Pembrolizumab-induced myositis and cardiogenic shock may be reversible with early immunosuppression and circulatory supportImmune therapy for thymoma can cause severe heart and muscle damage

Immune checkpoint inhibitor (ICI)-associated toxicities are uncommon but may rapidly progress to cardiogenic shock and death. Patients with thymoma are considered particularly vulnerable to severe immune-related adverse events because thymic malignancy is closely associated with impaired central immune tolerance and autoimmune dysregulation. We report a 34-year-old man with stage IV type B2 thymoma who developed initial myalgia and fatigue approximately 15 days after the most recent pembrolizumab-containing treatment cycle and progressed to fulminant ICI-associated myositis with respiratory failure and secondary cardiogenic shock by day 19. He presented with severe immune-mediated myositis, concurrent myocardial injury, left ventricular systolic dysfunction, cardiogenic shock, metabolic acidosis, acute kidney injury, acute liver injury, coagulopathy, and respiratory failure. Coronary angiography excluded obstructive coronary disease, and microbiological and virological investigations did not identify an infectious cause. Endomyocardial biopsy and cardiac magnetic resonance imaging were not feasible because of profound hemodynamic instability, VA-ECMO support, and coagulopathy. The patient was treated with early venoarterial extracorporeal membrane oxygenation (VA-ECMO), high-dose methylprednisolone, intravenous immunoglobulin, continuous renal replacement therapy, mechanical ventilation, and comprehensive organ support. Cardiac function improved from a left ventricular ejection fraction of 25% to 51%, VA-ECMO was discontinued on hospital day 7, and the patient survived to discharge. This case and a focused literature review suggest that thymoma-associated ICI myositis with secondary cardiac dysfunction may represent a high-risk phenotype characterized by early onset, overlap neuromuscular toxicity, severe hemodynamic compromise, and potential reversibility when early immunosuppression and advanced circulatory support are implemented.