HCM, or hypertrophic cardiomyopathy, is a condition where the walls of the heart become abnormally thick without an obvious external cause like high blood pressure. It’s the most common inherited heart muscle disease, affecting roughly 1 in 500 people. The thickened muscle can make it harder for the heart to pump blood efficiently and, in some cases, increases the risk of dangerous heart rhythms.
What Happens Inside the Heart
Your heart muscle is made up of tiny contractile units called sarcomeres, which slide past each other to make the heart squeeze and relax. In HCM, genetic mutations alter proteins in these units, essentially putting them into overdrive. The muscle fibers generate more force than normal and cycle faster, creating a “hypercontractile” state. Over time, this excessive mechanical stress causes the heart wall to grow thicker.
Two proteins account for the bulk of the problem. Mutations in one (called MYBPC3) mean the heart produces too little of a protein that normally keeps contraction in check. Mutations in the other (MYH7) directly increase the force each muscle fiber generates. Either way, the result is the same: a left ventricle that works harder than it should, thickens in response, and gradually stiffens.
Obstructive vs. Non-Obstructive HCM
About two-thirds of people with HCM have the obstructive form, where the thickened muscle physically blocks blood flowing out of the left ventricle. This obstruction forces the heart to work even harder to push blood into the body. People with obstructive HCM are at higher risk for heart failure and irregular heart rhythms like atrial fibrillation.
The remaining third have non-obstructive HCM. Most of these individuals have mild symptoms or none at all, and they generally face a lower risk of serious complications. The distinction is made by measuring the pressure difference across the outflow tract of the heart. A gradient of 30 mmHg or higher signals obstruction.
Genetics and Inheritance
HCM follows an autosomal dominant inheritance pattern, meaning you only need one copy of the mutated gene (from one parent) to develop the condition. If a parent has HCM, each child has a 50% chance of inheriting the mutation. Mutations in MYBPC3 and MYH7 together account for about 70% of genetically identified cases, with MYBPC3 alone responsible for roughly 61% and MYH7 for about 23%.
Not everyone who carries an HCM-causing mutation develops visible heart thickening. Some people are gene-positive but never show the physical signs on imaging. Genetic testing can identify these carriers, which is especially useful for screening family members of someone already diagnosed.
Common Symptoms
Many people with HCM have no symptoms at all, particularly those with the non-obstructive form. When symptoms do appear, the most common is shortness of breath during physical activity. Some people also notice breathlessness after meals or when bending forward, which is more characteristic of HCM than the classic lying-flat breathlessness seen in other forms of heart disease.
Chest pain, lightheadedness, and fainting affect a smaller but significant group. About one in five HCM patients experiences at least one episode of brief loss of consciousness. Fainting can result from the outflow obstruction itself, abnormal blood pressure responses during exertion, or irregular heart rhythms. Any unexplained fainting episode in someone with HCM is taken seriously because it may signal elevated risk.
How HCM Is Diagnosed
The primary diagnostic tool is an echocardiogram (heart ultrasound). HCM is diagnosed when the left ventricular wall measures 15 mm or thicker in someone without another explanation for the thickening, such as uncontrolled high blood pressure or intense athletic training. For relatives of known HCM patients, the threshold is slightly lower at 13 mm.
Cardiac MRI plays a complementary role. It provides more detailed images and can detect scarring (fibrosis) within the heart muscle that ultrasound may miss. The amount of scarring matters because extensive fibrosis, specifically 15% or more of the left ventricle’s mass, is one of the markers used to assess sudden cardiac death risk. Together, echocardiography and cardiac MRI give a complete picture of both the structure and the tissue health of the heart.
Sudden Cardiac Death Risk
The most feared complication of HCM is sudden cardiac death from a dangerous heart rhythm. While relatively uncommon, it can strike without warning, sometimes during exertion. Doctors assess risk by looking at a specific set of markers:
- Extreme wall thickness: 30 mm or greater in any segment of the heart
- Family history: a first-degree relative who died suddenly from HCM before age 50
- Unexplained fainting: particularly episodes within the past six months
- Reduced pumping function: an ejection fraction below 50%
- Abnormal heart rhythms: runs of rapid heartbeats detected on a portable heart monitor
- Left ventricular aneurysm: a thin, scarred bulge at the tip of the heart
- Extensive scarring on MRI: fibrosis covering 15% or more of the left ventricle
For people who have already survived a cardiac arrest or dangerous arrhythmia, an implantable cardioverter-defibrillator (ICD) is recommended. This small device monitors heart rhythm continuously and delivers a shock if it detects a life-threatening arrhythmia. For those who haven’t had an event but carry one or more of the risk markers above, an ICD may still be considered as a preventive measure. People with HCM who have none of these risk factors are generally not candidates for a defibrillator.
Treatment for Obstructive HCM
For people whose symptoms are caused by obstruction, the first line of treatment is medication that reduces how forcefully the heart contracts and slows the heart rate, giving the ventricle more time to fill. A newer class of drugs works differently: cardiac myosin inhibitors directly reduce the number of muscle fibers actively generating force during each heartbeat. One of these, mavacamten, received FDA approval for symptomatic obstructive HCM. A second, aficamten, works through a similar but distinct mechanism, binding to a different site on the same motor protein to prevent the muscle from entering its force-generating state.
When medications don’t provide enough relief, two procedures can physically reduce the obstruction. Septal myectomy is open-heart surgery in which a surgeon shaves away a portion of the thickened muscle. It has been the gold standard for over 50 years and produces strong, lasting results. Long-term follow-up shows it reduces the outflow pressure gradient by an average of about 58 mmHg, and over 94% of patients move from severe to mild symptom categories.
The alternative is alcohol septal ablation, a catheter-based procedure that uses a small injection of alcohol to shrink the obstructing tissue. Recovery is faster, typically requiring only a couple of nights in the hospital compared to the longer stay after open-heart surgery. However, it is somewhat less effective long-term: the gradient drops by about 48 mmHg on average, and re-intervention rates are higher (roughly 10% vs. less than 1% for myectomy). It also carries a higher chance of needing a permanent pacemaker afterward, around 12% compared to 4% with surgery. Ablation tends to be favored for older patients or those at high surgical risk, while myectomy is preferred for younger, otherwise healthy individuals.
Exercise and Physical Activity
The approach to exercise in HCM has shifted in recent years. Older guidelines broadly restricted competitive sports for anyone with the diagnosis. Current recommendations take a more individualized approach. After a comprehensive evaluation by a specialist, competitive athletes with HCM can be considered for continued sports participation through a shared decision-making process that weighs the benefits of activity against the potential risks, including sudden cardiac death.
People who carry an HCM gene mutation but have no structural heart changes (no thickening visible on imaging) can participate in competitive sports without restriction. For those who choose to step away from competition, regular recreational physical activity is still encouraged for overall cardiovascular health and longevity.