Get access to everything we publish when you sign up for Outside+.
My wife and I have only one rule when it comes to running through pain. If either of us ever feels any unusual sensation in our heart, we promise to stop and walk home.
Initially, that philosophy was motivated not by science, but by fear. We had read the news stories about cardiac arrest in otherwise healthy athletes, and the tragedies stuck with us. You may have read about Micah True—also known as “Caballo Blanco,” of Born to Run fame—who died of cardiac arrest on a trail run in 2012, and every so often a similar story crops up in the local or national press.
In reality, sudden cardiac arrest in runners is much rarer than the hype machine suggests, but it is a serious risk that is important to understand.
Most cases arise from a heart condition known as hypertrophic cardiomyopathy, or HCM. “Many runners think they’re in the clear from chronic diseases solely because they run,” says Emily Kraus, a sports-medicine doctor at Stanford University. “Although physical activity is an excellent cardio-protective practice, it can’t always shield from those underlying, non-modifiable risk factors.”
Here are five questions to help you understand your risk of sudden cardiac arrest from HCM.
What causes cardiac arrest in healthy athletes?
HCM is a genetic condition that causes progressive thickening of the heart tissue. According to the American College of Sports Medicine (ACSM), when the muscle gets large enough, during exercise it “can start to quiver in ventricular fibrillation, which is the cause of sudden death.”
“Imagine the heart as an inflatable bouncy castle,” says Robyn Reed, a pathologist at Children’s Hospitals and Clinics of Minnesota. “The kids ricocheting around inside are the blood. If you get the castle too pumped up, the doorway gets squeezed shut and the kids have trouble squeezing their way out. It can even get so pumped up that it cuts off its own air supply (or blood supply), leading to tissue damage.”
Reed adds that HCM can interfere with heart-muscle contraction and even result in the muscle cells no longer “correctly conducting the electrical signals that tell your heart how to beat. They’ll still try to contract, but they’ll do it in a disorganized way.”
How common is HCM?
According to the ACSM, about one in 500 people have HCM. The condition is more common in men and in families with a history of the diagnosis.
How often does HCM lead to sudden death?
A 2012 study in the New England Journal of Medicine found that one out of every 259,000 marathon and half-marathon participants died of sudden cardiac arrest. For cases where data was available, 23 of 31 were due to HCM.
To put that in perspective, your odds of being struck by lightning this year are 1 in 960,000. So even though both events seem unlikely, just as you wouldn’t venture outside with a metal umbrella during a thunderstorm, you shouldn’t expose yourself to unnecessary risk from HCM.
While no statistics are available for lifetime mortality risk, most people with HCM never know. It’s a time bomb with a fuse that may never be lit. But when it is, it often ends quickly and tragically.
What can I do to understand my risk?
You wouldn’t drive 50,000 miles without taking your car in for a checkup. The same goes for your body—the ACSM recommends regular physical exams to screen for things like HCM. People with HCM “may develop a heart murmur or arrhythmia,” according to the organization.
Not all people with HCM get a “check-engine-light” warning, but some do. Symptoms include “chest discomfort with exertion, unreasonable breathlessness (this is not the same as the breathlessness experienced from hill or sprint repeats—that’s normal), dizziness, fainting or blacking out,” says Kraus. If one of those lights pops on, see a doctor before continuing activity.
Kraus identifies additional heart-related risk factors as “a family history of a blood relative who had a heart attack before the age of 55 years (father or brother) or age 65 years (mother or sister), or an unexplained sudden death before age 50 (including drowning, unexplained car accident or sudden-infant-death syndrome).”
A sure diagnosis requires an electrocardiogram or echocardiogram. Some universities, like Rice, now require these tests before an athlete competes in intercollegiate athletics. Consider having your heart tested before embarking on a grueling training regimen.
What should I do if I have HCM?
If you have HCM, it’s not the end of the world. The risk of death for people with HCM is 2 to 4 percent per year.
There are a few different approaches to living with HCM. In general, says Kraus, “If a runner has been diagnosed with HCM, they should not run due to the risk of sudden cardiac death.” She adds that “depending on the severity of the diagnosis,” a doctor may prescribe low-to-moderate-intensity activity or install an implantable defibrillator. Treatment plans are highly personal and should be determined by a cardiologist.
David Roche is a two-time USATF trail national champion, the 2014 U.S. Sub-Ultra Trail Runner of the Year and a member of team Hoka One One and Team Clif Bar. He works with runners of all abilities through his coaching service, Some Work, All Play. Follow David’s daily training on Strava here, and follow him on Twitter here.