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Four children seen with H1N1 influenza infection at a San Diego hospital developed acute myocarditis, which killed one of them, researchers said.

"Our observations warrant a high index of suspicion for myocarditis in children with H1N1 influenza A infection," according to András Bratincsák, MD, PhD, and colleagues at Rady Children's Hospital in San Diego, in a research letter published online in the Journal of the American College of Cardiology.

"Early detection and aggressive management are paramount," they wrote.

The four children, ranging in age from 3 months to 9 years, were brought to Rady last October. They were among 80 children hospitalized in that month with H1N1 flu infections, the researchers said.

Three were diagnosed with fulminant myocarditis on the basis of echocardiographic and clinical signs of severely impaired left ventricular systolic function -- or, in the fatal case, lymphocytic infiltration in the heart muscle found at autopsy.

The death probably resulted from acute atrioventricular block, the researchers indicated.

The two surviving children with fulminant myocarditis were treated with extracorporeal membrane oxygenation. Complications included intracranial hemorrhage in one and ischemic encephalopathy in another.

There was no evidence of sepsis or bacterial infection, according to Bratincsák and colleagues.

The fourth case was less severe, with normal ejection fraction in the initial echocardiography and at follow-up. That child, a nine-year-old African-American boy with a history of asthma symptoms, was also in less distress at presentation, with just a cough and runny nose.

But the boy nevertheless developed respiratory distress with elevated cardiac enzymes and pericardial effusion.

Bratincsák and colleagues noted that their hospital services a region with about 800,000 children. Over the past three years, they indicated, there were a total of six cases of acute myocarditis thought to be related to viral infections.

The four cases occurring in conjunction with H1N1 in a single month "raise the possibility that the novel H1N1 influenza A virus is more commonly associated with a severe form of myocarditis than previously encountered influenza strains," the researchers concluded.

They added that "timely intervention with circulatory support" may improve outcomes in such cases.

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