Case Report: Immune checkpoint inhibitor-induced myositis/myocarditis with myasthenia gravis-like symptoms in a colon cancer patient with suspected thymic metastasis.

Background: The Widespread application of immune checkpoint inhibitors (ICIs) has led to a noticeable increase in patients developing the triad of severe myasthenia gravis (MG), myocarditis, and myositis. However, this overlapping triad syndrome is still very rare, characterized by a subtle onset, a rapid progression, and a high mortality rate. Due to its insidious onset and diagnostic challenges, there is currently limited knowledge of this overlapping triad syndrome in clinical settings. Case presentation: We report the case of a male colon cancer patient with suspected thymic metastasis who developed the overlapping immune-related myocarditis, myositis, and MG-like symptoms after one cycle of ICI treatment. The patient received timely administration of corticosteroids, oxygen therapy, immunoglobulin, and other treatments, following which he recovered and was discharged. No discernible adverse impact on his subsequent quality of life was observed. Conclusions: Vigilance is warranted for the risk of developing a triad of severe MG, myocarditis, and myositis when patients receiving ICIs present with symptoms such as exertional dyspnea, swallowing weakness, diplopia, dysarthria, limb weakness, or inability to walk due to muscle pain, particularly for patients with thymic masses. Such patients should be monitored in the hospital and, if necessary, admitted to the intensive care unit, where they will be managed by a multidisciplinary team of specialists.