We present a rare case of a triad of myasthenia gravis (MG), myositis, and myocarditis following a single cycle of pembrolizumab, highlighting the importance of early recognition and aggressive management of refractory neuromuscular immune-related adverse events (irAEs).

Immune checkpoint inhibitors (ICIs) have transformed oncologic care, but their neurological complications remain underappreciated. ICI-induced myasthenia gravis (MG) is uncommon yet far more dangerous, presenting with oculo-bulbar predominance and requiring ventilatory support up to seven times more frequently. The concurrent development of myositis and myocarditis, the IM3OS triad, escalates mortality from approximately 30% to 38–60%. No prospective treatment trials exist, and clinicians must rely on retrospective data and expert consensus to guide management.

This case illustrates a rare but life-threatening occurrence of ICI-induced multi-system immune injury, where Myasthenia gravis emerged as the most refractory component. Early recognition of the IM3OS triad, stepwise escalation of immunosuppression for refractory neuromuscular symptoms, and coordinated multidisciplinary care are essential to improving survival outcomes in this challenging syndrome.