Hypertrophic Cardiomyopathy (HCM)

Hypertrophic cardiomyopathy is usually caused by a pathogenic variant in a sarcomere protein gene. The clinical diagnosis is not simply “a thick heart.” It is left ventricular hypertrophy that cannot be explained by hypertension, aortic stenosis, or another cardiac or systemic loading condition. The hypertrophied myocardium is often disorganized and fibrotic. Its thick walls contract forcefully but relax poorly, so the ventricle fills under higher diastolic pressure. A patient may therefore have a relatively preserved ejection fraction while still developing exertional dyspnea, pulmonary congestion, or reduced exercise tolerance. A high ejection fraction does not prove that ventricular function is normal. Some patients develop dynamic left ventricular outflow tract (LVOT) obstruction. During systole, the narrowed outflow region and abnormal flow can draw the anterior mitral leaflet toward the septum, producing systolic anterior motion and often mitral regurgitation. The obstruction is dynamic rather than fixed: it changes as ventricular volume, vascular resistance, and contractility change. Reduced preload leaves a smaller ventricular cavity and gives the septum and mitral leaflet less room, so obstruction becomes worse with dehydration, standing, or straining. Reduced afterload...