Fatal Myocarditis Relapse in a Patient with Myasthenia Gravis Treated with Efgartigimod Alfa: A Case Report.

Myasthenia gravis (MG) is an autoimmune neuromuscular disorder primarily caused by autoantibodies against acetylcholine receptors (AChR), leading to muscle weakness. In 2021, efgartigimod was FDA-approved as the first neonatal Fc receptor (FcRn) blocker for generalized MG, reducing pathogenic IgG autoantibodies and improving symptoms. We present a 39-year-old woman with AChR-positive generalized MG and a history of resolved inflammatory myocarditis. Owing to persistent weakness despite intravenous immunoglobulin, she transitioned to efgartigimod. Efgartigimod induced significant neuromuscular improvement, reducing her myasthenia gravis activities of daily living (MG-ADL) score from 9 to 4. However, 3 months later, she developed fatal fulminant heart failure (LVEF 20%-25%) without concurrent skeletal muscle weakness. Despite emergency intervention with veno-arterial extracorporeal membrane oxygenation, the patient died after anoxic brain injury. Although efgartigimod effectively reduces pathogenic IgG, it may not adequately suppress cellular or cytokine-mediated inflammation driving MG-associated myocarditis. Clinicians should exercise caution and maintain cardiac monitoring when prescribing FcRn blockers to patients with a history of myocardial involvement.