There are many diseases which affect the mitral valve and its supporting structures.  The most common disorder of the mitral valve is the partial backflow (regurgitation) of blood through the valve.  Regurgitation is usually the result of valvular degeneration, which occurs most often in older members of the smaller dog breeds.
        In valvular degeneration, the leaflets or cusps which make up the valve, may have contracted and curled back on themselves.  This allows the valve to leak.  Valvular degeneration can also be a secondary result of an infection of the valve (endocarditis), or as a congenital (present at birth) malformation of the valve.
        With mitral regurgitation the blood backflows (leaks) from the left ventricle into the left atrium of the heart.  This regurgitation frequently occurs which such velocity that it produces a turbulence.  This turbulence can usually be detected as a systolic murmur, heard between the first and second heart sounds.
        As a result of mitral regurgitation the excess blood accumulates in the left atrium causing it to enlarge.  The enlargement of the left atrium can lead to enlargement of the left ventricle.  The amount of regurgitation is directly correlated with the size of the left atrium and ventricle.
        Severe mitral regurgitation, not only produces significant increases in the left side of the heart, it is frequently accompanied by varying degrees of congestive heart failure.

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