Could you experience a serious cardiac event—and not know it? Yes. Up to one-half of myocardial infarctions (the medical term for heart attacks) are “silent,” meaning they either cause no symptoms or cause symptoms mild or atypical enough that the affected person doesn’t realize they could be dangerous. (The latter is sometimes called an “unrecognized heart attack” since it isn’t really silent, but for simplicity’s sake we’ll refer to both types as silent heart attacks in this article.) In either case, the person doesn’t seek medical attention.
Rather, the attack typically remains undiscovered until some future time when an electrocardiogram (ECG) or cardiac imaging test—such as a myocardial perfusion scan or echocardiogram—is done, showing or suggesting previous damage. This might happen, for example, at a routine doctor’s visit or when seeking care for cardiac symptoms such as chest pain or shortness of breath. Or the prior attack might be revealed if coronary artery disease worsens and leads to a new heart attack or to heart failure (in which the heart can’t pump enough blood with adequate force to meet the body’s needs). Sometimes, a silent heart attack remains a secret until an autopsy uncovers an area of scarring in the myocardium, the muscular tissue of the heart.
The silent type
Like any heart attack, a silent heart attack occurs when the flow of blood delivering oxygen and nutrients to the heart is interrupted, resulting in permanent damage to the heart muscle. Most commonly this is caused by a rupture of plaque (fatty deposits that accumulate within the lining of arteries) in a coronary artery, which leads to a blood clot that obstructs the artery. Less often, a spasm of a coronary artery can reduce blood flow to a part of the heart muscle. The extent and implications of the damage depend partly on which area of the heart the affected arteries go to.
Anyone can suffer a silent heart attack, especially if certain risk factors are present, such as smoking, high blood pressure or cholesterol, obesity, diabetes, a sedentary lifestyle, or a family history of heart disease. Evidence suggests that silent heart attacks are especially prevalent (compared to recognized heart attacks) in people who are older than 60 or who have diabetes or chronic kidney disease. Some, but not all, research suggests they’re more common in women.
Why is it that some people experience no symptoms when their heart muscle is deprived of oxygen, while others feel crushing chest pain or pressure or other classic heart attack symptoms such as pain that spreads to the jaw or arm? It’s not entirely clear and probably stems from a combination of factors. Among the possible explanations are a very high threshold for pain; higher-than-average levels of endorphins (substances the body releases to dampen pain) or anti-inflammatory chemicals (which may block pain transmission pathways); defects in the brain’s ability to perceive pain; and blockages of smaller coronary vessels as opposed to larger ones, resulting in a smaller area of tissue death. In people with diabetes, nerve dysfunction related to the disease (diabetic neuropathy) may play a role, by inhibiting the ability to feel pain.
Though a silent heart attack causes few or no symptoms, it can be as dangerous as a recognized heart attack or even more so, because the affected person doesn’t realize it happened and thus doesn’t get prompt (or any) evaluation or treatment. That equates to a missed or delayed opportunity to start the medications (such as daily low-dose aspirin) or make the lifestyle changes that are normally advised after a heart attack to help prevent a future event—as well as to receive immediate interventions like having a coronary stent inserted to reopen the artery.
How common are silent heart attacks?
Estimates vary widely, with population studies reporting rates from as low as 4 percent of all heart attacks to more than 60 percent, depending on the group studied and what method is used to detect heart damage. Cardiac imaging, for example, identifies more silent heart attacks but is less commonly used than an ECG, which is inexpensive, fast, and widely available. Even among studies using ECGs, there’s no agreed-upon standard for exactly which abnormalities should be considered indicative of a prior heart attack.
In an analysis in Circulation in 2016, researchers looked at data from about 9,500 middle-aged adults, all initially free of cardiovascular disease, who participated in the long-running Atherosclerosis Risk in Communities (ARIC) study. In the seven to 10 years after enrollment, nearly as many people (317) experienced a silent heart attack—as detected by an ECG at one of the scheduled study examinations—as a clinically documented one (386). Put another way, 45 percent of all the heart attacks that occurred during that period went undetected.
