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

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

• Conditions in infants born to mothers with uncontrolled diabetes during pregnancy, 
• Heart conditions experienced by babies born with inherited metabolic disorders, such as Pompe’s disease, and 
• 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. This is not so with HCM, where the thickening is present without these other diseases, and the heart does not respond to efforts to lower blood pressure.

Hypertrophic cardiomyopathy 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 is not the case with true HCM.
 
It is 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, about 75 percent of 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, HCM occurs in one in 500 people, making it the most common inherited heart defect. 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 the heart failure symptoms than the sudden cardiac death. 

Symptoms of Hypertrophic Cardiomyopathy

If blood flow is obstructed due to a thickened heart muscle, a number of symptoms may result, including:

• Shortness of breath during physical activity (heart failure)
• Fainting, also called syncope, with activities
• Dizziness 
• Chest pain, also called angina, which is caused by a reduced supply of blood to the heart 
• Abnormal heart rhythms, which, in some cases, can lead to sudden death unless the heart is electrically “shocked” to restart a normal rhythm 
• Heart murmur, a whooshing sound that your doctor may hear through a stethoscope 

Sometimes a patient will have no symptoms but may suddenly experience cardiac arrest. For this reason, even those without symptoms need to be followed up regularly to assess their risks for sudden death and to consider preventive treatments.

Progression and Possible Complications with Hypertrophic Cardiomyopathy

In some patients, the thickened heart muscle may block blood flow. In others, it may not. Progressive hypertrophic cardiomyopathy 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.

Hypertrophic cardiomyopathy 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. Sometimes these are single extra beats from the upper or lower heart chambers. Sometimes 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.

Treatments for Hypertrophic Cardiomyopathy

Treatment for hypertrophic cardiomyopathy aims to reduce any obstruction of blood flow from the left ventricle, improve overall heart function and maintain normal heart rhythm. Reducing the obstruction typically improves symptoms, but does not necessarily decrease the risks of a sudden cardiac event, however. 

Treatments for HCM include:

• Medications, such as beta blockers to calm any excessive “adrenaline surges,” reduce the resting heart rate, eliminate obstruction and help prevent the development of arrhythmias. Sometimes more powerful antiarrhythmic medications or combinations of medications need to be used.
• Destruction of the excessive heart muscle tissue that may be blocking blood outflow. A cardiac catheterization procedure is performed by a specially trained cardiologist who inserts a thin, flexible tube (catheter) into a blood vessel and injects alcohol into the small branch heart arteries that supply blood to the area of obstructing muscle. This causes the muscle to die. These alcohol septal ablation procedures are not typically performed in children, but they have been shown to be effective in adults with HCM. They must be performed by doctors with experience in this procedure.
• Removal of some of the thickened heart muscles through open-heart surgery (surgical myectomy).
• Placement of an implantable cardiac defibrillator that monitors for abnormal and dangerous heart rhythms. These devices can shock the heart back to a normal rhythm if a dangerous arrhythmia develops.