Pembrolizumab-Associated Myocarditis and Myasthenic Crisis

BACKGROUND: Immune checkpoint inhibitors (ICI) can cause myocarditis with overlapping neuromuscular toxicity. CASE SUMMARY: A 77-year-old man developed biopsy-supported ICI myocarditis with biventricular dysfunction (left ventricular ejection fraction: 36%) after 1 pembrolizumab cycle. The course was complicated by seropositive myasthenia gravis (MG) requiring mechanical ventilation and an unexpected nonocclusive saddle pulmonary embolism. Corticosteroid-refractory troponin elevation prompted escalation to abatacept, while prednisone was reduced for worsening myasthenic weakness. Follow-up cardiac magnetic resonance during sepsis showed improved left ventricular ejection fraction of 53%, but myocardial T2 remained elevated with a persistent nonischemic pattern of late gadolinium enhancement. Troponin remained elevated, and ventilator dependence persisted. DISCUSSION: ICI myocarditis may show improving ventricular systolic function while myocardial tissue abnormalities persist, and coexisting MG may limit corticosteroid use. TAKE-HOME MESSAGES: Myocarditis and MG may coexist and require multidisciplinary management. Troponin T may remain elevated for months and is not diagnostic of ongoing myocardial inflammation.