Mitral valve prolapse--also known as billowing mitral valve syndrome, Barlow syndrome, floppy valve syndrome, myxomatous mitral valve disease, and click-murmur syndrome--refers to a condition in which the leaflets of the heart's mitral valve are not stiff enough, prolapsing during heart contractions. This sometimes causes mitral valve regurgitation where blood flows backwards into the left atrium from the left ventricle. Though usually manageable without treatment, it can sometimes cause lightheadedness, arrhythmias, breathing difficulties, and fatigue.