Progressive Conduction Disease on Serial ECGs Revealing Immune Checkpoint Inhibitor–Associated Myocarditis and Myositis

BACKGROUND: Immune checkpoint inhibitors are effective treatments for cancer but also carry cardiovascular toxicity risks. CASE SUMMARY: An 83-year-old male receiving cemiplimab for advanced squamous and basal cell carcinoma presented with severe back pain. One week earlier, electrocardiogram (ECG) demonstrated atrial flutter with new right bundle branch block, slowed atrial activity, and possible complete heart block with a ventricular rate of 48 beats/min, alongside rising liver enzyme abnormalities. On admission, ECG demonstrated complete heart block. Troponin and creatine kinase were elevated. He developed Society for Cardiovascular Angiography and Interventions (SCAI) Stage C cardiogenic shock treated with transvenous pacing. Progressive ophthalmoplegia, proximal weakness, and respiratory decline followed. He received high-dose methylprednisolone, intravenous immunoglobulin, and permanent pacemaker implantation. Muscle biopsy confirmed immune-mediated necrotizing inflammatory myopathy. DISCUSSION: Serial ECG evolution suggested early immune checkpoint inhibitor myocarditis. Despite rapid immunosuppression, his disease was steroid-refractory and fatal. TAKE-HOME MESSAGES: Progressive conduction abnormalities on electrocardiogram may be an early manifestation of immune-checkpoint inhibitor myocarditis, before troponin elevation. Baseline and surveillance electrocardiograms can detect progressive conduction abnormalities as manifestations of early immune-checkpoint inhibitor toxicity and guide emerging mechanism-matched immunosuppression.