The incidence of primary (idiopathic) hypertrophic cardiomyopathy (HCM) is much lower in dogs than it is in cats. Males appear to be predominantly affected and there may be a higher incidence in boxers and German shepherd dogs. The disease is characterized by hypertrophy of the left ventricular free wall and interventricular septum. The aetiology of primary hypertrophic cardiomyopathy has not been determined.
Myocardial hypertrophy results in decreased left ventricular compliance (increased myocardial *stiffness*) and diastolic dysfunction. Systolic function is usually adequate. Myocardial hypertrophy results in increased myocardial tension and afterload. This in turn leads to progressive myocardial isehaemia and the development of cardiac arrhythmias which are usually ventricular in origin. In most cases the hypertrophy is asymmetric with disproportionate thickening of the inter ventricular septum compared to the left ventricular free wall (at necropsy, septal to left ventricular free wall ratio in affected cases is greater than 1.1:1). Asymmetric hypertrophy may cause functional left ventricular outflow obstruction. Ventricular hypertrophy may also be associated with focal or diffuse endocardial fibrosis which further reduces ventricular compliance.
Dogs with hypertrophic cardiomyopathy may be asymptomatic or present with a history of weakness, exercise intolerance and syncope; a few cases may show more obvious signs of congestive heart failure such as coughing, dyspnoea, pleural effusion, hepatomegaly and ascites. A low-grade ejection-type systolic heart murmur is occasionally detected over the region of the aortic valve.
The ECG may be unremarkable. Conduction disturbances such as atrioventricular block and bundle branch block have been reported.
In many cases thoracic radiographs may be normal. Occasionally there is evidence of mild left atrial and left ventricular enlargement but signs of left-sided congestive heart failure rarely occur.
Echocardiography typically shows hypertrophy of the left ventricular free wall and inter ventricular septum and a reduction in the internal dimension of the left ventricle during systole and diastole. The left ventricular papillary muscles appear hypertrophied. Indices of left ventricular function may be normal or increased. Evidence of mitral regurgitacion may be present. Echocardiography can be used to rule out subaortic stenosis as a cause of myocardial hypertrophy in the younger dog.
The major differential diagnosis in a young dog is congenital subaortic stenosis which usually results in concentric symmetrical hypertrophy of the left ventricle and interventricular septum. Left ventricular hypertrophy may occur in athletic or working dogs; echocardiographic evidence of left ventricular hypertrophy has been reported in healthy fit greyhounds. It may also occur secondary to arterial hypertension in dogs with chronic renal disease.
The prognosis for hypertrophic cardiomyopathy is extremely guarded. The condition is progressive and although medical management may be palliative, sudden unexplained death is common even in animals which have previously been asymptomatic.
Medical management should be directed towards the signs of congestive heart failure or controlling any arrhythmia which maybe present. Beta blocking agents may increase ventricular compliance and thereby improve diastolic filling although there have been no controlled studies to confirm that this is the case. The use of calcium blocking drugs such as diltiazem has not been fully evaluated.