Mitral insufficiency common cause of primary mitral regurgitation in the United States, causing about 50% of cases. Myxomatous degeneration of the mitral valve is more common in women as well as with advancing age, which causes a stretching of the leaflets of the valve and the chordae tendineae. Such elongation prevents the valve leaflets from fully coming together when the valve closes, causing the valve leaflets to prolapse into the left atrium, thereby causing MI. Ischemic heart disease causes MR by the combination of ischemic dysfunction of the papillary muscles, and the dilatation of the left ventricle. This can lead to the
Mitral valve prolapse "atrialis", "fibrosa", and "spongiosa". Patients with classic mitral valve prolapse have excess connective tissue that thickens the spongiosa and separates collagen bundles in the fibrosa. This is due to an excess of dermatan sulfate, a glycosaminoglycan. This weakens the leaflets and adjacent tissue, resulting in increased leaflet area and elongation of the chordae tendineae. Elongation of the chordae tendineae often causes rupture, commonly to the chordae attached to the posterior leaflet. Advanced lesions—also commonly involving the posterior leaflet—lead to leaflet folding, inversion, and displacement toward the left atrium. Echocardiography is the most useful method of diagnosing a prolapsed mitral valve.