Nivolumab-induced myositis with myocarditis
B. Measure troponin and natriuretic peptides, and perform an echocardiogram
The key to the correct diagnosis was the presence of muscle weakness, elevated CK, and MRI changes consistent with myositis, occurring after nivolumab administration. Nivolumab-associated myositis may be associated with myocarditis, which can be asymptomatic. The ECG showed ST-segment elevations in the anterior (V1-V3) and anterolateral (V4-V6) leads with PR depressions in lead II and V4-V6, consistent with myopericarditis-type changes. Intravenous immune globulin (IVIG) is an effective treatment for myositis associated with immune checkpoint inhibitor therapy and IV fluids would treat rhabdomyolysis, but both could cause volume overload and acute heart failure without first evaluating left ventricular function. While muscle biopsy would definitively diagnose myositis, this is not the most appropriate next step due to its invasive nature.