When a family member has a sudden cardiac death, it can be devastating. For some people, it may also be the first sign that a heart condition called hypertrophic cardiomyopathy (HCM) runs in their family. Many people with HCM — a type of heart disease where the heart muscle thickens and grows bigger than normal — aren’t aware they have it. But 60 percent of those with the condition inherit it from a parent. This is called familial HCM.

HCM can put people at risk for complications, such as cardiac arrest and stroke. Although familial HCM can’t be prevented, genetic screening can help detect it early and prevent complications. Here, we explore familial HCM, including what causes it, possible symptoms, and how it’s inherited.

Understanding Familial HCM

When hypertrophic cardiomyopathy is caused by a gene mutation (change) that’s passed down through a family, it’s called familial HCM. It’s the most common type of genetic heart disease. Nonfamilial HCM refers to HCM that isn’t caused by a gene change or passed on from a parent to a child. The cause of nonfamilial HCM is unknown.

In familial HCM, changes within genes that are in charge of heart muscle growth are passed down from parent to child. These gene mutations cause the heart to thicken and get larger. Doctors refer to this enlargement as cardiac hypertrophy. However, this thickening does not make the heart stronger. Instead, the heart muscle cells are not lined up neatly but are arranged in a messy, uneven way. Because of this abnormal structure, the heart may be weaker and less efficient, and the disrupted muscle pattern can sometimes interfere with the heart’s electrical signals, increasing the risk of dangerous heart rhythm problems.

Depending on what part of the heart is affected and whether blood flow is reduced, the condition is defined as either obstructive or nonobstructive HCM. About two-thirds of people with HCM are reported to have obstructive HCM, which affects the heart’s septum or septal wall (vertical line that divides the heart in two).

Causes of Familial HCM

Familial HCM is passed down through family generations. Specifically, it’s inherited through changes within the genes that regulate how the heart muscle grows. Most of these mutations are inherited in what’s called an autosomal dominant pattern. In other words, it’s only necessary for one parent to have an HCM gene mutation for a couple to pass it on to their child. In rare cases, a child can inherit a copy of an HCM mutation from each parent, which can manifest as a more severe disease.

Familial HCM is inherited through changes within the genes that regulate how the heart muscle grows

Familial HCM can be more severe depending on which gene is involved. Many gene variations have been identified as causes for HCM. Certain gene changes cause HCM more often than others, including:

• MYH7 and MYBPC3 — Make proteins that provide structural support to heart muscles and regulate muscle contraction.
• TNNT2 and TNNI3 — Make troponins, proteins that control muscle contraction and relaxation.

Having a gene mutation linked to HCM doesn’t ensure you’ll get it, though. It’s possible to inherit a mutated gene but not develop the condition.

Symptoms of Familial HCM

Although the causes are different, the symptoms of familial HCM are the same as nonfamilial HCM. People experience symptoms of hypertrophic cardiomyopathy in different ways, even within the same family. Some people don’t have any symptoms at all.

Symptoms of HCM may occur more commonly during or immediately following physical activity. Common HCM symptoms include:

• Chest pain (especially with physical activity)
• Difficulty catching your breath
• Heart palpitations (a sensation that your heart is skipping beats or beating quickly)
• Tiredness or fatigue
• Lightheadedness or fainting
• Swelling of the neck veins or lower body

HCM can also lead to serious complications. Some people with HCM may experience arrhythmias — changes in the heart’s rhythm. Ventricular arrhythmias are the primary cause of sudden cardiac death in people with HCM. The risk of sudden death is increased for people with HCM, even those without any symptoms.

Understanding Your Risk

According to a 2019 study, about 70 percent of sudden cardiac deaths from HCM happen in people not previously diagnosed with HCM. Your family history may provide some insight into your risk of HCM. But, because many people with HCM don’t have any symptoms, it can be difficult to know if it runs in your family.

If you have a parent, sibling, or child with HCM, your risk of inheriting an HCM gene variant increases from 1 in 500 to 1 in 2. However, having a gene mutation doesn’t mean that you will definitely develop HCM. Although having a first-degree relative with HCM increases your inheritance risk to 50 percent, that doesn’t mean you have a 50 percent chance of developing HCM.

If you have a parent, sibling, or child with HCM, your risk of inheriting an HCM gene variant increases from 1 in 500 to 1 in 2.

Talk to Your Doctor if You Have a Family History of HCM

Providing a detailed family history can help your cardiologist diagnose HCM. Include any family members you’re biologically related to, such as:

• Parents
• Children
• Siblings
• Grandparents
• Aunts and uncles
• Grandchildren
• Nieces and nephews

While collecting your family history, ask about other health conditions your family members may have, including:

• Heart problems such as heart failure, heart attacks, arrhythmias, or coronary artery disease (CAD)
• A relative who needed a pacemaker or heart-related surgery
• High blood pressure
• Diabetes
• Stroke
• Exercise-induced asthma, which can sometimes be mistaken for symptoms of HCM

Genetic Screening and Counseling

Tell your healthcare provider if you’ve had family members that have had sudden cardiac deaths, especially at an early age. Your doctor may recommend that you and your family members undergo genetic screening for hypertrophic cardiomyopathy. Genetic screening for HCM involves using a blood sample to test for the mutated genes that can cause HCM.

Your healthcare provider may recommend undergoing genetic screening for HCM if you have a family history of the condition or if you had a family member that suffered from sudden cardiac death.

If your doctor suggests you undergo genetic screening, they will likely refer you for genetic counseling. A genetic counselor can help you understand what a positive screening result — and a diagnosis of HCM — would mean for you.

Genetic counseling can help you decide whether genetic testing is right for you. A counselor can help you understand important points, such as:

• How genetic testing for HCM works
• What your family’s risk is for familial HCM
• The benefits and limitations of genetic testing for HCM
• What you should expect from the testing process
• How to interpret your testing results, including how a negative result may not exclude HCM for you or your family

If you plan to have genetic screening for HCM, your provider may recommend you receive genetic counseling both before and after your test. During your meetings, your counselor can answer any other questions you may have about how the genetic condition might affect your life, such as whether you can continue working and enjoying your hobbies and whether it’s safe to have a child.

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