From immunotherapy to autoimmunity: reactive arthritis following intravesical BCG.

Reactive arthritis (ReA) is recognised as a sterile inflammatory arthritis triggered by infections or other antigens, including intravesical BCG used to treat non-muscle-invasive bladder cancer. Although BCG immunotherapy is highly effective, its side effects can extend beyond the genitourinary tract. We present a woman in her 60s with high-grade Ta urothelial cell carcinoma (UCC) and carcinoma in situ (CIS) who developed ReA 1 week following BCG instillation. She experienced fevers, blurred vision, mild conjunctivitis and oligoarthritis predominantly affecting her knees. Infection was ruled out by negative cultures, while elevated inflammatory markers pointed to an autoimmune aetiology. Prompt discontinuation of BCG and corticosteroid therapy led to significant clinical improvement. This case underscores the importance of recognising ReA as a potential post-BCG complication requiring multidisciplinary management. It highlights the need for intervention and follow-up.