Nearly half of all heart attacks may be silent, occurring without any symptoms like chest pain, shortness of breath and cold sweats, a U.S. study suggests.

Silent heart attacks, discovered during exams by cardiologists, were more common in men, researchers found. (Symptomatic heart attacks are more common in men, too.)

But in the ensuing years, women with silent heart attacks were 58 percent more likely to die compared to women with no heart attacks, whereas the mortality rate for men with a silent heart attack was only 23 percent higher than for men without a heart attack.

Overall, silent heart attacks were associated with a roughly tripled risk of dying from heart disease and a 34 percent increased risk of dying from any cause.

With silent heart attacks, people typically don't seek medical advice because they don't realize anything is wrong, said lead study author Dr. Zhu-Ming Zhang of Wake Forest Baptist Medical Center in Winston-Salem, North Carolina.

"There could be many reasons for people not realizing they are having a heart attack," Zhang added by email.

To assess the prevalence of silent heart attacks, Zhang and colleagues studied nearly 9,500 middle-aged adults.

Half the participants were followed for more than 13 years. During that time, 317 people had silent heart attacks and another 386 had heart attacks with classic symptoms that are easier to spot.

Out of 1,833 deaths from all causes during the study period, 189 were tied to heart issues, researchers report in the journal Circulation.

The relatively young age of participants may have influenced the gender disparity in deaths from silent heart attacks, said Dr. Laxmi Mehta, director of the women's cardiovascular health program at the Ohio State University Wexner Medical Center.

"We know rates of heart attacks in young women are lower than in young men," Mehta, who wasn't involved in the study, said by email.

"The death rates may be higher in women due to underuse of guideline-derived medical therapies and lower referral rates to cardiac rehab in women," Mehta added.

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There were no significant differences between blacks and whites, however.

One limitation of the study is that researchers lacked data on other races and ethnicities, the authors note. It's also possible that they underestimated the number of silent heart attacks because these episodes can be difficult to detect after the fact.

Other studies have also found women are more likely to have silent heart attacks than men, noted Dr. Leslie Cho, head of preventive cardiology at the Cleveland Clinic in Ohio.

"No one really knows why people have silent (heart attacks), however, when patients are further probed, they have atypical symptoms which they thought were not related to the heart but turned out to be a (heart attack)," Cho, who wasn't involved in the study, said by email.

Because silent heart attacks are by definition very hard to detect, it's crucial that people take any symptoms of discomfort seriously, whether it's slight pain in the chest or jaw or difficulty breathing or heartburn, said Dr. Sheila Sahni, chief fellow in cardiology at the Ronald Reagan University of California, Los Angeles, Medical Center.

Too often, patients downplay symptoms and fail to seek help.

"When it comes to matters of the heart, time equals muscle," Sahni, who wasn't involved in the study, said by email.

"When you feel something out of the ordinary, get it checked out," Sahni added. "It's better to be wrong than to find out a few days later you suffered a silent heart attack."