To report a case of immune checkpoint inhibitor (ICI)-associated myositis and myasthenia gravis successfully treated with rituximab.

With the broader application of ICIs in the field of oncology, an increasing number of neurological complications have emerged, with a significant proportion being myositis and severe myasthenia gravis. Cemiplimab is an ICI used for the treatment of squamous cell skin cancer and has been associated with neuromuscular complications. Evidence for the best treatment of these complications remains unclear.

A 69-year-old female with squamous cell carcinoma of the skin presented with acute-onset diplopia. She had completed two cycles of cemiplimab with the last dose administered two weeks prior to symptoms of diplopia. During the neurological examination, she exhibited gaze limitation in all directions. Laboratory results showed elevated CK (3485 U/L) and elevated troponin (450 ng). Myasthenia gravis-related antibodies were negative. MRI scans of the brain and orbit, cerebrospinal fluid analysis, electromyography, echocardiogram, and cardiac MRI were unrevealing. In spite of receiving 5 days of intravenous immunoglobulin (IVIG) and thymoma resection, the patient continued to deteriorate over the next ten days, resulting in upper and lower extremity weakness, dysphagia, and dyspnea that eventually required intubation. She underwent plasma exchange followed by a second round of IVIG and was started on prednisone with minimal improvement. 

Rituximab was initiated at a dose of 500mg/m2 every two weeks for two doses. Over the following three months, the patient exhibited significant improvement, ultimately discontinuing ventilation and tube feeds. She was discharged home with the ability to perform activities of daily living independently.

This case, with an overlap of ICI-associated myositis and myasthenia gravis, presents a diagnostic and treatment challenge, but ultimately resulted in a favorable treatment outcome. It underscores the importance of early application of the correct treatment and the role of rituximab in such cases.