Subcellular structure of the atrial myocardium of children in cases of atrial septal defect.

In order to study the initial development of myocardial ultrastructural changes owing to right atrium volume overload, myocytes have been studied in specimens taken from the right atrial wall and auricle of four children aged 1 to 6 years with ostium secundum atrial septal defect undergoing cardiac surgery. The younger patients (1 to 4-year-old children) we observed did not show diffuse and significant myocardial ultrastructural damages. The most significant myocardial changes were observed in the 2 older patients (six years old) as we found subcellular signs of myocardial hypertrophy such as an increased number of mitochondria, increased glycogen inclusions, areas of new sarcomerogenesis and nuclei lobulated and variably shaped. Focal degenerative changes, such as rupture of mitochondrial cristae and intercellular fibrosis were also noted. These changes may be considered as the initial features of myocardial hypertrophy because they were not as severe and diffuse as those usually seen in a marked functional failure.