Chapter 150 Cardiomyopathy
OVERVIEW
• Chronic tachyarrhythmias cause a cardiomyopathy with loss of myocardial contractility that is reversible if the arrhythmia is resolved.
• Regression of left ventricular hypertrophy may occur after successful treatment of systemic hypertension or hyperthyroidism.
Cardiomyopathies often are classified by the postmortem anatomic appearance of the left (or right) ventricle and by the correlative echocardiographic features of ventricular anatomy and function. The most important forms of cardiomyopathy can be classified as follows (see Table 150-1):
• Myocarditis—An inflammation of the heart muscle observed most often in cats. It may be responsible for premature ventricular complexes, sudden death, or progressive heart failure.
• Dilated cardiomyopathy (DCM)—A dilated, poorly contracting left ventricle (LV) usually associated with development of congestive heart failure (CHF), cardiac arrhythmias, and sudden death. DCM is a common disease of dogs but is very uncommon in cats.
• Hypertrophic cardiomyopathy (HCM)—A thickening of the LV walls of unknown or genetic cause and displaying considerable phenotypic heterogeneity. HCM is mainly a disorder of cats and often leads to cardiac murmurs, CHF, or thromboembolic disease.
• Restrictive cardiomyopathy (RCM)—A heterogeneous and poorly characterized disorder defined by extensive fibrosis in the LV. It is encountered mainly in mature or older cats and is a recognized cause of arrhythmias, CHF, and arterial thromboembolism.
• Right ventricular cardiomyopathy—A disorder that affects mainly (or initially) the right ventricle resulting in either CHF or ventricular arrhythmias.
• Unclassified cardiomyopathy—Primary LV diseases that are not easily classified as HCM, DCM, or RCM. Some cases are probably related to myocardial infarction.
• Cardiotoxicity—The heart also can be damaged by a number of cardiotoxins, some of which are listed in Table 150-1. The outcome of cardiotoxicity is often an arrhythmia, conduction disturbance, sudden death, or development of a secondary dilated cardiomyopathy.
This chapter will next describe the clinical features of feline cardiomyopathies and the therapy of related complications. Following this is a consideration of canine DCM and arrhythmogenic cardiomyopathy.
FELINE HYPERTROPHIC CARDIOMYOPATHY
Overview and Pathophysiology of Feline HCM
• Feline idiopathic HCM is characterized by hypertrophy and thickening of the left ventricle unexplained by congenital heart disease, systemic hypertension, or an endocrinopathy.
• The condition is genetic in a number of feline breeds, including the Maine coon cat, Persian cat, and the Ragdoll. Thus far, one sarcomeric mutation has been identified.
• The pattern of ventricular hypertrophy in this disease is variable as demonstrated at necropsy or by 2D echocardiography.
• Symmetrical concentric hypertrophy of the LV walls and papillary muscles is considered typical of feline HCM, but there is substantial variation in the location and severity of hypertrophy in this disease.
• The main histologic finding is of myocardial cell hypertrophy with fiber disarray. Small, intramural coronary arteries are often narrowed. Microscopically, there is fibrosis between myocytes. Focal areas of infarction or inflammation may be observed.
• The natural history of untreated feline HCM is quite variable following a benign or lethal course; a brief or protracted clinical disease; and sometimes remarkable recovery from life-threatening complications. Some cats remain asymptomatic for many years before succumbing (if ever) to the disease.
• Clinical signs in HCM are explained by left-sided CHF, complications of arterial thromboembolism (ATE), LV outflow tract obstruction, or arrhythmias capable of causing syncope or sudden cardiac death.
• The differential diagnosis for the clinical signs of HCM in cats is extensive (Tables 150-2 and 150-3), and includes congenital heart disease, myocarditis, and other forms of primary cardiomyopathy (RCM, DCM). When an echocardiogram demonstrates thickening of the LV, specific disorders causing LV hypertrophy must be excluded including
• Mitral valve malformation causing dynamic obstruction of the LV outflow tract with secondary hypertrophy.
• The pathophysiology of feline HCM (Figure 150-1) is relevant to the diagnosis and drug therapy of this disease. The presumptive cause of CHF is ventricular diastolic dysfunction, because most cats with HCM have a normal to hyperdynamic LV ejection fraction.
