasonographic examination revealed not only intra-articular synovitis in the bilateral wrist joint, wrist flexor tenosynovitis, and wrist extensor tenosynovitis, but also enthesitis of the flexor carpi radialis tendon (FCR). The severe enthesitis of the FCR in both wrists was considered to be an important finding that supported the diagnosis in this patient. Thus, we diagnosed this case as ReA induced by intravesical BCG therapy. In clinical practice, ReA is often difficult to distinguish from seronegative rheumatoid arthritis and other rheumatic diseases. Ultrasonographic findings of enthesitis may support a definitive diagnosis for these patients.