Immune checkpoint inhibitor-related myositis with hepatitis in HER2-positive metastatic breast cancer after trastuzumab deruxtecan plus sintilimab: a case report.

Immune checkpoint inhibitors (ICIs) have improved outcomes across multiple solid tumors, but they may trigger immune-related adverse events involving the nervous system, skeletal muscle, myocardium, and liver. ICI-related myositis is uncommon but clinically important and may overlap with myocarditis or other neuromuscular toxicities. Reports in breast cancer remain limited, particularly in HER2-positive disease outside standard immunotherapy settings. We describe a 67-year-old woman with HER2-positive metastatic breast cancer who had received multiple anti-HER2 regimens and had limited tolerance to chemotherapy and endocrine therapy. After multidisciplinary discussion and informed consent, her regimen was adjusted to trastuzumab deruxtecan plus sintilimab because of progressive pulmonary and intracranial disease. After the first cycle, she developed acute bilateral ptosis with mild bilateral lower-extremity weakness, marked creatine kinase elevation (peak CK 8075.3 U/L), CK-MB elevation (206.4 U/L), and severe transaminase elevation. Repetitive nerve stimulation did not show a typical decremental response, left orbicularis oculi fatigue testing was negative, myasthenia gravis-related antibody testing was recommended but declined, and the response to cholinesterase inhibitor therapy was limited. Troponin testing, repeat echocardiography, and cardiac magnetic resonance imaging were not performed. Therefore, the condition was reinterpreted as immune-related myositis with predominant ocular involvement and concomitant hepatitis; concomitant myasthenia gravis remained unproven and myocarditis could not be definitively excluded. Sintilimab was discontinued, and high-dose methylprednisolone was initiated and tapered. Muscle and liver enzymes decreased substantially, and ptosis partially improved. The patient received no further anticancer treatment and died in December 2025; telephone follow-up suggested progressive metastatic breast cancer as the most likely cause. This case highlights that new ocular symptoms accompanied by marked CK elevation after PD-1 inhibitor-containing therapy should prompt early consideration of immune-related myositis, even when classical myasthenia gravis is not supported by the available evidence.