Myxomatous Mitral Valve Disease: Beyond the Valve Leaflet

Myxomatous mitral valve disease is the most common heart disease in dogs. There has been focus on the pathological features of the valve leaflets and chordae tendineae and their underlying causes. However, the mitral valve apparatus also consists of the papillary muscles, which function is vital for normal valve morphology and integrity. Recent research has focused on changed mitral valve morphology in normal and MMVD diseased dogs and it is apparent that the valve, which is described saddle-shaped become flatter as a consequence of MMVD. Also, the valve is described flatter in dogs with no signs of MMVD but belonging to breeds at risk for developing MMVD. Recently, mutational variants near a gene (NEBL), which is exclusively expressed in the heart and encodes a protein that is important for stabilizing and strengthen muscle fibres, were recently identified in dogs of the CKCS breed, and two of the variants could also be linked to early disease onset in the Dachshund breed and reduced expression in the papillary muscles of affected MMVD dogs. Considering the pivotal role of the papillary muscle in maintaining mitral valve geometry and integrity, suggesting that papillary muscle weakening could be a significant component of MMVD development.