Feline Cardiomyopathy Christian Weder DVM, MS, DACVIM ...

Feline Cardiomyopathy Christian Weder DVM, MS, DACVIM (Cardiology)

Great Lakes Veterinary Conference

Introduction Cardiomyopathies are a heterogenous group of diseases of the heart muscle that can vary widely in their phenotypic expression and clinical manifestations. Hypertrophic cardiomyopathy (HCM) is the most common form diagnosed in cats and is characterized by pathologic thickening of the heart muscle (primarily the left ventricle). Other cardiomyopathies diagnosed in cats include restrictive cardiomyopathy (RCM), unclassified cardiomyopathy (UCM), dilated cardiomyopathy (DCM), and arrhythmogenic right ventricular cardiomyopathy (ARVC). While all of these conditions can be specifically diagnosed in cats, there can be significant overlap in the structural and functional impairments seen in these conditions. It is also important to realize that cardiomyopathy can be either primary or secondary in origin. Primary cardiomyopathy refers to disease that is inherent in the myocardium and not due to an increase in the afterload or hormonal stimulation.1 Primary cardiomyopathy is heritable in origin and is often progressive in nature. With that said, cardiac hypertrophy can also occur as the result of systemic hypertension, hyperthyroidism, acromegaly, and infiltrative disease (i.e. lymphoma). In these cases, the disease is referred as a secondary cardiomyopathy. It is an important distinction as secondary cardiomyopathies can often stabilize or even regress as a result of treatment of the underlying condition. Therefore, a minimum database (CBC/Chem/UA/T4) and blood pressure are recommended in all patients prior to making a diagnosis of primary HCM. The causes of secondary cardiomyopathy will be presented briefly, and then primary diseases of the myocardium will be discussed in depth.

Secondary Cardiomyopathies Left ventricular hypertrophy can be due to factors that are distinct from those that are genetic in origin. Thyrotoxic cardiomyopathy is a well-documented cause of myocardial disease in cats.2 Thyrotoxicosis is a condition in which there is an excess of circulating thyroid hormone and can result from an overactive thyroid gland or, less commonly, thyroid carcinoma. Cardiovascular abnormalities are common in cats with hyperthyroidism and can include left ventricular hypertrophy, tachycardia, arrhythmias, a gallop sound, heart murmurs and systemic hypertension. These findings are related to direct effects of thyroid hormone on the myocardium and indirect effects mediated through the adrenergic nervous system.3 While chronic thyrotoxicosis is most common, an acute form ("thyroid storm") has also been described.4 Systemic hypertension is another important cause of secondary cardiomyopathy in cats. The most common causes of systemic hypertension in cats are chronic kidney disease, diabetes mellitus, hyperthyroidism, obesity and primary hyperaldosteronism.5 While chronic systemic hypertension has a wide variety of effects on target organs, cardiovascular changes can include heart murmurs, gallop sounds, and left ventricular hypertrophy.6 There are published guidelines for the diagnosis and management of systemic hypertension in dogs and cats and should be referenced.5 Acromegaly (hypersomatotropism) is an endocrine disorder caused by oversecretion of growth hormone and is a documented cause of left ventricular hypertrophy in cats.7 Finally, cardiac hypertrophy can be the result of infiltrative disease, most commonly lymphoma in cats.8

Hypertrophic Cardiomyopathy Hypertrophic cardiomyopathy (HCM) is the most common form of primary cardiomyopathy diagnosed in cats.9 Specific genetic mutations that lead to the development of HCM have been identified in both the Maine Coon and Ragdoll breeds.10 These mutations are typically within the genes that encode for the proteins of the sarcomere, the basic structure that makes up cardiac muscle. These mutations lead to progressive left ventricular hypertrophy and all of sequelae of HCM. Chronic and progressive left ventricular hypertrophy leads to an increased stiffness of the myocardium and a subsequent rise in the intraventricular diastolic pressure. This can then result in left atrial dilation and, in some cases, congestive heart failure. Some cats can also develop systolic anterior motion (SAM) of their mitral valve and associated mitral regurgitation, which can also hasten progression of their disease (see below).

