Anomalous right ventricular muscles: A clinicopathologic study

Six cases of anomalous muscle bundles are reported and compared to 52 cases collected from the literature. A striking association of this anomaly with ventricular septal defect (56 per cent) and pulmonic stenosis (17 per cent) was noted in the previously reported cases. Four of the 6 cases in this report had a ventricular septal defect; none had pulmonic stenosis. None of the anomalous muscles was obstructive and none was diagnosed antemortem, although, in retrospect, the diagnosis could have been made from the selective right ventricular angiocardiogram. All patients but 1 were under 2 months of age, whereas in the earlier reports in which the muscles were obstructive, the patients were uniformly older. Gross pathologic examination showed large whorls of muscle that crossed the cavity of the right ventricle from apex to crista supraventricularis. Microscopically, the bundles were composed of normal myocardium with the minor degenerative changes often found in congenital heart disease. It is suggested that these muscles, by deranging flow through the primitive ventricle, may be a cause of the associated ventricular septal defect or pulmonic stenosis.