Hypertrophic cardiomyopathy, a potentially lethal disease of the heart, is characterized by a thickened heart muscle, in which the muscle fibers have become jumbled and tangled. This overdeveloped heart muscle is less efficient at pumping blood throughout the body. Although HCM primarily affects the thickness of the heart muscle, the electrical activity of the heart may also be affected, which can lead to abnormal heart rhythms or even sudden death.
As the most common inherited disease of the heart, HCM occurs in about 1 in 500 people, making it more prevalent than muscular dystrophy, cystic fibrosis, and AIDS in the United States. HCM is the most common identifiable cause of sudden death in young people and is the leading cause of death in competitive athletes. Sudden death occurs in approximately 1 percent of the people diagnosed with HCM and sudden death is frequently the only symptom.
First recognized in the 1950s, this disease has acquired many different names, the most common being idiopathic hypertrophic subaortic stenosis and hypertrophic obstructive cardiomyopathy. Although HOCM is technically a misnomer because HCM is not necessarily associated with an obstruction in the heart, this term is still used in the United Kingdom. In the United States, this disease is now referred to as hypertrophic cardiomyopathy, where hypertrophic literally means "excessive thickening" and cardiomyopathy means "heart muscle disease."
While HCM can lead to serious health complications in some individuals, most people experience few or no limitations on the quality or length of life.
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Hypertrophic cardiomyopathy frequently affects the function of the ventricles, myocardium, ventricular septum, and mitral valve. These structures of the heart play a vital role in circulating blood throughout the body.
The heart consists of muscle tissue known as the myocardium. Muscle fibers are the building blocks of the tissue. They not only provide the power for pumping but also transmit electrical signals throughout the heart.
In general, the structure of the heart can be thought of as a two-story house with four rooms or chambers. On the main floor are the two largest rooms—the left and right ventricles, which are the main pumping chambers of the heart. The right ventricle pumps blood into the lungs to be oxygenated; the left ventricle pumps blood to the body. The wall between the left and right ventricles is called the ventricular septum. On the upper story are two smaller rooms—the left and right atriums. The atriums function primarily as receiving chambers for blood entering the heart from the lungs or the rest of the body, but they also contribute slightly to pumping.
The valves of the heart function like one-way doors. The valves help control the direction of blood flow; this keeps the heart working efficiently. The four valves in the heart are the tricuspid, mitral, pulmonary, and aortic valves. Of particular interest in HCM are the mitral and the aortic valves. The mitral valve is the valve between the left atrium and the left ventricle. Blood from the lungs enters the left atrium via the pulmonary veins, then exits through the mitral valve into the left ventricle. The mitral valve keeps the oxygenated blood flowing from the left atrium into the left ventricle. The aortic valve is adjacent to the mitral valve; it helps keep the blood flowing from the left ventricle into the aorta, the main artery that channels oxygen-rich blood to the rest of the body.
Recent advances in genetic research have revealed that hypertrophic cardiomyopathy is a genetic disease. People who have defects in the genes that form the blueprints for the growth of individual heart muscle fibers are predisposed to developing HCM. These genetic defects or mutations make the heart muscle develop abnormally, causing thickening that can interfere with its pumping efficiency.
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Genes carry the sets of instructions that guide biological development. If these instructions are jumbled by a genetic mutation, development may be abnormal. Several hundred mutations have been identified in more than a dozen genes that have been linked to hypertrophic cardiomyopathy. These mutations are in genes that control how the myocardium, or heart muscle, contracts.
Not everyone with HCM has one or more of these mutations. There is about a 50 percent chance that a person who has HCM will have a normal genetic screen. Additional mutations are likely to be discovered as the genetic code continues to unravel for HCM.
Most people with HCM inherited the genetic mutations from their parents, but the mutations can also occur spontaneously. These mutations can pass directly from parent to child; they do not skip a generation. The likelihood of a parent passing the mutation on to a child is about 50 percent. The likelihood of a child spontaneously developing the mutation is small, between 1 in 500 and 1 in 1,000. In either case, subsequent environmental or lifestyle factors can influence how and when the disease manifests in an individual.
Even within families, the same genetic mutation may affect people in different ways. Individuals may have different degrees of thickening in the heart muscle and severity of obstruction and different susceptibilities to developing symptoms. The disease may develop at a different rate and age in each individual. Family members who are only mildly affected or completely symptom free may unknowingly pass the gene on to descendants, who in turn can be severely affected as children or young adults—sudden death included.
