Incidence depends on how hard you look
Most published incidence estimates come from patients tested only after myocarditis was suspected. Systematic testing of every treated patient finds more cases.
In our Michigan Medicine cohort, 2,606 patients receiving checkpoint inhibitors underwent systematic serial testing of aminotransferases, creatine phosphokinase and lactate dehydrogenase during therapy, and 27 of them (1.0%) were diagnosed with myocarditis (Vasbinder, JACC CardioOncol 2022). Incidence estimates from clinical trial safety reporting run several times lower, because trials of that era did not screen for myocarditis and captured mainly the patients who became severely ill.
The same study found signs of myocarditis in routine tests that were not being interpreted for it. Creatine phosphokinase, an inexpensive marker of muscle injury, was elevated at diagnosis in 89% of patients with myocarditis. In multivariable analysis, each doubling from baseline was associated with incident myocarditis (HR 1.83, 95% CI 1.59–2.10) and with all-cause mortality (HR 1.10, 95% CI 1.01–1.20). As a diagnostic test, an elevated level had a sensitivity of 99% and a specificity of 23% (Vasbinder, JACC CardioOncol 2022). That specificity is too low for a stand-alone test, but in a disease this easily missed an elevated level warrants further evaluation, and the test is already being drawn. Its sensitivity follows from the myotoxicity: the process that targets the myocardium also targets skeletal muscle, which releases the enzyme when injured. The combination was most informative: 95% of patients with myocarditis had at least three of these biomarkers elevated, against 5% of those without (Vasbinder, JACC CardioOncol 2022).
Troponin T misses fewer cases than troponin I
Cardiac troponin is the standard biomarker of myocardial injury. Laboratories measure either troponin T or troponin I as though the two were interchangeable, and in this disease they diverge.
Among 60 patients treated at cardio-oncology units in Paris and Heidelberg, more than 1,700 troponin T and 900 troponin I measurements were available. Within 72 hours of admission, troponin T was above its upper reference limit in 98% and troponin I in 88%. In 87 independent cases from an international registry, the corresponding figures on admission were 93% and 64% (Lehmann, Circulation 2023).
The discordance matters most in patients who deteriorate. Major adverse cardiomyotoxic events in this study were heart failure, ventricular arrhythmia, conduction block requiring a pacemaker, respiratory muscle failure requiring ventilation and sudden cardiac death. Within 72 hours of a patient's first such event, troponin T was elevated in all 23 patients who had one. Troponin I was still below its upper reference limit in 11%, and creatine kinase in 27% (Lehmann, Circulation 2023). So a normal troponin I does not exclude the disease.
Troponin T also stratifies risk. The peak ratio of troponin T to its upper reference limit in the first 72 hours discriminated a major event within 90 days with an area under the curve of 0.84 (95% CI 0.72–0.93), against 0.70 for creatine kinase. A ratio of 32 or more carried an age- and sex-adjusted hazard ratio of 11.1 (95% CI 3.2–38.0), and a ratio below 32 identifies a low-risk group (Lehmann, Circulation 2023).
Checkpoint myocarditis can mimic an acute coronary syndrome
The diagnosis is most often missed because a different one fits the presentation just as well.
Chest pain, a rising troponin and electrocardiographic changes in a patient in their seventies describe an acute coronary syndrome, and usually that is the correct diagnosis. Endomyocardial biopsy is the criterion standard for myocarditis, but the diagnosis often also requires clinical suspicion, cardiac biomarkers (troponin in particular) and cardiac imaging (Lehmann, JAMA Cardiol 2021). Suspicion has to come first, because treatment is difficult enough without a delayed start.
In an international registry, 22.6% of 261 patients with suspected checkpoint myocarditis who underwent coronary angiography had obstructive coronary artery disease (stenosis of 70% or more). A third of those were revascularized during the index admission. Revascularized patients were less likely to have received corticosteroids within 24 hours of admission. Their myocarditis-related 90-day mortality was 52.7%, against about 16% both in patients with coronary disease who were not revascularized and in those without it. Revascularization remained associated with myocarditis-related death at 90 days (HR 4.03, 95% CI 1.84–8.84) after adjustment for age and sex (Nowatzke, Eur J Cancer 2022).
