Troponin T/I discordance in immune checkpoint inhibitor myositis in a patient with ischaemic cardiac disease: a case report.

Background: Immune checkpoint inhibitors (ICIs) are increasingly used in oncology and can cause immune-related adverse events, including cardiotoxicity. Discrepancies between cardiac troponin T (cTnT) and cardiac troponin I (cTnI) have been described in ICI-associated myositis with otherwise normal cardiac findings and may help differentiate myositis from myocarditis. In this case, cTnT elevation was first detected on routine troponin testing performed because myocarditis is a recognized immune-related adverse event of nivolumab, despite the absence of typical cardiac symptoms. Case summary: A 72-year-old man receiving nivolumab developed marked cTnT elevation. He was referred to cardiology for suspected ICI-related myocarditis. Cardiac MRI was unremarkable. Because of cardiovascular risk factors and unexplained troponinaemia, coronary angiography was performed and revealed an 80% stenosis of the LAD, which was treated with a drug-eluting stent. Post-intervention, cTnT continued to rise while cTnI remained normal on repeated measurements. Differential diagnoses included myositis, myocarditis, and in-stent thrombosis. Taken together, the clinical, imaging, and biomarker findings, in particular the troponin T/I discrepancy, made a cardiac cause of cTnT elevation unlikely. Immune-mediated myositis was confirmed, and mycophenolate mofetil therapy led to clinical and biochemical improvement. Conclusion: This case may expand current knowledge by showing that, in an ICI-treated patient, troponin T/I discrepancy may not only help differentiate myositis from myocarditis but also assist in distinguishing skeletal muscle involvement from other cardiac causes, including ACS and in-stent thrombosis. Recognition of this pattern may improve diagnostic accuracy and guide appropriate therapeutic strategies in oncological patients presenting with troponinaemia.