It is the most 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 MR.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 subsequent displacement of the papillary muscles and the dilatation of the mitral valve annulus.Rheumatic fever (RF), Marfans syndrome and the Ehlers–Danlos syndromes are other typical causes. Mitral valve stenosis (MVS) can sometimes be a cause of mitral regurgitation (MR) in the sense that a stenotic valve (calcified and with restricted range of movement) allows backflow (regurgitation) if it is too stiff and misshapen to close completely. Most MVS is caused by RF, so one can say that MVS is sometimes the proximal cause of MI/MR (that is, stenotic MI/MR) and that RF is often the distal cause of MVS, MI/MR, or both. MR and mitral valve prolapse are also common in Ehlers–Danlos syndromes.Secondary mitral regurgitation is due to the dilatation of the left ventricle that causes stretching of the mitral valve annulus and displacement of the papillary muscles.
If the patient is hypotensive prior to the surgical procedure, an intra-aortic balloon pump may be placed in order to improve perfusion of the organs and to decrease the degree of MR.

Medicine
If the individual with acute MR is normotensive, vasodilators may be of use to decrease the afterload seen by the left ventricle and thereby decrease the regurgitant fraction. The vasodilator most commonly used is nitroprusside.Individuals with chronic MR can be treated with vasodilators as well to decrease afterload. In the chronic state, the most commonly used agents are ACE inhibitors and hydralazine. Studies have shown that the use of ACE inhibitors and hydralazine can delay surgical treatment of MR. The current guidelines for treatment of MR limit the use of vasodilators to individuals with hypertension, however. Any hypertension is treated aggressively, e.g. by diuretics and a low sodium diet. In both hypertensive and normotensive cases, digoxin and antiarrhythmics are also indicated. Also, chronic anticoagulation is given where there is concomitant mitral valve prolapse or atrial fibrillation. Surgery
Surgery is curative of mitral valve regurgitation. There are two surgical options for the treatment of MR: mitral valve replacement and mitral valve repair. Mitral valve repair is preferred to mitral valve replacement where a repair is feasible as bioprosthetic replacement valves have a limited lifespan of 10 to 15 years, whereas synthetic replacement valves require ongoing use of blood thinners to reduce the risk of stroke.