ABSTRACT
Hypertrophic cardiomyopathy (HCM) is the most commonly occurring genetic cardiovascular disease with a prevalence in the general population estimated to be 0.2%1,2 It is caused by mutations in any one of 10 genes encoding cardiac sarcomeric proteins with structural, regulatory, or contractile functions.3-5 Phenotypic expression is influenced not only by genetic mutation, but also by modifier genes and environmental factors, and may result in substantial molecular, anatomic, pathophysiologic, and clinical heterogeneity.3,5 Patients may have global hypertrophy, apical hypertrophy, mid cavity obliteration, and/or basal septal hypertrophy. Diastolic dysfunction,myocardial ischemia, and obstruction to late blood flow out of the left ventricle appear to be pathophysiologic features which are responsible for reduced exercise capacity, functional limitations, and other symptoms.3-7 Twenty-five percent of patients with HCM have evidence of obstruction of the left ventricular outflow tract (LVOT).3,8 As stated in the American College of Cardiology/European Society of Cardiology Clinical Expert Consensus Document on Hypertrophic Cardiomyopathy:5
It is important to distinguish between obstructive and nonobstructive forms of HCM, based on presence or absence of a LV outflow gradient at rest and/or with provocation since management is tailored to the hemodynamic state. Outflow gradients are responsible for a loud apical systolic ejection murmur associated with a constellation of unique clinical signs, hypertrophy of the basal portion of ventricular septum and small outflow tract, and an enlarged and elongated mitral valve in many patients. Obstruction may either be subaortic or mid-cavity in location. Subaortic obstruction is caused by systolic anterior motion (SAM) of the mitral valve leaflets and mid-systolic contact with the ventricular septum.This mechanical impedance to outflow occurs in the presence of high velocity ejection in which a variable proportion of the forward blood flow may be ejected early in systole. Systolic anterior motion is probably attributable to a drag effect or possibly aVenturi phenomenon and is responsible not only for subaortic obstruction, but also the concomitant mitral regurgitation (usually mild-to-moderate in degree) due to incomplete
leaflet apposition, which is typically directed posteriorly into the left atrium.When the mitral regurgitation jet is directed centrally or anteriorly into the left atrium or if multiple jets are present, independent abnormalities intrinsic to the mitral valve should be suspected (e.g. myxomatous degeneration, mitral leaflet fibrosis, or anomalous papillary muscle insertion). Occasionally (perhaps in 5% of cases), gradients and impeded outflow are caused predominately by muscular apposition in the midcavity region – usually in the absence of mitralseptal contact – involving anomalous direct insertion of anterolateral papillary muscle into the anterior mitral leaflet, or excessive mid-ventricular or papillary muscle hypertrophy and malalignment.
