Jubb, Kennedy & Palmer's Pathology of Domestic Animals: Volume 3 (eBook)

Degenerative lesions

Myxomatous valvular degeneration (“endocardiosis”) in dogs

Myxomatous valvular degeneration, commonly termed endocardiosis, is the most common cardiovascular lesion in dogs. It is a significant cause of clinically apparent left-sided heart failure, but is also frequently encountered as an incidental finding at postmortem. Advanced endocardiosis of the left atrioventricular (AV) (mitral) valve leads to mitral insufficiency and regurgitant flow into the left atrium, noted clinically as a systolic murmur, and culminates in left-sided heart failure. The shrunken, distorted AV valves are seen with greatest frequency in toy, small, and medium breeds of dogs, especially in males of breeds such as the Poodle, Pomeranian, Schnauzer, Chihuahua, Doberman Pinscher, Whippet, Fox Terrier, Boston Terrier, Cavalier King Charles Spaniel, Dachshund, and the English Cocker Spaniel. There are significant negative associations for the Labrador Retriever and the German Shepherd dog. The prevalence of the disease increases with age, from 5% at <1 year of age to >75% at 16 years of age. Endocardiosis is also reported in the left AV valves of normal market-weight pigs, but appears to be clinically inconsequential.

Endocardiosis affects chiefly the left AV valve (Fig. 1-30A-C). The right AV valve is less severely and less frequently affected. The aortic and pulmonic semilunar cusps are only occasionally involved. Grossly, the affected AV cusps are shortened and thickened. The thickening of the leaflet may be more or less uniform with a rounded edge, or with prominent nodular thickenings on the free margin. The valves are opaque and white, but the surface is smooth and glistening, without any evidence of inflammation. The chordae tendineae may also be thickened and occasionally are ruptured, allowing eversion of the leaflet into the atrium and leading to acute ventricular failure. The left AV valvular annulus may be enlarged.

The least severe gross change consists of a few small, discrete nodules at the line of closure of the valve. These progress to multiple larger nodules that tend to coalesce in the area of contact. There may be irregular areas of opacity in the proximal portions of the leaflet. The nodules may coalesce to form plaque-like deformities in the area of cusp contact. The chordae tendineae are thickened at their points of insertion into the valve, and there are clearly defined areas of opacity on the basal portions of the leaflet. The most severe change is characterized by a gross distortion of the valve by gray-white nodules and elevated plaques. The cusps are contracted, thickened, and irregular. There may also be fixed upward displacement of the free margin of the valve. Enlargement and redundancy of the valve may be seen as doming or hooding of the body of the valve toward the atrium; this prolapse of the body of the redundant left AV valve into the atrium (“mitral valve prolapse”) may be seen both echocardiographically and at postmortem. Chordae tendineae may be ruptured, in which case the free edge of the valve may prolapse as a flail leaflet. Care should be taken in the assessment of the valves, because both the left and right AV valve leaflets thicken with increasing age, and the free margin of the septal leaflet of the right AV valve is normally distinctly thicker than the free leaflets.

Changes secondary to AV valvular insufficiency are dilation of the atria, particularly the left atrium, and dilation of the ventricles. The ventricles may be eccentrically hypertrophied. The left atrial and ventricular subendocardium may be diffusely thickened by fibrosis following prolonged dilation. There may also be evidence of regurgitation in the form of focal elevated streaks and plaques of subendocardial fibrosis in the atria (“jet lesions”). Left atrial tears may also be seen in advanced cases, and these tears may rupture and lead to hemopericardium. Mural thrombi can form in the dilated left atrium; dislodgement of the thrombi can result in thromboembolism, for instance, of coronary arteries leading to myocardial infarction.

Microscopically, the earliest changes are present on the atrial side of the valves. There is proliferation of the endothelium, increased numbers of subendothelial fibroblasts and macrophages, and splitting and separation of elastic fibers between the atrialis and spongiosa. However, it is the thickening of the spongiosa and degeneration of the fibrosa that are the most prominent features of endocardiosis. The spongiosa is greatly thickened by the proliferation of loose fibroblastic tissue and the deposition of the proteoglycans, hyaluronic acid, and chondroitin sulfate (Fig. 1-31A-C). In a few cases, amyloid may be deposited. There is no increase in collagen or elastic tissue in the spongiosa. The changes in the fibrosa are also marked. Collagen bundles become swollen and hyalinized, fragment, and disappear. In advanced cases, only scattered remnants of the fibrosa remain. Similar changes are seen in chordae tendineae. Intramural coronary arteriosclerosis and focal myocardial necrosis and fibrosis are commonly seen in the left ventricular myocardium, especially the papillary muscle, if the ventricle is hypertrophic.

The cause of endocardiosis is not known, but there is clearly a genetically influenced degeneration of connective tissue at its center. This suggestion is supported by the observation that the most frequently affected breeds are of the chondrodystrophoid type. Endocardiosis is inherited in Dachshunds and Cavalier King Charles Spaniels; a polygenic mode of inheritance is suggested.

Recent research suggests that alteration in tensile...