Case 9 — Text
Text from “Small Animal Cardiovascular Medicine”
Myxomatous Atrioventricular Valvular Degeneration
Introduction
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 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.
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 is a common sequel.
Pathology
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. 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. In class II, the free edges are thickened and irregular, and some rough zone chordae tendineae are thickened. In class III, the valve edges are grossly thickened and nodular, with redundant tissue that may prolapse into the left atrium, and chordal rupture may be evident resulting in mitral valve flail.
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. On occasion, a full-thickness left atrial tear occurs resulting in hemopericardium, pericardial tamponade, and usually death.