Describe the anatomic abnormalities in HOCM

Describe the anatomic abnormalities in HOCM.

Hypertrophic cardiomyopathies usually result from asymmetric hypertrophy of the basal ventricular septum, and occur in either obstructive or nonobstructive forms. A dynamic pressure gradient is present in the obstructive forms. Other conditions can also produce the picture of an obstructive cardiomyopathy due to massive infiltration of the ventricular wall, as in Pompe’s disease, where a massive accumulation of cardiac glycogen in the ventricular wall produces obstruction to ventricular outflow. The following discussion concentrates on the obstructive form.

HOCM, asymmetric septal hypertrophy (ASH), and idiopathic hypertrophic subaortic stenosis (IHSS), are all terms applied to the same disease process. The main anatomic feature of HOCM is hypertrophied ventricular muscle at the base of the septum in the outflow tract of the left ventricle. Histologically, this is a disorganized mass of hypertrophied myocardial cells extending from the left ventricular septal wall and may involve the papillary muscles. Intramural (“small vessel”) coronary artery disease has been identified in autopsy specimens, especially in areas of myocardial fibrosis. This may play some role in the etiology of myocardial ischemia in these patients.

Obstruction to left ventricular outflow is caused by the hypertrophic muscle mass of the interventricular septum and systolic anterior motion (SAM) of the mitral valve’s anterior leaflet. It was thought that SAM is caused by a Venturi effect of the rapidly flowing blood in the left ventricular outflow tract (LVOT). Recently, echocardio-graphic data has revealed that a different mechanism might be the cause of SAM and consequently LVOT obstruction. The hypertrophied ventricular septum causes the mitral valve to be positioned more anteriorly in the left ventricular cavity. This brings the leaflet coaptation point closer to the interventricular septum than normal. Excessive anterior mitral valve tissue in combination with the more anterior position of the mitral valve causes the anterior mitral valve leaflet to protrude into the LVOT. Additionally, the hypertro-phied ventricular septum changes blood flow in the LVOT, redirecting it behind and lateral to the enlarged anterior mitral valve leaflet, and thus also pushing it into the septum.

Consequently, a dynamic subaortic pressure gradient is present. The outflow tract obstruction can result in hyper-trophy of the remainder of the ventricular muscle, second-ary to increased pressures in the ventricular chamber. As the ventricle hypertrophies, ventricular compliance decreases, and passive filling of the ventricle during diastole is limited. Therefore, the ventricle increasingly depends on atrial sys-tole (“kick”) to maintain ventricular end-diastolic volume. Furthermore, most patients with HOCM and dynamic LVOT obstruction present with some degree of mitral regurgitation. Occasionally, HOCM is associated with right ventricular outflow tract obstruction as well.