A national survey on myocarditis associated with influenza H1N1pdm2009 in the pandemic and postpandemic season in Japan

An influenza pandemic occurred in 2009. We performed a retrospective national questionnaire survey about H1N1pdm2009 myocarditis to compare influenza A H1N1pdm2009 myocarditis in the pandemic (2009/2010) and postpandemic seasons (2010/2011) by collecting data from 360 hospitals. The diagnosis of myocarditis was performed using the guidelines for Diagnosis and Treatment of Myocarditis published by the Japanese Circulation Society (JCS 2009). Twenty-nine patients with influenza A H1N1pdm2009 myocarditis were reported, with 25 from the 2009/2010 season and only 4 patients from the 2010/2011 season. Morbidity and mortality was 28 % (8/29) among all the myocarditis patients. Six patients with myocarditis were complicated by pneumonia. Myocarditis was proved by endomyocardial biopsy or autopsy in 9 patients, although histological findings showed mild myocarditis even in clinically defined fulminant myocarditis cases. Seventeen patients were diagnosed with fulminant H1N1pdm2009 myocarditis with fatal arrhythmias or varying degrees of cardiogenic shock. Fifteen fulminant myocarditis patients were seen in the 2009/2010 season and only 2 in the 2010/2011 season. Ventilators were used in 16 patients. Mechanical circulatory support with intraaortic balloon pumping or percutaneous cardiopulmonary support (IABP/PCPS) was emergently inserted in 13 patients. Of these, 9 patients were rescued with mechanical circulatory support, and 4 patients died. Four fulminant myocarditis patients treated without IABP/PCPS died. We described the clinical features of patients with myocarditis associated with influenza H1N1pdm2009 in the pandemic and postpandemic seasons and demonstrated the high prevalence of fulminant myocarditis (17/29, 59 %). The number of patients with myocarditis associated with influenza A virus seemed to increase in the pandemic season.