Hypertrophic Cardiomyopathy (HCM) occurs if heart muscle cells enlarge and cause the walls of the ventricles (usually the left ventricle) to thicken. The ventricle size often remains normal, but the thickening may block blood flow out of the ventricle. If this happens, the condition is called obstructive hypertrophic cardiomyopathy.

Hypertrophic cardiomyopathy often goes undiagnosed because many people with the disease have few, if any, symptoms and can lead normal lives with no significant problems. However, in a small number of people with HCM, the thickened heart muscle can cause shortness of breath, chest pain or problems in the heart's electrical system, resulting in life-threatening abnormal heart rhythms (arrhythmias).

Dilated cardiomyopathy is a disease of the heart muscle, usually starting in your heart's main pumping chamber (left ventricle). The disease occurs when the heart's ability to pump blood is lessened because its left ventricle, is enlarged and weakened.

The ventricle stretches and thins (dilates) and can't pump blood as well as a healthy heart can. The term "cardiomyopathy" is a general term that refers to the abnormality of the heart muscle itself.