Hypertrophic cardiomyopathy (HCM) is a condition where the heart muscle becomes abnormally thick, making it harder for the heart to pump blood efficiently. It affects roughly 1 in 500 people, making it the most common inherited heart disease. Many people live with it for years without knowing, while others experience symptoms that range from mild breathlessness to, in rare cases, sudden cardiac arrest.
What Happens Inside the Heart
In a healthy heart, the walls of the left ventricle (the main pumping chamber) are typically about 10 to 12 millimeters thick. In HCM, one or more segments of that wall thicken to 15 millimeters or more. That extra muscle tissue stiffens the chamber, reducing its ability to relax and fill with blood between beats. The thickened wall can also narrow the outflow tract, the path blood takes as it leaves the heart and heads to the rest of the body.
About two-thirds of people with HCM have some degree of outflow tract obstruction, where the thickened muscle and the mitral valve partially block blood flow during each heartbeat. This is called obstructive HCM. The remaining third have non-obstructive HCM, where the muscle is thick but doesn’t significantly block the outflow path. Both forms can cause symptoms, but obstructive HCM tends to produce more noticeable problems during physical effort.
Why It Happens: Genetics and Inheritance
HCM is caused by mutations in genes that control the proteins inside heart muscle cells, specifically the sarcomere, the microscopic unit responsible for muscle contraction. Two genes account for the vast majority of cases. Mutations in MYBPC3 (which encodes a protein that helps organize muscle fibers) are found in about 50% of genetically confirmed cases. MYH7 (which codes for a key contractile protein) accounts for roughly 24%. Together, these two genes explain about three-quarters of all identified mutations.
The condition follows an autosomal dominant pattern, meaning you only need one copy of the mutated gene (from one parent) to develop it. Each child of an affected parent has a 50% chance of inheriting the mutation. That said, inheriting the gene doesn’t guarantee you’ll develop significant thickening or symptoms. Some people carry the mutation their entire lives with minimal changes to their heart, while others develop pronounced thickening in adolescence or early adulthood. In some cases, HCM doesn’t appear until later in life.
Symptoms to Recognize
Many people with HCM have no symptoms at all, especially early on. The condition is often discovered incidentally during a routine physical exam or a screening test prompted by a family member’s diagnosis. When symptoms do appear, they typically include:
- Shortness of breath during exercise or exertion, caused by the stiff ventricle struggling to fill properly
- Chest discomfort during activity, sometimes mimicking angina
- Lightheadedness or fainting (syncope), particularly during or just after physical effort
- Heart palpitations from abnormal rhythms triggered by the thickened tissue
- Fatigue that seems disproportionate to the level of activity
These symptoms tend to worsen with dehydration, heavy meals, alcohol, and anything that increases the heart’s workload or reduces blood volume. Some people notice that standing up quickly or straining (like during a bowel movement) triggers lightheadedness. In obstructive HCM, symptoms often intensify during exercise because the faster heart rate gives the ventricle even less time to fill, and the obstruction worsens.
How HCM Is Diagnosed
The cornerstone of diagnosis is an echocardiogram, an ultrasound of the heart. A wall thickness of 15 mm or greater in any segment of the left ventricle, when it can’t be explained by another cause like long-standing high blood pressure or intense athletic training, meets the diagnostic threshold. In Black patients, the threshold is sometimes set at 20 mm because of natural variation in wall thickness across populations.
An electrocardiogram (ECG) often shows abnormalities even before symptoms appear, including unusual patterns in the electrical signals that suggest thickened muscle. Cardiac MRI provides more detailed images and can reveal scarring within the muscle, which is useful for assessing risk. Genetic testing can confirm the specific mutation and is especially valuable for screening family members who don’t yet show physical signs of the disease.
Sudden Cardiac Death Risk
The most feared complication of HCM is sudden cardiac death, typically caused by a dangerous heart rhythm that arises from the abnormal muscle tissue. This is what sometimes makes headlines when a young athlete collapses during competition. While the overall annual risk is relatively low (around 1% per year for most patients), certain factors push the risk higher.
Five established clinical markers are used to assess this risk: a family history of sudden cardiac death, unexplained fainting episodes, episodes of rapid heart rhythm picked up on monitoring (nonsustained ventricular tachycardia), extreme wall thickness of 30 mm or more, and an abnormal blood pressure response during exercise testing. If you have one or more of these risk factors, your cardiologist may recommend an implantable cardioverter-defibrillator (ICD), a small device placed under the skin that can detect and correct dangerous rhythms within seconds.
Treatment Options
For people with mild or no symptoms, treatment may simply involve regular monitoring and avoiding specific triggers. Beta-blockers and calcium channel blockers are the traditional first-line medications. They slow the heart rate and reduce the force of contraction, giving the ventricle more time to fill and easing the obstruction.
A newer class of medication, cardiac myosin inhibitors, represents a significant shift in how obstructive HCM is treated. Mavacamten, approved by the FDA, works by reducing the excessive interaction between the two proteins (actin and myosin) responsible for muscle contraction. It essentially dials back the overactive contraction that causes the obstruction. Because it can lower the heart’s pumping strength too much in some cases, it requires regular echocardiogram monitoring and is only available through a special safety program. Patients on this medication have their heart function checked at specific intervals, starting four weeks after beginning treatment, to make sure the pumping strength stays in a safe range.
When medications don’t provide enough relief, two procedures can physically reduce the obstruction. Septal myectomy is open-heart surgery where a surgeon removes a portion of the thickened muscle. At experienced centers, five-year survival after myectomy is around 96.5%, and roughly 73% of patients report minimal or no symptoms afterward, compared to only about 6% who reach that level on medication alone. The alternative, alcohol septal ablation, is a catheter-based procedure that intentionally creates a small, controlled injury to shrink the obstructing tissue. Both procedures are most effective when performed at centers that specialize in HCM.
Exercise and Daily Life
For decades, people diagnosed with HCM were told to avoid all vigorous exercise and were automatically disqualified from competitive sports. That guidance has shifted substantially. The 2024 guidelines from the American Heart Association and American College of Cardiology recommend that all HCM patients without symptoms engage in regular mild to moderate aerobic exercise, and note that more vigorous exercise is reasonable for many. Short-term studies have not linked light, moderate, or even vigorous exercise to dangerous heart rhythms in HCM patients.
Disqualification from competitive sports is no longer automatic. Instead, it’s handled on a case-by-case basis after a thorough evaluation and a conversation about the potential risks of extreme training. Strength and resistance training are also now suggested, a reversal from earlier blanket restrictions. This doesn’t mean all exercise is safe for every patient, but the days of advising everyone with HCM to live a sedentary life are over.
Screening Family Members
Because HCM is inherited, first-degree relatives (parents, siblings, children) of anyone diagnosed should be screened. This typically involves an echocardiogram and an ECG, and may include genetic testing if a specific mutation has been identified in the family. For children and adolescents in affected families, screening is usually repeated every one to two years during the growth period, since the thickening often develops during puberty. Adults who test negative genetically generally don’t need ongoing screening, but those who carry the mutation without yet showing thickening should continue periodic monitoring.