What Is Hypertrophic Cardiomyopathy?

Hypertrophic cardiomyopathy (HCM) is a genetic condition in which the muscle of the heart is abnormally large in the absence of an apparent cause. When the muscle of the left ventricle of the heart becomes thicker than normal, blood flow is obstructed to the rest of the body.

The thickening of the muscle can also affect the heart's mitral valve, which separates the left atrium and the left ventricle. When this happens, the mitral valve leaflets become leaky, allowing blood to move backwards from the left ventricle into the left atrium, instead of forward to the rest of the body.

Hypertrophic cardiomyopathy (HCM) is also known as hypertrophic obstructive cardiomyopathy (HOCM) and idiopathic hypertrophic subaortic stenosis (IHSS). In patients with hypertrophic cardiomyopathy, the heart muscle is often stiff and relaxes poorly.

This makes it difficult for the heart to pump blood to the rest of the body and requires higher pressures than normal to expand the inflow of blood. The amount of blood that the heart can hold is then reduced as a result of the disease, which limits the amount of blood that can flow to the rest of the body. Hypertrophic cardiomyopathy can also affect the heart's mitral valve, causing blood to leak backward through the valve.

Normally, the heart wall measures from 0.08 to 1.2 cm in width. In hypertrophic cardiomyopathy, the heart wall measures 1.3 cm or greater. The pattern of muscle thickening varies by individual and is identified using an echocardiogram (ultrasound of the heart).

The distribution of muscle thickening in patients with hypertrophic cardiomyopathy is variable:

  • Typically occurs in the septum with or without left ventricular outflow tract obstruction.
  • May occur in the free wall, apex, or globally.

In 70% of individuals affected by hypertrophic cardiomyopathy, there is an increase of pressure called "obstruction" that occurs either at rest, or with exercise when the blood is ejected out of the pumping chamber of the heart. If this pressure increase is present, the term "hypertrophic obstructive cardiomyopathy" is used.

In addition, when cells from the hypertrophic cardiomyopathy heart muscle are viewed under a microscope, they will most often show an abnormal pattern called "myocardial cell disarray." The hypertrophic cardiomyopathy heart muscle cells align in a disorganized pattern, and this can result in irregular heart rhythms and problems with the electrical system.

Clinical Trials

Clinical trials are research studies that evaluate a new medical approach, device, drug, or other treatment. As a Stanford Health Care patient, you have access to the latest, advanced clinical trials.

Open trials refer to studies currently accepting participants. Closed trials are not currently enrolling, but may open in the future.

Our Clinic


See a Stanford specialist to learn about your treatment options. Visit our clinic to make an appointment.

Call 650-723-6459 to make an appointment