By Amy Norton
"Almost everyone knows what a heart attack is. When we hear about it, we think of chest pain and [emergency] medical care," said Dr. Andrea Ohrn, the lead researcher on the new study.
"But what's less known is, many people experience heart attacks without knowing it -- without ever receiving a diagnosis," said Ohrn, a fellow at the University of Tromso in Norway.
No one knows why that is. But the new findings suggest that pain tolerance might be a factor.
Using a standard test of pain sensitivity, Ohrn's team found that people who had a silent heart attack in the past generally had a higher pain tolerance than people who'd sought treatment for heart attack symptoms.
When the researchers dug deeper, the connection appeared to be stronger in women than men.
That's an interesting finding, but it's unclear what to make of it at this point, said Dr. Nieca Goldberg, medical director of the Women's Heart Program at NYU Langone Medical Center in New York City.
The main takeaway, Goldberg said, is that people should know the "atypical" symptoms of heart attack.
"We need to be more vigilant in educating people that chest pain is not the only symptom of heart attack," said Goldberg, who wasn't involved in the study.
The findings, published Dec. 21 in the Journal of the American Heart Association, came from a long-term health study of people living in Tromso, Norway.
One study visit involved a standard pain-sensitivity test where people plunge a hand into cold water for as long as they can stand it, for up to two minutes.
At the next study visit, participants underwent electrocardiograms -- which can detect signs of a past heart attack, even if it was unrecognized at the time.
Of over 4,800 adults who had both tests, it turned out that 8 percent had previously had a silent heart attack. Just under 5 percent had ever had a diagnosed heart attack.
When the researchers compared the two groups, they found that people who had a silent attack had, on average, greater pain tolerance.
It's possible that people who tolerate cold-induced pain are also less sensitive to pain caused by reduced blood flow to the heart, Ohrn said.
But, Goldberg said, it's not clear how often "silent" heart attacks truly caused no pain or other symptoms. Some people may simply have not realized the seriousness of their symptoms, and chose to bear them instead of seeking help.
There were differences between men and women in the study. Overall, more men had suffered a heart attack -- 19 percent versus 7 percent of women.
But silent attacks accounted for three-quarters of all heart attacks among women, compared with 58 percent among men.
According to Ohrn's team, that's in line with past studies: Women are more likely than men to have atypical heart attack symptoms, and their heart attacks are more likely to go undiagnosed.
It's not clear how pain sensitivity fits into that picture.
In this study, women generally had less pain tolerance than men. But the connection between greater pain tolerance and silent heart attack was stronger among women than in men.
It's possible, Ohrn said, that women with diagnosed heart attacks have particularly severe symptoms. "It may be that it 'takes more' symptoms to have your heart attack recognized if you are a woman," she speculated.
The bottom line, according to Ohrn, is that it's important for doctors to be on the lookout for less-obvious heart attack symptoms, especially in women.
"Absence of chest pain should not lower doctors' alertness toward heart disease," she said.
Sometimes the red flags pop up in the longer term, Ohrn noted. Breathing problems and swelling in the legs, for instance, can be signs of heart muscle damage from a previous, undiagnosed heart attack, she said.
Silent attacks, Ohrn said, are just as serious as ones that cause obvious chest pain -- carrying similar risks of death or repeat heart attack over the long term.
And that, Goldberg said, underscores the importance of prevention. A healthy diet, regular exercise and controlling risk factors like high blood pressure and high cholesterol are all critical, she said.