The bizarre septal architecture of idiopathic hypertrophic subaortic stenosis (IHSS) may be secondary to a small systolic cavity with late systolic isometric contraction. We examined ventricular muscle for IHSS-type muscle-fiber disarray in infant hearts in which isometric contraction would occur during development — namely, pulmonary or aortic-valve atresia with intact ventricular septum and normal atrioventricular valves. Fifteen patients with aortic atresia and 10 with pulmonary atresia were compared to 25 normal controls of matched age and heart weight. Aortic atresia showed disorganization of muscle-fiber alignment of left ventricle, particularly septum, and intramural coronary-artery changes virtually identical to IHSS. Pulmonic atresia had similar right ventricular disarray and vessel changes, again most marked in the septum. Thus, cardiac muscle-cell disorientation similar to IHSS occurs in infant ventricles with out-flow-tract obstruction. This IHSS-type myocardial-fiber disarray may result from altered wall stresses related to isometric systolic contraction. (N Engl J Med 296:135–139, 1977) Hypertrophic cardiomyopathy or idiopathic hypertrophic subaortic stenosis (IHSS) with or without obstruction is characterized by an asymmetrically thickened interventricular septum with a striking histologic pattern of disorganized hypertrophied myocardial fibers arranged in a seemingly random manner rather than running parallel to each other as in the normal heart1 2 3 4 5 (Fig. 1). Although this marked cellular disorganization has been described as a relatively specific feature of hypertrophic cardiomyopathy, such myocardial architecture has also been found in the infundibular muscle in tetralogy of Fallot,6 embryonic hearts7 and salamander hearts contracting in tissue culture.8 Thus, the question whether the bizarre architecture of the septum.
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