• Early diastolic dysfunction is characterized by abnormal myocardial relaxation with a vigorous LA contraction to support late diastolic filling. This situation is associated with the atrial (S4) gallop so often detected in these cats.
• Progressive ventricular disease is associated with reduced chamber compliance with loss of passive LV ventricular distensibility. Tissue injury and fibrosis increase myocardial and chamber stiffness, requiring elevated LA filling pressures. This creates a situation of rapid early diastolic filling that becomes restricted as the stiff ventricle is distended. This correlates to a ventricular (S3) or summation (S3+4) gallop.
• As the left atrium distends, a loss of atrial contractility may develop. This reduces filling effectiveness and also predisposes to stagnant flow, formation of atrial thrombi, and atrial fibrillation.
• The roles of myocardial ischemia, infarction, and neurohormonal activation in the pathogenesis of progressive tissue injury and cardiac dysfunction are possible therapeutic targets because β-blockers and angiotensin system inhibitors blunt these detrimental responses.
• Demand ischemia—the sudden increase in myocardial oxygen demand that outstrips coronary blood supply—may contribute to the sudden development of left-sided CHF (flash pulmonary edema) so often observed in stressed cats with HCM. Ischemia impairs myocardial filling and contraction. For this reason, drugs with anti-ischemic effects (atenolol, diltiazem) are often prescribed in this disease.
• Systolic abnormalities are identified in some cats with HCM. These include subtle abnormalities detected only by tissue Doppler echocardiography; identification of apical or free wall-infarcts; regional wall motion abnormalities; or rarely, a global loss of LV systolic function.
• Dynamic and labile pressure gradients between the LV and aorta are often identified during ejection across the LV outflow tract. In most cases, systolic gradients stem from either septal and papillary muscle hypertrophy (midventricular obstruction) or systolic anterior motion (SAM) of the mitral valve causing mitral septal contact. These abnormalities can be documented on high-quality echocardiographic studies. The presence of significant SAM is invariably associated with an eccentric jet of mitral regurgitation (MR), readily seen by color Doppler examination.
• Some chronic cases of HCM appear to evolve into a restrictive form of cardiomyopathy with either regional wall dysfunction (suggestive of myocardial infarction) or globally reduced LV systolic function (suggesting fibrosis or ischemia). Rarely, a cat will progress to a grossly dilated form of disease.
Table 150-2 DIFFERENTIAL DIAGNOSIS OF FELINE CARDIOMYOPATHY AND HEART FAILURE*
Other Causes of Dyspnea/Tachypnea
Other Causes of Cardiac Murmurs/Gallops/Arrhythmias/Cardiomegaly
Clinical Findings in Feline HCM
The clinical presentation and examination findings in feline HCM are variable.
• Male cats are predisposed in some reports, and cats of any age, including young cats, may be affected. As previously noted, certain breeds are at genetic risk for this disease, and it is not uncommon to examine affected cats that are related.
• Most often, the idea of HCM is prompted by auscultation of a murmur or gallop sound in a cat that has no other signs of heart disease. Nonspecific signs such as lethargy or anorexia may be reported.
• When symptomatic for left-sided CHF, a cat will demonstrate tachypnea and dyspnea, signs attributable to pulmonary edema or pleural effusion. Cough can occur but is an inconsistent sign.
• Stress, fever, moderate-to-severe anemia, thyrotoxicosis, anesthesia, surgical procedures, trauma, or fluid therapy may precipitate CHF in a previously stable cat.
• Syncope or sudden cardiac death can occur but are less common than in the human disease. Sudden death may be explained by a coronary embolus, a ventricular arrhythmia, or if signs of CHF are unrecognized and the cat succumbs to hypoxia.
• Typical physical examination features of HCM include various combinations of the following:
• Systolic murmur—Murmurs are commonly due to MR or dynamic LV outflow obstruction. These murmurs can vary, often increasing in intensity with higher heart rates (and higher sympathetic tone). This finding is not specific because functional ejection murmurs also become more intense with increasing sympathetic drive, and functional murmurs are extremely common in cats of all ages.
• Auscultatory evidence of pulmonary edema or pleural effusion include increased bronchovesicular sounds, crackles or a fluid line, indicating CHF.
Diagnostic Tests in HCM
A number of routine diagnostic tests are helpful in recognizing and staging HCM.
