Cardiomyopathies are diseases affecting heart muscle. In hypertrophic cardiomyopathy (HCM, aka idiopathic hypertrophic subaortic stenosis, asymmetric septal hypertrophy, hypertrophic obstructive cardiomyopathy) sarcomeres (units of heart muscle) enlarge, causing heart muscle cells to hypertrophy (increase in size). This causes the heart muscle to thicken unevenly and changes the way different parts of the heart move, which then requires it to work harder to pump blood. In HCM, hypertrophy usually involves the septum (wall) between the ventricles (lower heart chambers), which differs from other types of cardiomyopathy. As part of the hypertrophy process, the normal pattern of muscle cells is upset and can lead to problems with electrical function of the heart, resulting in arrhythmias (abnormal heartbeat). The first case of HCM was diagnosed 50 years ago. It has since been found to affect 0.2% to 0.5% of the general population and appears in all racial groups. It is the most common cause of sudden death in young people, including trained athletes, and causes heart failure disability at any age. This disorder is different from left ventricular hypertrophy, which is usually caused by undertreated hypertension. The October 21, 2009, issue of JAMA includes an article about HCM transmitted by sperm donation.