Hypertrophic Cardiomyopathy (HCM) is a form of heart disease where the heart muscle becomes abnormally thick. In most patients, the thickening occurs in the septum, the muscular wall that divides the left and right sides of the heart, and in the outer wall of the left ventricle.

HCM is a common disease that affects up to 600,000 people in the United States. It affects people of both genders, all ages, and all races. It may be diagnosed in childhood or late in life, depending on when symptoms develop.

HCM is a genetically-determined disease, which means it is often passed down through families. The American Heart Association estimates that 50-60% of children with HCM have a relative with the disease.

The image below shows a heart with hypertrophic cardiomyopathy. The left ventricle is small and the surrounding heart muscle is too big. The blood flow leaving the heart is restricted and turbulent.


In contrast, this image shows a healthy heart without hypertrophic cardiomyopathy. The left ventricle and the heart muscle are a healthy size and the blood flow has a steady pattern.


Obstructive Hypertrophic Cardiomyopathy

In obstructive HCM, the heart muscle is thick and the mitral valve behaves abnormally. The mitral valve is on the left side of the heart and sits between the left atrium and the left ventricle.

Part of the mitral valve touches the thickened septum each time the heart beats. This blocks blood from leaving the left ventricle and causes blood to leak backwards to the lungs. Almost two-thirds of HCM patients have obstructive HCM.

Non-obstructive Cardiomyopathy

In non-obstructive HCM, the heart muscle is thick, but blood is not blocked from leaving the heart.


There is a wide range of symptoms among patients with HCM. Some patients experience symptoms that limit their activity, while others may have no symptoms at all. Symptoms can include the following:

  • Shortness of breath
  • Light-headedness
  • Chest pain, often with exercise
  • Unexplained fatigue or tiredness
  • Fainting


HCM is caused by a genetic abnormality, or mutation, that makes heart muscle cells grow too large. There are multiple different genetic abnormalities that cause HCM.

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