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

Background and 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 hospitalisation and subsequent myocarditis recurrence. Methods We conducted a retrospective cohort study in New South Wales (NSW), Australia using the Admitted Patient Data Collection (APDC) of all hospitalised 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. Results 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 hospitalisation 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.