Immune Checkpoint Inhibitor-Induced Myocarditis, Myositis, and Concern for 3-Ms Syndrome in a Patient With Metastatic Melanoma: A Case Report.

Immune checkpoint inhibitors (ICIs), including ipilimumab and nivolumab, have revolutionized treatment for advanced malignancies such as metastatic melanoma. While effective, these agents can trigger immune-related adverse events (irAEs) that may involve multiple organ systems. Myocarditis, myositis, and myasthenia gravis - collectively referred to as the "3 Ms" overlap syndrome - are rare but potentially fatal irAEs. These conditions can present concurrently, often with nonspecific symptoms, making diagnosis challenging and potentially delayed. This case describes a 79-year-old male with stage IV metastatic melanoma who developed myocarditis and myositis following a single cycle of combination ICI therapy, with clinical features raising concern for an overlapping myasthenia-like syndrome, although a definitive diagnosis was not made. Early recognition and prompt initiation of high-dose corticosteroids resulted in clinical improvement and stabilization. This case underscores the importance of maintaining a high index of suspicion for severe irAEs in patients receiving ICIs, as timely intervention is critical to preventing life-threatening complications.