Signs and symptoms of hypertrophic cardiomyopathy may include one or more of the following:

• Shortness of breath, especially during exercise
• Chest pain, especially during exercise
• Fainting, especially during or just after exercise or exertion
• Sensation of rapid, fluttering or pounding heartbeats (palpitations)
• Heart murmur, which a doctor might detect while listening to your heart

Hypertrophic cardiomyopathy is usually caused by abnormal genes (gene mutations) that cause the heart muscle to grow abnormally thick. People with hypertrophic cardiomyopathy also have an abnormal arrangement of heart muscle cells (myofiber disarray). This disarray can contribute to arrhythmia in some people.

The severity of hypertrophic cardiomyopathy varies widely. Most people with hypertrophic cardiomyopathy have a form of the disease in which the wall (septum) between the two bottom chambers of the heart (ventricles) becomes enlarged and restricts blood flow out of the heart (obstructive hypertrophic cardiomyopathy).

Sometimes hypertrophic cardiomyopathy occurs without significant blocking of blood flow (nonobstructive hypertrophic cardiomyopathy). However, the heart's main pumping chamber (left ventricle) may become stiff, reducing the amount of blood the ventricle can hold and the amount pumped out to the body with each heartbeat.

Hypertrophic cardiomyopathy is usually inherited. There's a 50 percent chance that the children of a parent with hypertrophic cardiomyopathy will inherit the genetic mutation for the disease. First-degree relatives — parents, children or siblings — of a person with hypertrophic cardiomyopathy should ask their doctors about screening for the disease.

Many people with hypertrophic cardiomyopathy (HCM) don't experience significant health problems. But some people experience complications, including:

• Atrial fibrillation. Thickened heart muscle, as well as the abnormal structure of heart cells, can disrupt the normal functioning of the heart's electrical system, resulting in fast or irregular heartbeats. Atrial fibrillation can also increase your risk of developing blood clots, which can travel to your brain and cause a stroke.
• Sudden cardiac death. Ventricular tachycardia and ventricular fibrillation can cause sudden cardiac death. People with hypertrophic cardiomyopathy have an increased risk of sudden cardiac death, although such deaths are rare. Sudden cardiac death is estimated to occur in about 1 percent of people with HCM each year. Hypertrophic cardiomyopathy can cause heart-related sudden death in people of all ages, but the condition most often causes sudden cardiac death in people under the age of 30.
• Obstructed blood flow. In many people, the thickened heart muscle obstructs the blood flow leaving the heart. Obstructed blood flow can cause shortness of breath with exertion, chest pain, dizziness and fainting spells.
• Dilated cardiomyopathy. Over time, thickened heart muscle may become weak and ineffective in a very small percentage of people with HCM. The ventricle becomes enlarged (dilated), and its pumping ability becomes less forceful.
• Mitral valve problems. The thickened heart muscle can leave a smaller space for blood to flow, causing blood to rush through your heart valves more quickly and forcefully. This increased force can prevent the valve between your heart's left atrium and left ventricle (mitral valve) from closing properly. As a result, blood can leak backward into the left atrium (mitral valve regurgitation), possibly leading to worsening symptoms.
• Heart failure. The thickened heart muscle can eventually become too stiff to effectively fill with blood. As a result, your heart can't pump enough blood to meet your body's needs.