Influenza-Associated Cardiac Manifestations: A Systematic Review of Case Reports and Case Series

Background: Influenza is primarily a respiratory illness, but it is increasingly linked to acute cardiovascular complications, including myocarditis and sudden cardiac death. Evidence on influenza-associated cardiac involvement remains fragmented and largely derived from case-based reports. Methods: This systematic review followed PRISMA guidelines and was registered with PROSPERO (CRD420251249130). PubMed, Web of Science, and Google Scholar were searched for eligible studies. Case reports and case series were included to characterize rare, severe, or atypical cardiac manifestations that are not captured in larger epidemiological studies. Data on demographics, influenza subtype, cardiac manifestations, investigations, management, and outcomes were extracted. Results: Thirty-three studies (32 case reports and one case series) comprising 36 cases were included. Influenza A was reported in 26 cases and influenza B was reported in 9 cases. Myocarditis was the most common manifestation (19 cases, 53%), including fulminant myocarditis in 9 cases (25%). Cardiogenic shock occurred in seven cases (19%), malignant arrhythmias in four(11%), myocardial infarction-like presentations in three (8%), and cardiac arrest or sudden cardiac death in six cases (17%). Cardiac involvement occurred across all age groups, frequently in patients without pre-existing coronary artery disease. Overall mortality was 44% (16/36), with fatalities reported in both influenza A and B infections. These findings predominantly reflect severe and published cases and may not represent the full clinical spectrum of influenza-associated cardiac involvement. Conclusions: This review highlights that influenza infection has been reported in association with a range of acute cardiac manifestations, particularly myocarditis, including severe and fulminant presentations. However, given the reliance on case reports and case series, the findings are subject to significant publication bias and do not permit the estimation of incidence, risk, or causal inference. These observations should therefore be interpreted as hypothesis-generating.