When the mitral valve does not function properly, it can cause excessive leakage of blood back into the left atrium. In its most severe form, it can cause blood to back up into the lungs and cause deterioration of heart function. Together, these conditions are called congestive heart failure.
Leakage of the mitral valve, called mitral valve regurgitation, can be produced by a number of different pathologic conditions. Most commonly, excessive leaflet tissue prevents the leaflets from closing normally, leading to leakage of blood through the valve. Alternatively, the muscles or chordae that support the leaflets can rupture, which prevents normal valve closure. Finally, different adaptations to chronic heart failure can cause the annulus of the mitral valve to enlarge, thereby producing leakage.
Less commonly, other factors can prevent the mitral valve from opening enough, actually restricting the amount of blood that can flow through the valve. This condition, usually associated with rheumatic fever, is called mitral valve stenosis.