Disproportionate ventricular septal thickening in the developing normal human heart.

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Abstract
A disproportionately thickened ventricular septum containing numerous disorganized cardiac muscle cells is the most characteristic anatomic feature of hypertrophic cardiomyopathy. Since information concerning ventricular wall thicknesses and cellular arrangement in the developing heart may be pertinent to understanding the genesis of hypertrophic cardiomyopathy, morphologic observations were made in 151 normal human embryos, fetuses and term infants. Disproportionate ventricular septal thickening (septal-free wall ratio greater than or equal to 1.3) was present in 94% of embryos and young fetuses; in over one-third disproportionate thickening was particularly pronounced (septal-free ratio greater than or equal to 2.0). Disproportionate septal thickening was also present in 65% of older fetuses, but in only 12% of infants over two weeks of age. Septal-free wall ratio decreased in a curvilinear fashion with increasing age and apporixmated unity in the newborn. This phenomenon occurred because, while both ventricular septal and left ventricular free wall thicknesses increased directly with age, free wall thickness increased at a greater rate than septal thickness, particularly after birth. Marked cellular disorganization in the septum was not a feature of the hearts studied.