Rapid onset pembrolizumab-induced inflammatory arthritis diagnosed using musculoskeletal ultrasound.

Immune checkpoint inhibitors have revolutionised the treatment of cancer. While very effective, they commonly cause a wide spectrum of immune-related adverse events. These immune-related adverse events can be fatal and often have significant effects on quality of life. They therefore require prompt recognition and management. We report the case of a woman presenting with widespread joint pain and stiffness 6 hours after her first pembrolizumab infusion. She had no joint swelling on physical examination but an ultrasound scan revealed widespread musculoskeletal inflammation, confirming the diagnosis of inflammatory arthritis. To the best of our knowledge, this is the fastest reported inflammatory arthritis onset following immune checkpoint inhibitor treatment. It highlights the importance of timely imaging in patients on immune checkpoint inhibitors who present with new non-specific musculoskeletal pain. Her symptoms improved dramatically with intramuscular triamcinolone injection.