The presence of numerous abnormally arranged cardiac muscle cells in the ventricular septum has been considered a characteristic anatomic feature of hypertrophic cardiomyopathy. However, it has been suggested that the ventricular septum of infants with certain congenital cardiac diseases (such as aortic or pulmonic valve atresia) contains disorganized cardiac muscle cells similar to those in patients with hyper-trophic cardiomyopathy. To test the validity of this concept and the true specificity and sensitivity of septal disorganization for hypertrophic cardiomyopathy, sections of ventricular septum were obtained at necropsy from 276 patients and the extent of ventricular septal disorganization was determined quantitatively. Disorganiza-tion was most marked in infants, children and adults with hypertrophic cardiomyopathy (ie, present in 95% of 60 patients); the mean area of septum disorganized was 31 ą 3%. Although disorganized cells were present in 64% of 33 infants with aortic or pulmonic valve atresia, these cells occupied extremely small areas of ven-tricular septum (mean area of septum disorganized 2.8 ą 0.7%; p< 0.001). Furthermore, the minimal septal disorganization present in aortic or pulmonic valve atresia was similar to that found in 91 infants with other congenital heart malformations and in 92 normal fetuses or infants (mean area ofseptum disorganized was 1.4 ą 0.6 and 0.3 ą 0.1%, respectively). Hence, extensive ventricular septal disorganization is a highly sensitive and specific finding for hypertrophic cardiomyopathy, although small areas of disorganization may occur in in-fants with other heart diseases, including aortic or pulmonic valve atresia.

IN THE INITIAL REPORT of hypertrophic car-diomyopathy, Tearel described a bizarre arrangement of cardiac muscle cells in the asymmetrically hyper-trophied ventricular septum. Subsequently, other investigators made similar observations regarding the histologic appearance of ventricular septal myocardium in this disease. 29 A recent report, based on a qualitative histologic analysis, emphasized that the ventricular septum of infants with aortic or pulmonic valve atresia contains disorganized cardiac muscle cells similar to those in patients with hypertrophic car-diomyopathy. 10 Recent experience witha quantitative method for assessing the extent of cellular dis-organization in the ventricular septum has shown that it is not the presence or absence but rather the extent of septal disorganization that distinguishes patients with hypertrophic cardiomyopathy from those with other heart diseases histologically.'1 Hence, we under-took the present study to determine the relative extent