Cardiac disease myocarditis has historically affected more men than women, and that holds true for pediatric populations, according to a study published this week in the Journal of the American Heart Association.

A nationwide analysis of children admitted to Finnish hospitals for myocarditis, an inflammatory disease commonly caused by viral infections, determined that, barring infancy, the illness is more prevalent in boys and men than in girls and women, first author Anita Arola, MD, PhD, and colleagues wrote. While the epidemiology of myocarditis has been studied in adult populations—consisting largely of those 50 years old and up—the same can’t be said for pediatric studies of the same topic. Because of this, Arola et al. wrote, this trial focused on defining myocarditis in a younger population and evaluating the sex differences that are present in older age groups.

“True incidence of myocarditis is unknown,” the authors wrote. “On the one hand, it is probably underdiagnosed because it may be asymptomatic in a considerable number of patients. On the other hand, congestive heart failure, ventricular arrhythmias and sudden death can be the presenting symptoms. Diagnosis is challenging because symptoms are frequently nonspecific, especially in infants and children, masquerading as respiratory and gastrointestinal infections.”

Indeed, Arola and co-authors said, most cases of myocarditis start as unsuspecting viral infections, making the real problem difficult to pinpoint.

The team identified 213 Finnish children and adolescents who’d been hospitalized for myocarditis-related illness between 2004 and 2014. While Arola and colleagues disqualified any patients whose primary, secondary or tertiary causes for admission weren’t myocarditis, it was the primary cause for admission in 86 percent of the study population, translating to an overall incidence rate of 1.95 in 100,000 person-years.

Seventy-seven percent of the studied patients were boys, the researchers reported, but sex differences in the data were age-dependent. Children ranging from newborns to 5-year-olds saw no gender differences when it came to myocarditis diagnosis, but six-year-olds through adolescents showed a clear gap—boys were exhibiting more cases than girls. The sex difference increased exponentially with age.

“In both clinical and experimental studies, male sex has been shown to be a risk factor for myocarditis,” the authors wrote.

Despite these new findings, Arola and her team wrote there’s still a lot of research that needs to be done on the subject, including better assessment of long-term prognosis of childhood myocarditis and more precise diagnostic criteria, to form a more comprehensive etiology of the disease.

“It is impossible to estimate the incidence of asymptomatic myocarditis during common viral infections,” they wrote. “Viruses that most often cause myocarditis are highly prevalent in the general population but cause myocarditis only rarely, indicating a yet-unknown genetic predisposition in some individuals.”