Hypertrophic obstructive cardiomyopathy (HOCM) is a disease that causes thickening of the heart muscle. When the muscular septum that divides the right and left side of the heart becomes too think, it begins to block the flow of blood out of the heart to the body, and may cause the mitral valve to leak (regurgitation). HOCM is a genetic disease that commonly runs in families, but can also occur spontaneously in people without a family history. Symptoms include shortness of breath, light-headedness, chest pain, palpitations, and fatigue. It is a common cause of sudden unexplained death, especially in young people.