Mitral valve dysplasia
Congenital malformation of the mitral valve leaflets or associated chordae tendineae and papillary muscles is a relatively rare disorder in the dog but is probably the most common congenital cardiac defect reported in the cat. The affected valve cusps are often thickened and incomplete closure of the valve during systole results in regurgitation of blood into the left atrium. Clinical signs of left-sided congestive heart failure develop at an early age (in most cases before 6 months of age). German shepherds, great Danes, bulldogs, bull terriers and chihuahuas appear predisposed with a higher incidence in males.
The pathophysiology and clinical signs of mitral dysplasia are similar to those of severe decompensated acquired mitral insufficiency. A harsh holosystolic murmur is present over the left atrioventricular valve. Electrocardiography shows changes consistent with left atrial and left ventricular enlargement and arrhythmias are common. Radiographic abnormalities include generalized or left-sided cardiomegaly (the left atrium in particular is often markedly enlarged) and pulmonary venous congestion or oedema. Echocardiography may demonstrate thickened or fused mitral valve leaflets and wide excursions of the valve leaflets during systole and diastole. Initially, fractional shortening may increase as the preload increases but eventually chronic volume overload lends to dilation of the left ventricle and a progressive decrease in myocardial contractility.
The prognosis for mitral dysplasia is poor. Medical management of the congestive heart failure in the form of a low salt diet, cardiac glycosides (if myocardial contractility is impaired), diuretics and vasodilators may reduce the volume of the regurgitant fraction and temporarily alleviate the clinical signs. Mitral valve replacement with a biprosthetic valve is possible with cardiopulmonary bypass.
Congenital mitral stenosis occurs infrequently in dogs often in association with subaortic stenosis; Newfoundlands and bull terriers appeared predisposed. A genetic basis may exist in bull terriers.
Mitral stenosis is usually caused by thickening and fusion of the mitral valve leaflets resulting in obstruction to the transmitral flow of blood. A diastolic pressure gradient is created across the mitral valve. Mean left atrial pressure increases resulting in left atrial enlargement and pulmonary venous congestion. Clinical signs include coughing, dyspnoea, exercise intolerance and syncope. Unlike humans where mitral stenosis is usually associated with a low-grade diastolic murmur, dogs with mitral stenosis often have a murmur typical of mitral regurgitation since most dogs develop concurrent mitral insufficiency. Radiographs show pronounced left atrial enlargement. Supraventricular arrhythmias are relatively common and echocardiography reveals abnormal diastolic motion of the mitral valve and thickened valve cusps with poor leaflet separation. Echocardiography may also detect diastolic doming of the septal mitral valve leaflet into the left ventricle in some dogs.
Dogs showing signs of congestive heart failure should be managed medically. Vasodilators and diuretics should be used cautiously since they may lead to hypotension.
Tricuspid valve dysplasia
Tricuspid dysplasia is less common than mitral dysplasia, the highest incidence occurring in male large breed dogs (great Danes. German shepherds. Labrador retrievers and weimaraners appear to be predisposed). In cats, tricuspid dysplasia is the second most common congenital defect next to mitral dysplasia. A harsh, regurgitant holosystolic murmur is present over the trucuspid valve region radiating across to the left side. Tricuspid regurgitation eventually leads to right ventricular volume overload and signs of right-sided congestive heart failure. A jugular pulse may be present in severe cases. An ECG may show changes consistent with right atrial and right ventricular enlargement. Radiographic evidence of tricuspid regurgitation (right-sided cardiomegaly and an enlarged caudal vena cava) is usually present. Echocardiography may show flattened or paradoxical septal motion and can be used to confirm right atrial and right ventricular dilation.