This observational comparison cannot separate delayed corticosteroid treatment from a more severe presentation in patients with both diseases. Still, an obstructive coronary lesion does not exclude myocarditis, and in a patient on checkpoint therapy the two diagnoses should be pursued in parallel rather than in sequence.
Thymoma and combination therapy mark risk before the first dose
Most treatment toxicities in oncology become apparent only after exposure. Susceptibility to this one can be identified beforehand, and the principal risk factor suggests a mechanism.
Developing T cells acquire tolerance to self-antigens in the thymus, which makes thymic disease a natural candidate when that tolerance fails. Checkpoint inhibitor treatment of thymic epithelial tumors, thymoma in particular, was associated with myotoxicity more often than treatment of other cancers. In patients with these tumors, myocarditis began earlier after treatment started and was more severe, with more life-threatening arrhythmias and more concurrent myositis leading to respiratory muscle failure (Fenioux, Nat Med 2023).
In an international registry of 748 patients admitted with checkpoint myocarditis in 17 countries, the primary outcome was a major cardiomyotoxic event: severe arrhythmia, heart failure, respiratory muscle failure or cardiomyotoxic death. Active thymoma remained associated with it (HR 3.6, 95% CI 1.7–7.7) in a multivariable Cox model (Power, Eur Heart J 2026). The other risk factor known before the first dose is combined blockade of CTLA-4 with PD-1 or PD-L1, which raises the risk of developing myocarditis.
Five findings at admission predict deterioration
Treatment requires deciding how intensely to suppress the immune response, which depends on identifying at admission the patients who will deteriorate.
In the same 748 patients, five variables available at presentation predicted a major cardiomyotoxic event: active thymoma, cardiomuscular symptoms, a Sokolow–Lyon QRS voltage of 0.5 mV or less on the presenting electrocardiogram, a left ventricular ejection fraction below 50%, and the magnitude of troponin elevation. In multivariable analysis, the hazard ratio was 1.8 (95% CI 1.4–2.4) for a troponin 20 times the upper reference limit and 4.6 (95% CI 2.3–9.3) for one 2,000 times the limit (Power, Eur Heart J 2026). Compared with the same patients' pre-treatment tracings, presenting electrocardiograms showed more conduction disease, a longer QRS and QTc, and lower Sokolow–Lyon voltage (Power, Circulation 2021).
Of 757 patients in the international registry, 707 had an ejection fraction recorded on admission, and in 35% of them it was below 50%. These patients were more likely to have received chest radiation, 24.2% against 13.5% (Chen, JACC CardioOncol 2025). Prior radiation may have compromised the myocardium before the myocarditis began.
A point score built from the five factors stratifies the 30-day cumulative incidence of major cardiomyotoxic events from 4% at a score of zero to 81% at four or more. Its C-index was 0.70 (95% CI 0.67–0.73) on internal validation (Power, Eur Heart J 2026). The score was externally validated in independent French and United States cohorts. In the French cohort it was applied prospectively: all seven patients with a score of zero had the checkpoint inhibitor stopped and received no immunosuppression, and none had a major cardiomyotoxic event or died within a month. A score of zero may identify patients who can be spared high-dose corticosteroids.
Treatment rests on experience, and every option has a cost
Early recognition matters because treatment is difficult. The regimen rests on experience rather than randomized evidence, every option carries a cost, and the disease extends beyond the myocardium.
Corticosteroids are first-line, and a substantial minority of patients do not respond adequately. The next step, derived from clinical experience, adds the JAK inhibitor ruxolitinib and abatacept. Abatacept is a CTLA-4 fusion protein, built from the checkpoint that one class of these drugs blocks, and its dose was adjusted by CD86 receptor occupancy on circulating monocytes (Salem, Cancer Discov 2023). No regimen has been established by a randomized trial. A randomized, double-blind, dose-finding phase II trial of abatacept, with seven patients per arm, has been completed (Salem, Arch Cardiovasc Dis 2025).
