Risk factors for myocarditis hospitalization and recurrence: a state-wide retrospective observational study.

Aims: Myocarditis is a potentially life-threatening condition with diverse aetiologies including viral infections, toxins, and autoimmunity. We aimed to quantify the risk factors of index myocarditis hospitalization and subsequent myocarditis recurrence. Methods and results: We conducted a retrospective cohort study in New South Wales (NSW), Australia, using the Admitted Patient Data Collection (APDC) of all hospitalized patients. Conditions temporally associated with myocarditis within 30 days of the index admission were identified using conditional logistic regression analysis. In patients with previous myocarditis, risk factors for recurrent myocarditis admission were calculated with both Cox regression using cause-specific hazards and competing risk analysis. There were 4071 cases of index myocarditis from 2004 to 2021. Over a median of 4.8 years of follow-up, there were 124 patients whose myocarditis recurred. Two-thirds of cases were male with an average age of 42 years. Index myocarditis cases were much more common within 30 days of a hospitalization for pericarditis, heart failure, ventricular arrhythmias, COVID-19, and several other cardiac, respiratory, and autoimmune conditions, compared to the baseline risk over the preceding 12 months. Similarly, myocarditis recurrence was more common within 30 days of pericarditis, ventricular arrhythmias, COVID-19, and autoimmune disease. Recurrence was not strongly predicted by any features of the index myocarditis admission. Our analysis is solely based on administrative coding, with limited clinical validation, which introduces potential for misclassification. Conclusion: In our cohort, myocarditis was more frequently diagnosed following presentations with acute respiratory illness (including COVID-19), autoimmune conditions, or cardiac events including ventricular arrhythmias, atrial fibrillation, and heart failure.