Hypertrophic Cardiomyopathy

Overview

Hypertrophic cardiomyopathy (HCM) is a disease in which the muscle of the lower left chamber of the heart (left ventricle) becomes abnormally thick and enlarged. This thickening typically affects some parts of the ventricle walls more than others (known as asymmetric thickening). The most commonly affected area is the wall (septum) between the two bottom heart chambers. HCM is a significant cause of both heart failure and sudden death. It’s the most common cause of sudden cardiac death among athletes who are believed to be healthy.¹ 

In the vast majority of cases, HCM is inherited from a parent, even though other close family members (such as parents or siblings) may not experience symptoms or even be aware they carry the disease. It’s important that HCM be differentiated from other causes of heart muscle thickening (known as hypertrophy). These causes include the following:

• Conditions in infants born to mothers with uncontrolled diabetes during pregnancy
• Heart conditions experienced by babies born with inherited metabolic disorders such as Pompe disease
• Heart muscle thickening caused by uncontrolled, severe high blood pressure (hypertension) or aortic stenosis

Like any other muscle, if the heart has to work harder because of increased blood pressure or a tight valve, it will become thickened. Once hypertension is controlled or the valve is replaced, it would be expected that this thickening should begin to resolve; however, this isn’t so with HCM, where the thickening is present without these other diseases, and the heart does not respond to efforts to lower blood pressure.

HCM is sometimes difficult to distinguish from an “athlete’s heart,” seen in those who have undergone particularly rigorous conditioning and exercise. In this situation, the heart may become more enlarged than is typical, and the left ventricle may become somewhat thickened. Usually, the extent of thickening in these individuals is symmetric, however, and not as thick overall as that seen in HCM. With athlete’s heart, the heart muscle will return to normal size and thickness once the conditioning stops. This also isn’t the case with true HCM.

It’s becoming increasingly clear that the most common type of HCM is caused by genetic abnormalities that affect the structural components of the heart muscle and the way that the heart muscle is regulated at the molecular level. Researchers are discovering reasons for HCM that used to be identified as idiopathic—that is, without a known cause. Currently, many patients with HCM have an identifiable genetic mutation that can be screened with a simple blood test. Once a defective abnormality is known, it becomes much easier to screen other family members.

According to the American Heart Association (AHA), HCM affects up to 500,000 people in the U.S., with 10% of cases occurring in children under age 12.² The most severe cases become evident in teenagers and young adults, but the disease may be first identified at any age, with older individuals typically suffering more from heart failure symptoms than sudden cardiac death.

Progression and possible complications

In some patients, the thickened heart muscle may block blood flow, while in others, it may not. Progressive HCM can lead to worsening heart failure, a condition in which the heart is unable to supply enough blood to the body, as well as exacerbation of any of the symptoms noted above.

The thickened heart muscle may also distort the mitral valve, which is located between the upper left and lower left heart chambers. This may cause the valve to “leak” or allow blood to flow backward into the lungs. When a valve “leaks,” the heart must work harder to pump blood to the body, resulting in even more heart failure symptoms.

HCM also may cause abnormal heart rhythms, which may be due to scarring of the severely thickened heart muscle. The blood supply to the thickened muscle may also be reduced. Since heart muscle cells have a natural electrical activity, diseased heart muscle may create inappropriate heart rhythms called arrhythmias. These are single extra beats from the upper or lower heart chambers, or there are multiple abnormal beats that can quickly degenerate into a severe rhythm problem. Sudden death can occur due to an abnormal heart rhythm or sudden reduction of blood flow out of the left ventricle caused by an increased obstruction. Due to the risk of arrhythmias, some people with HCM also may need to abstain from high-level competitive athletics.

References

1. National Center for Biotechnology Information (NCBI), National Institutes of Health (NIH)
2. American Heart Association (AHA)