Endocardiosis is the most common cause of cardiac disease in dogs. An overall incidence of 17-40% has been reported ; in one survey 58% of dogs over 9 years of age which were presented for necropsy were affected. The condition is characterized by chronic nodular fibrous thickenings of the free edges of the atrioventricular valve cusps which interfere with normal closure of the valves. Valvular incompetence results in regurgitation of blood from the ventricles into the atria during ventricular systole. The specific cause of the condition is not known. The lesions appear degenerative rather than inflammatory and primarily affect the mitral valve. Most cases of endocardiosis involve the mitral valve only; approximately one third of cases involve both the mitral and tricuspid valves. Isolated tricuspid valve disease is rare as are lesions involving the aortic and pulmonary valves. There is a higher incidence in older male dogs of small breeds such as chihuahuas, miniature poodles, miniature pinschers and whippets. Endocardiosis is particularly common in cavalier King Charles spaniels in which a multifactorial polygenic threshold trait has been demonstrated. The disease in this breed is unusual in that a significant percentage of animals develop murmurs at five years or younger. Endocardiosis is rare in cats.
With mitral insufficiency regurgitation of blood into the left atrium during systole causes the left atrium to dilate which causes displacement of the mitral valve leaflets and dilation of the atrioventricular annuli thereby increasing the degree of mitral regurgitation. Pulmonary venous hypertension develops secondary to the increased left atrial pressure until eventually pulmonary oedema occurs. The clinical signs of mitral insufficiency are those of left-sided congestive heart failure; involvement of both atrioventricular valves may be expected to produce signs of left- and right-sided failure (see previous sections on the pathophysiology and clinical signs of congestive heart failure). Rupture of the chordae tendineae occasionally occurs resulting in acute and ultimately fatal cardiac decompensation.
The first sign of endocardiosis is usually the production of a holosystolic systolic murmur with a point of maximal intensity over the affected atrioventricular valve although high-grade murmurs tend to radiate dorsally and crantally throughout the thoracic cavity. Echocardiographic studies in cavalier King Charles spaniels have shown a good correlation between the intensity of the murmur and the severity of chronic valvular disease and heart failure class. The intensity of the murmur does not, however, correlate with either the severity of clinical signs or the degree of regurgitation. With severe mitral regurgitation an S3 sound may occasionally be detected.
The radiographic findings vary according to the severity of the mitral incompetence and the size of the regurgitant fraction. The first chamber to enlarge is the left atrium; marked enlargement of the left atrium may result in compression of the left mainstem bronchus. Left ventricular or in severe cases biventricular enlargement may also be evident. Volume overload of the left atrium and left ventricle leads to pulmonary venous engorgement and eventually pulmonary oedema develops. The latter initially may be confined to the perihilar region or, in more severe cases, may manifest as a diffuse interstitial and / or alveolar lung pattern. Right ventricular failure is characterized by hepatomegaly with distension of the caudal vena cava, ascites and the presence of pleural effusion.
The ECG may be normal or may show evidence of left atrial, left ventricular or biventricular enlargement. Mean electrical axis is usually normal. With severe atrial dilatation a supraventricular arrhythmia may be present (for example premature atrial complexes or atrial fibrillation).
Echocardiography is useful for confirming thickened atrioventricular valve leaflets and defective valve function as well as left atrial and left ventricular enlargement. Several echoeardiographic studies have shown that myocardial contractility during the early stages of mitral regurgitation is cither normal or in some cases there may even be a compensatory increase in contractility. Echocardiography may also demonstrate thickened chordae tendineae and prolapse of the mitral valve into the left atrium. In cavalier King Charles spaniels it has been shown that mitral valve prolapse occurs as early as one to two years of age and predisposes to endoeardiosis and mitral regxirgitaion. Doppler studies show increased diastolic mitral inflow velocities. Rupture of the chordae tendineae results in a chaotic fluttering of the mitral valve.
Diagnosis of Endocardiosis
A diagnosis of mitral insufficiency is based on the clinical, radiographic, electrocardiographic and echocardiographic findings. The differential diagnosis of mitral (or tricuspid) endocardiosis includes the following:
Other causes of atrioventricular valvular insufficiency, for example that associated with dilated cardiomyopathy, congenital heart disease, bacterial endocarditis, ruptured chordae tendineae and atrial and ventricular dysrhythmias.
Other congenital and acquired causes of cardiac murmurs.
Chronic pulmonary disease and other causes of chronic coughing for example chronic bronchitis, bronchopneumonia, pulmonary foreign body, pulmonary neoplasia. Filaroides osleri infection, eosinophilic pneumonia (pulmonary infiltrate with eosinophils), collapsing trachea, dirofilariasis and systemic mycoses.
Cor pulmonale where primary disease of the lung and / or pulmonary vasculature can lead to right ventricular hypertrophy and ultimately signs of right-sided congestive heart failure.
Treatment of congestive heart failure associated with mitral insufficiency
The treatment and general management of mitral and / or tricuspid endocardiosis essentially involves treating the signs of congestive heart failure (congestive heart failure) as they arise on a stage by stage basis. The staging scheme drawn up by International Small Animal Cardiac Health Council is useful for establishing a therapeutic protocol although a certain amount of controversy exists concerning the time at which many of the cardiovascular drugs should be administered (see below and section on cardiovascular therapeutics). Client education is extremely important especially since most of the conditions associated with mitral insufficiency are slowly progressive and their management requires a great deal of owner compliance.