Hypertrophic Cardiomyopathy

  • Hypertrophic Cardiomyopathy is a rare disorder affecting 0.2% of the population and is characterized by asymmetric cardiac hypertrophy which typically affects the left ventricle. In such cases, left ventricular hypertrophy secondary to other common causes such as systemic hypertension or aortic valve stenosis must be ruled out.
  • Myocardial Hypertrophy is generally asymmetric and although global cases of hypertrophy have been described, pathology typically affects the left ventricle along with the interventricular septum. In the majority of patients hypertrophy of the interventricular septum appears out of proportion to that affecting the free left ventricular wall. Histologically, the pattern of ventricular Hypertrophy is not uniformly concentric (as observed when hypertrophy is secondary to hypertension or aortic stenosis) but is rather disorganized with substantial presence of fibrosis.
  • The vast majority of hypertrophic cardiomyopathy cases appear to be familial and caused by mutations in diverse genes critical for myocardial tension-generation.
  • Overview
    • The primary pathophysiological consequence of Hypertrophic Cardiomyopathy is the reduced compliance of the hypertrophied left ventricle, resulting in deranged diastolic filling. However, in those with prominent hypertrophy of the interventricular septum, physical obstruction of systolic blood outflow by the hypertrophied septum can cause additional and dramatic pathophysiological consequences. It should be noted that the myocardium in hypertrophic cardiomyopathy is often hyperkinetic and displays increased contractility although this is of little pathophysiological significance.
  • Reduced Compliance
    • The hypertrophied left ventricle displays a reduced capacity to relax during diastole and thus a reduced ventricular compliance. Consequently, increased left atrial pressures are required to achieve sufficient diastolic filling to maintain cardiac output. Increase pressures of the left atrium are transmitted retrogradely into the pulmonary circulation resulting in pulmonary edema and attendant symptomology of dyspnea.
  • Outflow Obstruction
    • Substantial hypertrophy of the interventricular septum narrows the left ventricular outflow tract and can cause systolic ejection of blood to suck the anterior leaflet of the mitral valve into the outflow tract. In such situations the anterior mitral valve leaflet can suddenly cause a substantial obstruction of left ventricular outflow. In addition, because the mitral valve leaflet does not close properly, mitral regurgitation ensues.
    • Because outflow obstruction can change from beat to beat, this is termed a "Dynamic Outflow Obstruction" and stands in contrast to a scenario like aortic stenosis where outflow obstruction is relatively constant over long periods of time. Such dynamic outflow obstructions can cause sudden but transient drops in the arterial pressure and thus cardiac blood flow. The increased cardiac oxygen demand of the hypertrophied myocardium along with transient drops in cardiac blood flow can lead to episodes of myocardial ischemia resulting anginal chest pain.
Clinical Consequences
  • Overview
    • Substantial variability in the course of disease is observed in patients with hypertrophic cardiomyopathy. However, most patients present with one or several of the following symptoms early in their second decade of life.
  • Symptomology
    • Dyspnea: Due to pulmonary edema caused by elevated left atrial pressures described above
    • Anginal Chest Pain: Due to elevated oxygen demand by the thickened myocardium combined with episodic reductions in cardiac blood flow due to dynamic outflow obstruction
    • Syncope: May occur especially during exertion when dynamic outflow obstruction causes a sudden reduction in cardiac output and thus systemic arterial pressure
    • Sudden Cardiac Death: Disorganization of myocardial fibers can lead to sudden, unpredictable derangements in cardiac electrophysiology and thus arrhythmias such as ventricular fibrillation
    • Infective Endocarditis: Repeated contact of the anterior leaflet of the mitral valve responsible for dynamic outflow obstruction against the adjacent endocardium results in endocardial injury and thus increased risk for infective endocarditis
  • Signs
    • S4: Caused by blood ejected by the left atrium encountering the stiffened left ventricle
    • Heart Murmur: Typically a sharply crescendo-decrescendo systolic murmur caused by blood flowing through the narrowed ventricular outflow tract