Another paper, published in the American Journal of Cardiology in 2014, found that silent heart attacks accounted for 35 percent of all heart attacks that had occurred among 6,534 adults ages 55 and older in Rotterdam, the Netherlands, as determined by ECGs taken at the start of the study in people with no clinical history of a heart attack. And a 2012 analysis in the Journal of the American Medical Association, which included 936 older adults in Iceland, found that silent heart attacks were even more common than recognized heart attacks when coronary MRI (rather than the less-sensitive ECG) was used to detect them—affecting 17 percent versus 10 percent of participants at baseline (meaning that more than 60 percent of heart attacks among the study participants were silent ones). When an ECG was used in the same participants, however, fewer silent heart attacks were detected.
Silent . . . but potentially deadly
Numerous studies have looked at the prognoses of people who have had silent heart attacks compared to recognized heart attacks or no heart attacks, examining mortality rates as well as the risk of future heart attacks and problems such as arrhythmias and heart failure, which can be triggered or exacerbated by a heart attack. Overwhelmingly, the studies show that, as with recognized heart attacks, silent heart attacks substantially increase the risk of future heart events and death from coronary causes, compared to not having a heart attack. Whether they increase those risks less than, more than, or comparably to a recognized heart attack varies by study, with many of the more recent analyses showing that risks are similar between the two—at least over the long term. Here’s a sampling of recent findings:
- In the 2014 Rotterdam study mentioned earlier, a prior silent heart attack was associated with a greater chance of dying from any cause or from cardiovascular disease among both men and women over nearly two decades, compared to no heart attack. In men, a silent heart attack raised mortality risk comparably to a recognized heart attack. In women, a silent heart attack wasn’t associated with quite as big an adverse effect as a clinically recognized heart attack, though both increased the risk of dying.
- Having ECG evidence of a silent heart attack was associated with triple the risk of death from coronary heart disease (CHD) and a 34 percent greater likelihood of dying from any cause over an average of nine years in the 2016 ARIC study, compared to no silent heart attack. Having a documented heart attack was associated with even higher risk of those outcomes—more than quadruple the risk of CHD death and a 55 percent higher risk of any-cause death. The authors noted a “potentially greater increased risk among women” than among men from both types of heart attacks.
- In an updated analysis of data from the Icelandic study, published in JAMA Cardiology in 2018, researchers tracked the participants’ outcomes over 13 years. During the first few years of the study, people who had a prior symptomatic heart attack were more likely to die of any cause than those who had a silent heart attack (whose death rates were comparable to people who didn’t suffer any heart attack). But by the 10-year mark, silent heart attack sufferers were as likely to have died as known heart attack sufferers—and mortality in both groups was significantly higher than in people who never had a heart attack. People with silent heart attacks also had an elevated risk (similar to that of known heart attack survivors) of having a subsequent heart attack or developing heart failure
So if it’s discovered—likely by accident—that you have experienced a silent heart attack, what’s next? You can expect your doctor to advise the same treatments given to people after a symptomatic, documented heart attack, to help prevent a future one. This entails a combination of lifestyle changes (such as losing excess weight, exercising, eating a healthy diet, and not smoking) and prescription medication (which may include drugs to lower blood pressure, cholesterol, or blood sugar). You will likely also be advised to start daily low-dose aspirin or another medication to help prevent blood clots. And you’ll probably be referred to a cardiac rehab program (a medically supervised program of exercise and education).
Silent heart attacks seem to be as dangerous as “regular” ones, and the fact that they’re so common should provide extra impetus to take steps now that are known to protect the heart: Keep your blood pressure, cholesterol, and blood sugar levels in the optimal range (using medication if warranted); maintain a healthy body weight; don’t smoke; eat a high-fiber diet that’s rich in fruits, vegetables, whole grains, and legumes, with moderate amounts of heart-healthy unsaturated fats (as found in fish and nuts, for example); limit your intake of saturated fat; get regular exercise; and follow good sleep practices.
Consult your doctor if you experience unexplained sluggishness, fatigue, or dizziness; heartburn that gets worse during exercise; or unusual nausea or vomiting. These are all less typical symptoms that might indicate that a heart attack has occurred or is impending.
This article first appeared in the UC Berkeley Wellness Letter.
Also see 13 Tips for Preventing Heart Disease.
Published August 11, 2020