Case Studies In Small Animal
Text from "Small Animal Cardiovascular Medicine"
MYXOMATOUS ATRIOVENTRICULAR VALVULAR DEGENERATION
The mitral and tricuspid valves are the valves that separate the left and right atria from the left and right ventricles respectively. They are also termed the atrioventricular (AV) valves. Both act as one-way valves, allowing blood to flow into the ventricles during ventricular diastole and preventing blood from flowing backward into the atria during ventricular systole. The mitral AV valve apparatus is made up of the mitral valve leaflets, the annulus of the mitral valve, the chordae tendineae, and the left ventricular papillary muscles. The tricuspid valve apparatus is composed of the tricuspid leaflets, chordae tendineae, and right ventricular papillary muscles. The function of the mitral valve can also be influenced by the left atrial wall and the left ventricle and the tricuspid valve is likely similarly influenced by the right atrial wall and right ventricle. Mitral regurgitation (MR), also known as mitral insufficiency, is the leakage of blood through the mitral valve, from the left ventricle to the left atrium, during systole. Tricuspid regurgitation (TR) is the same situation on the right side of the heart. Greater force is generated in systole in the left ventricle than in the right ventricle. Consequently, greater regurgitation is produced for a given size orifice in the valve in MR than in TR.
Atrioventricular regurgitation occurs secondary to numerous abnormalities. An abnormality in any component of the mitral or tricuspid valve apparatuses can result in valve leakage. Examples include: 1) annular dilation and a displaced coaptation point secondary to increased ventricular chamber size (dilation) as occurs with dilated cardiomyopathy, myxomatous valve degeneration, etc.; 2) abnormalities of the valve leaflets that include myxomatous degeneration and bacterial endocarditis; 3) abnormalities of the chordae tendineae that include lengthening and rupture in myxomatous degeneration; and 4) abnormalities of the papillary muscles that usually involves malorientation secondary to ventricular diseases such as hypertrophic cardiomyopathy. The most common primary abnormality that causes AV regurgitation is myxomatous degeneration of a valve. Most of the abnormalities that result in secondary regurgitation are discussed in other sections of this book. Myxomatous degeneration of the AV valves and associated lesions are the primary abnormality discussed in this chapter.
Myxomatous valve degeneration (MVD) is also known in the veterinary literature as endocardiosis, chronic degenerative valvular disease, chronic valvular disease, chronic valvular fibrosis, and simply as acquired mitral or tricuspid regurgitation/insufficiency. In human medicine this disease is commonly called mitral valve prolapse as this anatomical change in the valve (discussed below) is a common sequel. These are misnomers or incomplete definitions. Endocardiosis suggests degeneration of the endocardium. Although endocardium lines the valves, it does not appear that degeneration of this layer is the primary abnormality in this disease. Chronic degenerative valvular disease is a reasonable name for the disease but does not specifically address the type of degeneration. Chronic valvular disease is very nonspecific and theoretically could include numerous abnormalities including chronic bacterial endocarditis. Chronic valvular fibrosis is an incomplete description as it only describes the pathologic changes seen in the atrialis layer and does not describe the more prominent, and most likely primary, changes in the spongiosa and fibrosa layers of the valve. Acquired mitral or tricuspid insufficiency only distinguish the disease from congenital lesions of the AV valves. Mitral valve prolapse only describes one manifestation of the disease.
Myxomatous degeneration primarily affects the mitral and tricuspid valve leaflets and the chordae tendineae. Mitral valve lesions are more common. In about 60% of cases the mitral valve alone is affected while only the tricuspid valve is affected in about 10% of cases. The other 30% have both valves involved. The disease only rarely affects the aortic and pulmonic valves.
In myxomatous degeneration, the gross pathology of the valves is that of thickening and redundancy of the valve cusps. The pathologic features of the disease are most pronounced at the free margins of the valves. This area is thickened with prominent nodular thickenings in severely affected dogs. The affected regions of the valves are opaque and the surface is smooth and glistening with no evidence of inflammation. Whitney has classified the progression of the lesions into four classes and Kogure has modified it into three classes. In class I, the lesions usually start as small, discrete nodules along the edge of the valve leaflets, The lesions increase in size and coalesce to form larger deformities toward the free edges of the leaflet. In class II, the free edges are thickened and the edges of the leaflets become irregular and more thickened as the disease progresses. Some rough zone chordae tendineae are thickened where they attach to the valve. In class III, the valve edges are grossly thickened and nodular. The thickening extends part way and sometimes all the way to the base of the valve leaflets. There appears to be redundant tissue. This may result in some thickened tissue prolapsing into the left atrium. The rough zone chordae are thickened and chordal rupture may be evident, resulting in mitral valve flail. Chordae tendineae to the septal leaflet also elongate in dogs with class III disease.
On histopathology, the earliest changes occur along the atrial side of the AV valve. The endothelium proliferates and there is an increase in the number of subendothelial fibroblasts. The elastic fibers between the atrialis and spongiosa split and separate. This is followed by the spongiosa increasing remarkably in size while the fibrosa layer of the valve degenerates. When it is thickened, the spongiosa has the appearance of embryonic mesenchymal tissue, therefore the name myxomatous. This is characterized by widely separated stellate and spindle-shaped cells and a marked increase in the extracellular matrix without a significant increase in mature collagen or elastic fibers. The extracellular matrix is comprised primarily of glycosaminoglycans (GAGs), mostly hyaluronic acid and chondroitin sulfate. Fibroblasts proliferate in the spongiosa forming swirls and small nodules. In the fibrosa, the collagen bundles become swollen and hyalinized, fragment, and vanish. In severe cases, only scattered remnants of the fibrosa remain. Similar changes occur in the chordae tendineae.
Besides the valvular and chordal changes, the left atrium is dilated and the left ventricle is eccentrically hypertrophied. Jet lesions are commonly seen in the left atrium. They are fibrous plaques in the endocardium that occur in a region subjected to the impact of the high velocity mitral regurgitant jet. Endomyocardial splits or tears may also be identified. On occasion, a full thickness left atrial tear occurs resulting in hemopericardium, pericardial tamponade, and, usually, death. Rarely, a full thickness endomyocardial tear will involve the interatrial septum, causing an acquired atrial septal defect. Intramural coronary arteriosclerosis has been reported in dogs with chronic myxomatous mitral valve degeneration and has been used as an explanation for myocardial failure in this disease. This type of lesion is also observed in older dogs without mitral valve disease suggesting it is an incidental finding.
©Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.