Immune Checkpoint Inhibitor-Induced Myositis: a Case Report and Literature Review

We clarify clinical characteristics of patients with immune checkpoint inhibitor (ICI)-induced myositis. In 13 of 15 cases with ICI-induced myositis, the type of malignancy was melanoma. Eight, 4, and 3 patients received anti-PD-1 alone, anti-CTLA4 alone, and a combination of those, respectively. The mean period to the onset of ICI-induced myositis from the initiation of ICI was 4 weeks. Myocarditis was a complication in five patients. Seven of the patients died. The causes of death were myocarditis in three patients, respiratory muscle paralysis in two patients, and cancer progression in two patients. In patients without myocarditis or respiratory muscle paralysis, the prognosis for myositis was favorable with normalization of the CK levels occurring upon the cessation of ICI and the administration of immunosuppressive agents. Myocarditis and respiratory muscle paralysis are the major causes of death as immune-related adverse events in patients with ICI-induced myositis.