Hypertrophic cardiomyopathy (HCM) is primarily a genetic condition, meaning the people most at risk are those who inherited a specific gene mutation from a parent. It affects an estimated 1 in 200 to 1 in 500 people, which translates to roughly 750,000 Americans. Yet only 10% to 20% of those cases are ever identified clinically, leaving the majority of people with HCM unaware they have it.
Family History Is the Strongest Risk Factor
HCM follows an autosomal dominant inheritance pattern. If one of your parents carries the mutation, you have a 50% chance of inheriting it. Two proteins in the heart muscle’s contractile machinery account for the vast majority of cases: mutations in one gene are responsible for about 61% of genetically confirmed cases, and mutations in a second gene account for another 23%. Together, these two genes explain roughly 70% of all mutation-positive HCM.
Inheriting the mutation doesn’t guarantee you’ll develop noticeable heart thickening or symptoms. The condition has what geneticists call “variable expressivity and incomplete penetrance,” meaning some carriers develop severe thickening of the heart wall while others show little or no change on imaging. Two siblings with the same mutation can have very different outcomes.
Because of this unpredictability, major cardiology guidelines recommend that first-degree relatives of anyone diagnosed with HCM begin screening around age 12. In Europe, guidelines suggest starting at age 10, or even earlier if the family has a history of severe disease in childhood. Screening typically includes an electrocardiogram, an echocardiogram, and genetic counseling to determine whether cascade genetic testing makes sense for the family.
Age and When Symptoms Appear
HCM can develop at any age, but the timing of onset shapes the overall risk picture. People who develop the condition before age 40 tend to have more symptoms and complications over their lifetime. The heart wall can thicken gradually, so some people have no symptoms early on and only develop shortness of breath, chest pain with exertion, dizziness, or fainting years later.
Children, teenagers, and young adults face a particular danger because HCM can cause sudden cardiac death in people who have no idea anything is wrong. This is one of the leading causes of sudden cardiac death in young competitive athletes. The condition may produce no warning signs until the heart is pushed hard during intense exercise.
Sex and Racial Differences in Diagnosis
Men are diagnosed with HCM more often than women. In one analysis of cardiac patients, about 61% of HCM cases were in men. Whether this reflects a true biological difference or a gap in how aggressively the condition is evaluated in women remains an open question.
Racial disparities in HCM diagnosis are striking. One study of 366 cardiac patients found HCM was nearly 14 times more common among white patients than Black patients. Researchers point out this likely reflects a combination of factors: differences in genetic architecture across populations, but also significant gaps in access to the specialized cardiac imaging needed to diagnose HCM in the first place. If you don’t get an echocardiogram or cardiac MRI, HCM simply won’t be found.
Athletes and the “Gray Zone”
Competitive athletes occupy a unique risk category, not because exercise causes HCM, but because intense training can make a normal heart look somewhat similar to an HCM heart on imaging, and because undetected HCM can be fatal during high-intensity exertion.
A healthy athlete’s heart adapts to training by enlarging slightly and thickening its walls, but that thickening stays under 15 millimeters. HCM is diagnosed when the heart wall reaches 15 millimeters or more without another explanation. Some athletes fall into a gray zone where their wall thickness sits right at the borderline, making it hard to tell whether the thickening is a normal training response or early disease.
The key differences lie in how the heart relaxes between beats. An athlete’s heart fills normally or even better than average during the relaxation phase. An HCM heart, by contrast, has stiff walls that don’t relax properly, and this diastolic dysfunction shows up in the majority of HCM patients. Athletes with borderline wall thickness are often evaluated with stress echocardiography, where up to 70% of HCM cases will show obstruction of blood flow leaving the heart during exertion.
Conditions That Mimic HCM
Not every thickened heart wall is classic HCM. Several other genetic conditions produce a heart that looks nearly identical on imaging but involves completely different underlying biology. These are sometimes called phenocopies, conditions that copy the appearance of HCM without being caused by the usual sarcomere gene mutations.
One example is a rare storage disorder first described in 1981, where cells in the heart become filled with glycogen, a stored form of sugar. Patients with this condition develop severe heart thickening that closely resembles HCM but may also show abnormal electrical pathways in the heart. Another storage disorder involving fat buildup in cells can also cause the heart wall to thicken over time. These conditions matter because their treatment and prognosis differ from standard HCM, and genetic testing can distinguish between them.
Markers That Increase Risk of Complications
Among people already diagnosed with HCM, certain features signal a higher chance of dangerous heart rhythm problems or sudden cardiac death. The most established marker is extreme wall thickness. When the thickest part of the heart wall reaches 30 millimeters or more, the risk of sudden death rises significantly, and this finding alone is considered a strong reason to discuss a preventive implantable defibrillator.
Cardiac MRI adds another layer of information. When imaging reveals scarring in the heart muscle (detected by a technique that highlights damaged tissue), the risk climbs further. In a study of nearly 1,300 HCM patients, extensive scarring covering 15% or more of the heart muscle was associated with double the risk of sudden death. For every additional 10% of the heart affected by scarring, the risk increased by 36%. This type of imaging has become one of the most powerful tools for separating higher-risk patients from those who can be monitored more conservatively.
Other clinical features that raise concern include a history of fainting without a clear cause, a family history of sudden cardiac death at a young age, and abnormal blood pressure responses during exercise testing. No single factor determines risk in isolation. Doctors weigh all of these together when deciding how aggressively to monitor someone with HCM.