Definition and Etiology. Hypertrophic cardiomyopathy (HCM) is defined as hypertrophy of the myocardium more than 1.5 cm, without an identifiable cause (Figure 1). Other causes of left ventricular (LV) hypertrophy, such as long-standing hypertension, amyloidosis, and aortic stenosis must first be excluded before HCM can be diagnosed.