Hypertrophic cariomyopathy, or HCM, is a disease of the myocardium (the muscle of the heart) in which a portion of the myocardium is hypertrophied (thickened) without any obvious cause. Though perhaps most famous as a leading cause of sudden cardiac death in young athletes HCM's more important significance is as a cause of sudden unexpected cardiac death in any age group and as a cause of disabling cardiac symptoms.