Presentation and Diagnosis The age of cats at time of presentation is highly variable. HCM has been diagnosed in cats as young as 6 months of age and is seen throughout all subsequent stages of life. In the authors experience, purebred cats such as Maine Coons, Ragdolls, and Sphynx tend to be diagnosed at a younger age and tend to be more severely affected. Many

cats are asymptomatic at time of presentation and their disease is identified after a heart murmur, arrhythmia, or gallop sound is found. Other cats have no abnormalities on physical exam, even in the presence of significant underlying disease. Congestive heart failure (CHF) can also be the first sign of disease and often manifests as tachypnea/dyspnea and non-specific symptoms such as hiding, poor appetite, lethargy, etc. Coughing is an uncommon manifestation of heart failure in cats as compared to dogs. Although uncommon, some cats present with signs related to an aortic thromboembolism (acute hind limbs paralysis, vocalization, etc.) and an even smaller percentage experience sudden death.

The most commonly used diagnostic tool for feline HCM is echocardiography. Cats with HCM have varying degrees of left ventricular hypertrophy, papillary muscle enlargement, and left atrial enlargement. More advanced stages of disease can be characterized by systolic dysfunction and regional thinning of the myocardium often due to an infarction. Cats can also have systolic anterior motion (SAM) of their mitral valve, which subclassifies their condition as hypertrophic obstructive cardiomyopathy (see below). If respiratory signs are present, thoracic radiography is also recommended. With that said, a presumptive diagnosis of CHF can often be made on echocardiography with the appropriate accompanying clinical signs. Cats with congestive heart failure can manifest with pulmonary edema, pleural effusion, or both. Cats can be very hemodynamically unstable during an episode of CHF and, therefore, thoracic radiographs are not considered imperative in these cases. Minimum database testing is recommended to investigate for comorbidities and to evaluate renal function, if diuretics are to be used. Finally, systemic blood pressure and thyroid testing are also recommended to rule out causes of secondary cardiomyopathy.

Cardiac Biomarkers Cardiac biomarkers are a relatively new and emerging diagnostic tool. Although their utility is still limited, there are some important potential applications. N-terminal pro-B type natriuretic peptide (NTproBNP) is the most commonly and clinically useful biomarker in cats. There are two commonly used NTproBNP tests in cats, a quantitative measure and a SNAP test. The quantitative test provides an exact concentration, whereas, the SNAP test is reported as `negative' or `positive.' These tests are currently available through larger reference laboratories. In the authors opinion, there are two scenarios in which measurement of the NTproBNP is helpful: identification of cats with occult cardiomyopathy and distinguishing cardiac from non-cardiac causes of respiratory distress. There is a high prevalence of occult cardiomyopathy in the general feline population. Echocardiography is often considered the gold standard for diagnosing HCM, but it is an expensive diagnostic tool and can necessitate a significant amount of travel for the owner. Multiple studies have shown that the NTproBNP is higher in cats with echocardiographic evidence of cardiomyopathy than in cats without cardiac disease.11 There are also reported values for detecting occult cardiomyopathy in cats, if the quantitative test is used. A value of 100pmol/L has a high sensitivity (71%) and specificity (100%) for detecting occult cardiomyopathy in cats.12 A higher sensitivity can be achieved at lower cut-off values, however, this would likely identify cats with more mild and likely clinically insignificant disease. Given the available literature, the quantitative NTproBNP can serve as a useful screening tool in general practice and, particularly, for cats with a heart murmur. For example, if a murmur is detected in an adult cat during a routine health exam and the owner is unsure as to if they want to pursue and echocardiogram, an NTproBNP test can be performed. If the results come back as normal ( ................
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