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Idiopathic hypertrophic subaortic stenosis (IHSS) is characterized by subaortic obstruction to left ventricular (LV) outflow. However, the obstruction is variable and many patients have no resting gradient and no clinical evidence of IHSS. Although a characteristic systolic movement of the anterior mitral valve leaflet can be demonstrated echocardiographically in many patients with IHSS, this may be absent in patients without obstruction under baseline conditions. Several necropsy studies of patients with IHSS have demonstrated that the ventricular septum characteristically is asymmetrically hypertrophied in relation to the posterior left ventricular wall. Since echocardiography can measure the thickness of the ventricular septum and the posterior LV free wall during life, we have evaluated the sensitivity and specificity of asymmetric septal hypertrophy (ASH) in diagnosing IHSS. The ventricular septum and posterior LV free wall were measured in 15 patients with IHSS documented by catheterization, 11 patients with other forms of fixed LV outflow obstruction, 52 patients with miscellaneous forms of heart disease, and 16 normal subjects. The mean septal-free wall ratio in normal subjects was 1.03, in fixed LV outflow obstruction 1.03, in miscellaneous heart disease 1.03, while in IHSS it was 1.68. The ratio exceeded 1.3 in all 15 patients with IHSS (including seven patients without a gradient under baseline conditions), and it was less than 1.3 in 78 of 79 patients without IHSS. Thus ASH (1) can be diagnosed noninvasively, (2) appears specific for IHSS, and (3) is independent of the degree of obstruction. Moreover, since our findings indicate that the detection of ASH identifies the pathognomonic anatomic abnormality in IHSS and includes a wider spectrum of patients (i.e. those without obstruction as well as clinically normal family members of patients with IHSS), we suggest that a more appropriate term for this disease entity is asymmetric septal hypertrophy or ASH.