Feline hypertrophic cardiomyopathy, hypertension, and hyperthyroidism (Proceedings)
This session will focus on the three forms of feline cardiomyopathies: hypertrophic, dilated and restrictive. We will also discuss thromboembolic disease.
Myocardial disease, particularly hypertrophic cardiomyopathy, is the most common heart disease in the adult cat. Remember that valvular disease in the cat is very rare!!!! Cats do not develop endocardiosis and endocarditis is very rare, so always consider myocardial disease first when considering differentials for feline heart disease.
There are several forms of feline cardiomyopathy, hypertrophic (HCM), restrictive (RCM), dilated (DCM), arrhythmogenic (ARVC) and unclassified. In many cases, there is significant overlap between these different forms of cardiomyopathy with regard to clinical signs, ECG and radiographic findings. Echocardiography is needed to complete the diagnosis.Feline Hypertrophic Cardiomyopathy (HCM)
Hypertrophic cardiomyopathy is defined by left ventricular hypertrophy w/o causative systemic or other cardiac disease. It is the most common form of heart disease in the cat!
The etiology for the majority of cases is unknown. However, it is inherited in Maine Coon and Ragdoll breeds and is believed to be in the American Shorthair, Sphynx, Norwegian Forest and a few others. In the Maine Coon it has been shown to be an autosomal dominant trait (both genders are equally affected). In the Maine Coon and Ragdoll breeds a mutation has recently been identified as causative for the disease. Genetic tests based on a buccal swap are now available for these breeds (http://www.vetmed.wsu.edu/deptsVCGL/felineTests.aspx). It is a different mutation in both breeds and does not appear to be causative in other breeds.
Hypertrophic cardiomyopathy is typically characterized by hypertrophy of the left ventricular free wall and/or interventricular septum. This results in myocardial stiffness and decreased ventricular lumen size and therefore is a diastolic dysfunction disease. Mitral regurgitation may develop from distortion of the left ventricular cavity and from systolic anterior motion of the mitral valve (SAM). An increased left atrial pressure develops to fill the stiffened left ventricle. Elevated pulmonary venous pressure and pulmonary edema may result. In some cases, pulmonary hypertension and right ventricular enlargement may occur secondary to the left sided heart disease and pleural effusion may develop. Thrombi may develop in the stretched, dilated atria and subsequently break free and lodge in the systemic circulation (typically the distal aorta).
The clinical signs of affected cats are quite variable. Affected cats may be asymptomatic, but dyspnea, and shortness of breath may be presenting complaints due to congestive heart failure. Acute hindlimb paralysis suggests distal aortic embolization. Sudden death can occur.
Physical exam findings may include a systolic murmur consistent with left ventricular outflow tract obstruction and/or mitral regurgitation. A gallop rhythm may be ausculted indicating abnormal left ventricular filling. Since asymptomatic cats may be affected, we recommend screening with an echocardiogram if a murmur or gallop is ausculted. Tachypnea and dyspnea may be observed if heart failure is present but crackles are uncommon.
The ECG is often within normal limits but conduction disturbances and arrhythmias (ventricular and supraventricular) may be noted.
Radiographs may be useful to evaluate for cardiomegally, chamber enlargement patterns, and evidence of heart failure, but are NOT diagnostic for the specific form of feline heart disease. Radiographic findings may range from normal to significant cardiac enlargement depending on the stage of disease and may have evidence of heart failure with pulmonary venous distension and patchy pulmonary edema present.
Echocardiography is the best diagnostic test !!! Generalized concentric left ventricular hypertrophy may be observed and localized left ventricular free wall and / or interventricular septal hypertrophy may be noted. Most clinicians use a cut-off of a wall thickness at diastole of greater than or equal to 6 mm as diagnostic in the absence of other causes of left ventricular hypertrophy such as hyperthyroidism, or systemic hypertension. Wall thickness measurements may be made on M-mode echocardiogram or by measurement of the wall on long axis. Multiple modes of echocardiography should be used for a thorough study including 2D echo to scan for regions of asymmetric hypertrophy, M-mode for LV measurements and Doppler to look for evidence of SAM and document obstruction. Left atrial or biatrial dilation may be noted.
Since left ventricular hypertrophy can occur secondary to hyperthyroidism and systemic hypertension, these diseases should always be considered when evaluating a cat with left ventricular hypertrophy.
BNP is a test that is being discussed with increasing frequency in both canine and feline cardiology. Prohormone BNP is released when the ventricles are dilated, hypertrophic or subjected to increased wall tension. However it is most specific for identifying cats that are in heart failure. It has NOT been found to be reliable for assessing the presence or absence of heart disease in asymptomatic cats or cats with heart murmurs.