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.

Syncope refers to a generalized weakness of muscles with loss of postural tone, inability to stand upright, and loss of consciousness. Once the patient is in a horizontal position, blood flow to the brain is no longer hindered by gravitation and consciousness is regained. Unconsciousness usually lasts for seconds to minutes. Headache and drowsiness (which usually follow seizures) do not follow a syncopal attack. Syncope results from a sudden impairment of brain metabolism usually due to a reduction in cerebral blood flow.