Large apical muscular ventricular septal defect: asymptomatic due to anomalous muscle bundles in the right ventricle.

This case report demonstrated an apical muscular ventricular septal defect (VSD) that was a large defect but behaved like a small defect because of the restrictive flow across the anomalous muscle bundles in the right ventricular (RV) apex. The anomalous muscle bundles separated the RV sinus into two parts: the RV apex connecting with the left ventricle through the apical muscular VSD on one side, and the rest of the RV sinus connecting with RV inflow and RV outflow on the other side. These findings explained why the 11-year-old girl in this study remained asymptomatic without evidence of volume load. Thus far, it was not necessary to close her defect because of the hemodynamic insignificance.