Clinical meaning
Hypertrophic cardiomyopathy (HCM) is the most common genetic cardiovascular disease (~1:500 prevalence) and the most common cause of sudden cardiac death (SCD) in young athletes. It is caused by autosomal dominant mutations in sarcomere protein genes (most commonly beta-myosin heavy chain [MYH7] and myosin-binding protein C [MYBPC3]). The mutations cause myocyte disarray, interstitial fibrosis, and asymmetric septal hypertrophy (typically septum/posterior wall ratio >1.3) without a hemodynamic cause (not from HTN or aortic stenosis). The hallmark pathophysiology is dynamic left ventricular outflow tract (LVOT) obstruction: the thickened septum narrows the LVOT, and during systole, the Venturi effect pulls the anterior mitral valve leaflet toward the septum (systolic anterior motion [SAM] of the mitral valve), causing obstruction. This obstruction is DYNAMIC — it worsens with decreased preload (dehydration, standing, Valsalva), decreased afterload, and increased contractility (exercise, inotropes). The murmur of HCM is a harsh, crescendo-decrescendo systolic murmur that INCREASES with Valsalva and standing (decreased preload → decreased LV volume → worse obstruction) and DECREASES with squatting and passive leg elevation (increased preload → increased LV volume → less obstruction). This is OPPOSITE...