Like heart attacks that cause chest pain and other symptoms, these silent attacks can increase the risk of death from heart disease and other causes.
During a silent heart attack, blood flow to the heart muscle is severely reduced or cut off completely. However there are none of the classic symptoms, such as pain in your chest, shortness of breath and cold sweats.
"The outcome of a silent heart attack is as bad as a heart attack that is recognised while it is happening," said Elsayed Soliman, lead author of the study and director of epidemiological cardiology research at Wake Forest Baptist Medical Center, North Carolina.
"And because patients don't know they have had a silent heart attack, they may not receive the treatment they need to prevent another one."
Researchers analysed the records of 9,498 middle-age adults enrolled in the Atherosclerosis Risk in Communities (ARIC), a study analysing the causes and outcomes of atherosclerosis - hardening of the arteries.
They examined heart attack differences between ethnicities and genders.
Over an average of nine years after the start of the study, 317 participants had silent heart attacks while 386 had heart attacks with clinical symptoms.
Researchers continued to follow participants for more than two decades to track deaths from heart attack and other diseases.
They found that silent heart attacks:
- Made up 45% of all heart attacks
- Increased the chances of dying from heart disease by three times
- Increased the chances of dying from all causes by 34%
- And were more common in men but more likely to cause death in women.
"Women with a silent heart attack appear to fare worse than men," Soliman said.
Researchers accounted for many factors that could influence results including whether or not they smoked, body weight, diabetes, high blood pressure and cholesterol.
They did not adjust for access to care but did adjust for income and education, which could impact access to care.
Symptoms of silent heart attacks appear so mild that they are barely noticed, if at all.
They are detected later, usually when patients undergo an electrocardiogram (ECG) to check their heart's electrical activity, researchers said.
Soliman said that silent heart attacks, once discovered, should be treated the same as heart attacks with symptoms.
"The modifiable risk factors are the same for both kinds of heart attacks," he said.
"Doctors need to help patients who have had a silent heart attack quit smoking, reduce their weight, control cholesterol and blood pressure and get more exercise."
The study was published in the journal Circulation.