A congenital non-syndromic heart malformation characratized by an accessory mitral valve leaflet or various accessory mitral valve structures. It may be asymptomatic or present at various ages with symptoms of left ventricular outflow tract obstruction, low cardiac output due to subaortic obstruction or congestive heart failure. In some cases, it may be a source of cardioembolism. The malformation may be isolated or associated with other congenital heart malformations.

An abnormality of the mitral valve characterized by insufficiency or incompetence of the mitral valve resulting in retrograde leaking of blood through the mitral valve upon ventricular contraction.