If you’re an athlete, there’s one muscle that can become too big: your heart.

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Sure, it sounds counterintuitive. The purpose of training, after all, is to grow and strengthen your muscles. Doesn’t a strong heart fuel athletic performance? In most cases, yes. In general, it’s completely normal and healthy for your heart to adapt to intense exercise by becoming stronger, thicker, and larger — but in some cases, a big heart can be dangerous.

Called hypertrophic cardiomyopathy (HCM), excessive thickening of the heart muscle affects an estimated 600,000 Americans. HCM is a genetic disorder, meaning it’s usually inherited from one of your parents.

What happens when you have HCM

In HCM, as the heart becomes bigger, it also becomes stiffer and less effective at doing its job: pumping blood. Excessive thickening of the heart muscle most often strikes the septum, the muscular wall that separates the left and right sides of the heart. A too-thick septum narrows the heart’s chambers, obstructing blood flow out of the heart and through the body. To compensate, the heart has to work harder to get blood where it needs to go — to your brain, organs, and muscles.

HCM also can cause the heart’s mitral valve to leak, limiting blood flow exiting the heart and causing blood to push backward and into the lungs. This can result in shortness of breath during exercise and reduce athletic performance. One study found that athletes with HCM could only achieve about half the workload of elite athletes with similar heart wall thickness. However, an even bigger concern for athletes and their families is the risk of sudden cardiac death.

Hypertrophic cardiomyopathy is the leading cause of sudden cardiac death in young athletes. That’s because in HCM, as the heart muscle thickens, its cells become disorganized and irregular, causing changes in the heart’s electrical signals, which can result in abnormal heart rhythms (arrhythmias). Occasionally these abnormal heart rhythms can be very serious and may cause sudden cardiac arrest and even death. These changes differ from normal adaptive thickening, which occurs in athletes without HCM whose cells remain well organized with intact electrical circuits.

Is your heart too big?

While HCM often develops during puberty and is usually undetectable in childhood, its symptoms can present at any age — or not at all. The good news is that most people with HCM lead normal lives without symptoms. Those who do develop symptoms typically experience shortness of breath, chest pain, fatigue, lightheadedness, and occasionally syncope (fainting or passing out).

Identifying young athletes with HCM is one of the main reasons for pre-participation sports screenings. If your screening raises any concerns that you might have HCM (such as a family history of HCM or sudden cardiac death, typical symptoms or findings on physical examination), you will be referred to a cardiologist for further evaluation. Ideally, you should see a sports cardiologist, as they are well trained at recognizing the difference between HCM and normal changes to the heart due to intense exercise.

To detect any abnormalities associated with HCM, your cardiologist will likely listen to your heart and lungs and perform an electrocardiogram, echocardiogram, and possibly additional tests such as a cardiac MRI.

How to treat HCM

If you are diagnosed with HCM, it’s important to talk to your doctor about how it will affect your sports participation. In most cases, the diagnosis of HCM will prevent you from playing competitive, extreme, or intense sports, as well as heavy weightlifting.

The risk of sudden cardiac death in patients with HCM is highest during or just shortly after exercise. Unfortunately, experts have not yet determined what levels of physical exertion are safe in people with HCM, and this likely differs from person to person; your sports cardiologist will be able to provide some guidelines.

By limiting the stress you place on your heart through exercise, you can live with HCM safely and symptom-free. In rare and severe cases, medications, implantable devices, and procedures to remove obstructions may be necessary to help your heart work at its best.

This blog item is based on a series of articles produced by STACK in association with Cleveland Clinic, including Dermot Phelan, MD, PhD.