Because HCM is a genetic disease, family members of patients with HCM should also be tested for HCM. More information about screening family members is provided in the management section.
In the healthy heart shown on the left in this picture, the ventricular septum, separating the left and right ventricles, is relatively thin—approximately 8 mm to 12 mm thick. Blood flows freely into the left ventricle through the mitral valve when the heart relaxes between beats. When the heart muscle contracts, the blood is pumped out through the aortic valve into the aorta, beginning its journey to deliver oxygen and nutrients to the rest of the body.
In hypertrophic cardiomyopathy, the ventricular septum thickens, usually 20 to 25 millimeters but sometimes to over 60 millimeters. The thickening primarily occurs inside the heart and can be evenly distributed throughout the heart or limited to a specific region such as in asymmetric septal hypertrophy, shown in the heart on the right in this picture. Other types of hypertrophy include midcavity hypertrophy, characterized by thickening in the middle portion of the heart, or apical hypertrophy, in which thickening is limited to the bottom tip of the heart. Not only does the enlarged muscle occupy space formerly used for blood, but it also does not relax normally, further reducing the space for blood in the left ventricle.
If the myocardium thickens such that the flow of blood through the heart is obstructed, the condition is known as hypertrophic obstructive cardiomyopathy. Approximately 70 percent of people with HCM may develop an obstruction. In most people obstruction is transient; triggered by any circumstances that lead to decrease in blood pressure or an increase in heart rate such as salt content of food, body position, exertion level, or temperature of the room. In some people, the obstruction is always present, but the degree of obstruction varies over time.
Obstruction is an active process that decreases the space available for blood flow in the left ventricle. This, in turn, decreases the volume of blood flowing in the heart. The heart compensates by pumping harder and faster. In some people, this faster blood flow can force open the mitral valve during muscle contraction, when it should be closed—allowing blood to flow backwards into the left atrium. More importantly, the opened mitral valve can physically block the opening to the aorta, obstructing the flow of oxygenated blood to the rest of the body. The blood flow actually worsens the harder the heart squeezes, because the thickened septum gets even thicker during contraction, protruding even farther into the pathway of the blood. When the flow of blood is obstructed within the heart, the symptoms of heart failure can develop, sometimes suddenly.
In addition to thickening, the muscle fibers of the heart become disorganized in HCM. Normally, the fibers of the heart muscle interlock with one another forming a parallel array. In a hypertrophied heart, the fibers no longer interlock properly, forming a myocardial disarray.
The risk factors for developing hypertrophic cardiomyopathy include both the genetic and the environmental. If you have a genetic mutation for hypertrophic cardiomyopathy, you are predisposed to developing this disease. If you have a blood relative with the disease, you may have one of the HCM genetic mutations that increase your risk of developing the disease.
The exact relationship between the genetic material health and environmental influences that affect the way the disease manifests is just beginning to unfold. The abnormal growth of the heart muscle frequently occurs during periods of rapid growth, such as adolescence. Although HCM may be diagnosed in people of all ages, the course of the disease is usually more severe if onset occurs during adolescence.
Other factors known to affect the development of the disease and its symptoms include blood pressure and activity levels. When diagnosed in people over 60 years of age, HCM is often associated with high blood pressure or hypertension. High blood pressure is like weight lifting for the heart; a long history of high blood pressure can itself lead to the overgrowth of the heart muscle. Extensive physical training can also result in an overdeveloped heart. The difference between a professional athlete's heart and the heart of an individual with HCM can be subtle. Other environmental factors that increase one's risk of developing HCM will most likely become apparent as research continues.
Approximately 1 percent of people with hypertrophic cardiomyopathy will die suddenly, with little or no warning. Anyone who has hypertrophic cardiomyopathy should be evaluated and counseled for risk of sudden death by a cardiologist experienced with sudden death from HCM. Sudden death can occur in people who are symptom-free and don't realize they have HCM.
The risk factors for sudden death are:
- Previous resuscitation from a cardiac arrest
- Family history of sudden death
- Myocardium (heart muscle) thickness greater than 30 millimeters, especially in children and young adults
- Repetitive fainting spells that cannot be attributed to some other cause
- Abnormal heart rhythms recorded during continuous electrocardiography
- Abnormal changes in blood pressure during an exercise stress test
Last reviewed on 2/11/2009
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