Hypertrophic cardiomyopathy (HCM) is a genetic heart condition where the muscular walls of the heart become abnormally thick, making it harder for the heart to pump blood efficiently. It affects roughly 1 in 327 people, making it one of the most common inherited heart diseases. Many people live with it for years without knowing, while others experience symptoms that range from mild breathlessness to life-threatening heart rhythm problems.
What Happens Inside the Heart
In a healthy heart, the muscle contracts and relaxes in a coordinated rhythm, pumping blood out through a pathway called the left ventricular outflow tract. In HCM, the heart muscle, particularly the wall between the two lower chambers (the septum), grows too thick. This thickening stiffens the heart, which means it can’t relax and fill with blood as easily between beats. The diagnosis is typically made when the wall measures 15 millimeters or more on an imaging study, a threshold well beyond normal.
The root cause lies in the proteins that make heart muscle cells contract. HCM is caused by mutations in genes that encode these contractile proteins. Two genes account for about 75% of cases where a specific mutation is identified. The mutations cause the muscle fibers to form too many connections during each heartbeat, forcing the heart into a state of excessive contraction. Over time, this leads to thickening and disorganization of the muscle tissue. Abnormal calcium signaling within heart cells also plays a role, particularly in the stiffness that makes relaxation difficult.
Obstructive vs. Non-Obstructive HCM
Not all cases of HCM behave the same way. About two-thirds of people with the condition have the obstructive form, where the thickened muscle physically blocks or narrows the outflow tract that blood must pass through as it leaves the heart. This obstruction forces the heart to work harder with every beat and is the main driver of symptoms like fainting and severe breathlessness.
The remaining third have non-obstructive HCM. The muscle is still abnormally thick, but blood flows out of the heart without a significant blockage. These individuals may have fewer symptoms, though the stiffened heart muscle can still cause problems with filling and relaxation. The distinction between the two forms matters because it directly shapes treatment decisions.
Symptoms and How It’s Discovered
Many people with HCM don’t realize they have it because they experience few or no symptoms. The condition is sometimes found incidentally during a routine physical exam, when a doctor hears an unusual heart murmur, or during cardiac screening for another reason entirely.
When symptoms do appear, they tend to show up during physical exertion. The most common include:
- Shortness of breath, especially with exercise
- Chest pain, particularly during activity
- Fainting or near-fainting, often during or just after exertion
- Palpitations, a sensation of rapid, fluttering, or pounding heartbeats
These symptoms occur because the thickened, stiff heart can’t keep up with the body’s demand for blood during activity. In the obstructive form, the physical blockage worsens during exercise as the heart contracts more forcefully, which can cause a sudden drop in blood flow to the brain and lead to fainting.
The Risk of Sudden Cardiac Death
HCM is one of the most recognized causes of sudden cardiac death in young people, including athletes. The thickened, disorganized muscle tissue can trigger dangerous heart rhythm disturbances. While this risk gets significant attention, the actual likelihood varies enormously from person to person.
Doctors use a formal risk calculator developed by the American Heart Association to estimate an individual’s chance of a life-threatening event. The factors that go into this assessment include age, the maximum thickness of the heart wall, the size of the left atrium (the upper chamber that feeds into the thickened ventricle), the degree of outflow tract obstruction, and several specific warning signs: a family history of sudden cardiac death from HCM, episodes of abnormal heart rhythms detected on monitoring, unexplained fainting, reduced pumping function, and certain structural abnormalities visible on cardiac MRI. For people at elevated risk, an implantable defibrillator can be a lifesaving intervention, automatically correcting dangerous rhythms if they occur.
How It Runs in Families
HCM follows an autosomal dominant inheritance pattern, meaning a parent with the condition has a 50% chance of passing the gene mutation to each child. This makes family screening essential. When someone is diagnosed, their first-degree relatives (parents, siblings, and children) should undergo evaluation that includes an electrocardiogram and an echocardiogram.
If genetic testing identifies the specific mutation in the diagnosed person, their relatives can then be tested for that same mutation. Relatives who don’t carry it can be reassured and spared years of repeat screening. Those who do carry it need ongoing monitoring, typically every one to five years, because the heart thickening can develop at any age, from childhood through middle age. This approach, called cascade genetic testing, is one of the most efficient ways to catch HCM early in family members who don’t yet have symptoms.
Medications That Target the Problem
For decades, treatment relied on older medications like beta-blockers and calcium channel blockers, which slow the heart rate and reduce the force of contraction to ease symptoms. These drugs help many people but don’t address the underlying muscle dysfunction.
A newer class of medication works differently. These drugs (the first approved was mavacamten) directly target the overactive contractile proteins in heart muscle cells. They reduce the excessive cross-bridge connections that form between muscle fibers during contraction, essentially dialing down the heart’s tendency to squeeze too hard. This relaxes the thickened muscle, reduces obstruction, and improves the heart’s ability to fill between beats. For many patients with obstructive HCM, this has meant meaningful symptom relief without surgery. The medication requires careful monitoring because reducing contraction too much could weaken the heart’s pumping ability.
Surgical Options for Severe Obstruction
When medications don’t adequately control symptoms in obstructive HCM, two procedures can physically reduce the thickness of the septum to open up the blocked outflow tract.
Septal Myectomy
This is open-heart surgery where a surgeon trims away a portion of the thickened septum. It provides the most immediate and complete relief of obstruction and is considered the gold standard at experienced centers. Ten-year survival after myectomy is strong, with one large study reporting a 10-year mortality rate of 8.2%. The early procedural death rate is very low, around 0.3% at high-volume centers.
Alcohol Septal Ablation
This is a less invasive catheter-based procedure. A small amount of alcohol is injected into the artery supplying the thickened area, deliberately causing a controlled, localized injury that thins the muscle over time. Recovery is faster than with surgery, but the trade-offs are real: the obstruction may not be relieved as completely, and the procedure carries a higher rate of needing a permanent pacemaker afterward. A large comparative study found that 10-year mortality was 26.1% in the ablation group versus 8.2% in the myectomy group, a difference that persisted even after adjusting for age and other health conditions. Ablation is generally reserved for patients who aren’t good candidates for open-heart surgery or who prefer a less invasive approach.
Exercise and Physical Activity
The old advice for people with HCM was simple: avoid all vigorous exercise. Current guidelines take a more nuanced approach. The 2024 guidelines recognize that participation in vigorous recreational activities is reasonable after a thorough annual evaluation and a careful conversation with a cardiologist experienced in HCM. For high-level athletes with the condition, even competitive sports may be considered under the guidance of an expert provider who specializes in managing athletes with HCM.
This doesn’t mean there are no restrictions. The evaluation looks at the degree of obstruction, history of dangerous rhythms, and overall heart function. The goal is to find a level of activity that supports cardiovascular health and quality of life without creating unacceptable risk. Each person’s exercise plan looks different depending on their specific features.
Living With HCM Long-Term
HCM is a lifelong condition, but most people with it live full, normal-length lives. The dramatic stories of young athletes collapsing represent the most extreme end of a very broad spectrum. Many people are diagnosed incidentally and never develop significant symptoms. Others manage well with medication alone. The key to good outcomes is accurate risk assessment, consistent follow-up, and family screening to catch the condition early in relatives who may not yet know they carry it.