Accessory mitral valve tissue: report of two asymptomatic cases.

Accessory mitral valve tissue is a rare anomaly of embryologic development of the endocardial cushion and may cause substantial and progressive obstruction of the left ventricular outflow tract. Subaortic obstruction resulting from accessory mitral tissue is most likely due to systolic ballooning of the tissue into the outflow tract. The obstruction can occur in the early period of life as a result of mass effect or it can develop gradually due to the continued deposition of fibrous tissues within the left ventricular outflow tract. In patients with accessory mitral valve tissue, surgery is mandatory if there is a significant obstruction in the left ventricular outflow tract. We report two cases with accessory mitral valve tissue causing mild subaortic stenoses which did not require surgery.