The same autoimmune process targets skeletal muscle, and myositis frequently accompanies the myocarditis, which is why the field increasingly calls the syndrome cardiomyotoxicity. When the respiratory muscles are involved, a patient can die of respiratory failure while cardiac function recovers. Systematic screening for respiratory muscle involvement, with early mechanical ventilation when it was found, was half of the strategy that reduced fatality (Salem, Cancer Discov 2023).
Corticosteroids, abatacept and ruxolitinib all suppress the antitumor immune response that the checkpoint inhibitor was given to produce. Dampening it to protect the heart risks losing control of the cancer, no rule defines an acceptable trade-off, and the decision has to be made within days.
When to stop is as unsettled as what to start. Tapering immunosuppression too quickly risks recurrent myocarditis; tapering too slowly risks infection and leaves the cancer unchecked. We have worked on monitoring resolution with troponin and the non-cardiac biomarkers rather than relying on a fixed schedule (Vasbinder, Curr Cardiol Rep 2023). As with the risk score, the aim is to match the intensity of treatment to the severity of disease, here during de-escalation.
Fatality is far lower than early reports suggested
Early reports gave the disease a reputation for killing about half of those who developed it. Case fatality is far lower now, and the evidence attributes the decline to earlier recognition and treatment rather than to a milder disease.
Among the 748 registry patients, the 30-day incidence was 33% for a major cardiomyotoxic event, 13% for cardiomyotoxic death and 17% for death from any cause (Power, Eur Heart J 2026). These rates are well below the figures still widely quoted. Those came from the first case series and pharmacovigilance reports, which by their nature captured the patients ill enough to be recognized and reported (Mahmood, J Am Coll Cardiol 2018; Salem, Lancet Oncol 2018).
The clearest evidence that recognition and treatment account for the decline comes from a single center that changed its practice partway through a series. Of 40 patients with definite myocarditis, the first 10 were managed according to the guidelines then in use, and 60% died of the myotoxicity, consistent with historical controls. The next 30 underwent systematic screening for respiratory muscle involvement, early mechanical ventilation where it was found, and treatment with abatacept and ruxolitinib. One of the 30 died of the myotoxicity (Salem, Cancer Discov 2023).
A before-and-after comparison at one center is hypothesis-generating, as the authors state, and the later patients may also have been recognized earlier as awareness grew. Drift in case ascertainment is unlikely to explain a difference of that size, however, and every component of the change was a clinical decision. The troponin assay, the routine biomarkers and the risk score described above treat nobody, but each can bring treatment forward to a point where it still works.
Checkpoint inhibitors may accelerate atherosclerosis
Myocarditis is an acute and rare toxicity. The larger burden may be chronic and vascular, and the program is taking up that question next.
In the TriNetX federated health record network, we compared 30,720 propensity-matched patients starting either a checkpoint inhibitor or chemotherapy. Over two years, checkpoint therapy was associated with a 40% higher risk of hospitalization for a major adverse cardiovascular event (RR 1.40, 95% CI 1.26–1.56). The excess was consistent across heart failure, myocardial infarction and ischemic stroke, and the two-year cumulative incidence was highest with durvalumab, at 16.2% (Vosooghi, JCO Oncol Pract 2026). The chemotherapy cohort was historical, treated between 1997 and 2011, so background cardiovascular care differs between the groups and the excess cannot be attributed to the drugs alone. We report it as a signal that warrants a prospective study.
In mouse and cellular models, the checkpoints these drugs block (PD-1, PD-L1 and CTLA-4) are negative regulators of atherosclerosis, so systemic blockade is a candidate mechanism for the excess. In a matched cohort at one academic center, another group found a three-fold higher risk of atherosclerotic cardiovascular events after a checkpoint inhibitor was started (HR 3.3, 95% CI 2.0–5.5). In an imaging substudy, total aortic plaque volume progressed more than three times faster after treatment began (Drobni, Circulation 2020).
Our next questions are whether accelerated atherosclerosis is occurring in the patients we follow, how early it can be detected, and whether it can be modified. Answering them requires measuring inflammation, the subject of the lab's other program.
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