• The electrocardiogram may be abnormal, but results are very inconsistent. Increased amplitude R-waves in lead II (exceeding 0.7-1.0 mV.) or a left axis deviation compatible with concentric hypertrophy or left anterior fascicular block may be observed.
• Thoracic radiographs can be normal, but in moderate to severe disease will demonstrate abnormalities.
• Cardiomegaly (elongation), apex shifting (to the right or left), and LA enlargement (most evident as an auricular bulge on the DV view) are common findings.
• In cats with CHF, the cardiac silhouette may be further enlarged by a small to moderate pericardial effusion caused by CHF. Cardiac silhouette size may be reduced considerably after diuretic therapy.
• Prominent pulmonary vascular patterns may indicate pulmonary hypertension secondary to elevated LV diastolic pressure or fluid retention.
• Increased lung densities are compatible with pulmonary edema and may be focal, patchy, diffuse, and often in more dependent areas than is typical for dogs with CHF.
• Routine CBC and clinical chemistries are unremarkable in most cases unless there is thromboembolism or intercurrent disease. The creatine kinase (CK), AST, and ALT all derived from skeletal muscle origin are markedly elevated in thromboembolism to the limb(s).
• Renal function may become impaired in cats with concurrent renal disease or in those treated for heart failure with diuretics and ACEIs. In many cases of advanced, treated CHF there will be mild to moderate azotemia.
• Echocardiography—Definitive diagnosis and staging of HCM relies on echocardiography and Doppler studies interpreted in light of clinical findings.
• Typical HCM is characterized echocardiographically by papillary muscle thickening, generalized thickening of the LV walls (generally to 6 mm or more in diastole), normal to decreased intraluminal size, and normal or increased systolic shortening fraction.
• There is a marked heterogeneity to the pattern of hypertrophy, and not all wall segments may be involved.
There may be generalized involvement with greater involvement of the septal or free wall segments.
• Doppler studies may demonstrate MR, dynamic obstruction, or abnormal LV filling patterns. Early diastolic dysfunction is characterized by abnormal myocardial relaxation, shown on Doppler studies as delayed LV relaxation in early diastole with a vigorous LA contraction to support late diastolic filling.
• The differential diagnosis of feline cardiomyopathy is extensive (Table 150-2), including other cardiovascular and noncardiac disorders.
FELINE RESTRICTIVE CARDIOMYOPATHY (RCM)
Overview and Pathophysiology of Feline RCM
• The pathogenesis of these lesions is undetermined. Antecedent myocarditis seems a likely, though unproven, initiating cause. In other cats, RCM clearly represents a late stage of HCM complicated by myocardial fibrosis or myocardial infarction.
• A variety of necropsy lesions have been observed in cats demonstrating clinical features of RCM.
• The left ventricle can exhibit regional hypertrophy, but overall the walls are not thickened. Often there is regional thinning or infarction of the LV free wall or apex interspersed with focal hypertrophy. Prominent papillary muscle hypertrophy or fibrosis is evident in some cats.
• Extreme endocardial fibrotic scarring can involve the mitral valve apparatus, lead to mid-ventricular stenosis, or obliterate the LV apex.
• The pathophysiology of RCM in the cat is unresolved but in many ways fits the “intermediate” label initially suggested by Harpster (Figure 150-2).
• Echocardiography generally demonstrates mild systolic dysfunction, regional LV wall dysfunction, mild mitral or tricuspid valvular regurgitation, elevated LA pressures, and impaired LV distensibility with a “restrictive” filling pattern (tall but abbreviated E-wave; small A-wave).
• Progressive increases of LA pressure develop, and the combination of ventricular dysfunction, atrial stiffness, and renal retention of sodium elevate pulmonary venous pressures and predispose to CHF and to pulmonary hypertension.
• Pulmonary edema, pleural effusion, jugular venous distention, and hepatic congestion are typical manifestations of CHF in this disease. Clinically a diagnosis of “biventricular” CHF is often made.
Clinical Findings in Feline RCM
• The clinical findings and diagnostic studies of RCM are in many ways similar to those of HCM with a few notable differences.
• The most consistent physical auscultatory finding is a loud gallop sound; murmurs are variable and less common than in HCM.
• Premature ventricular or atrial beats are common, leading to an irregular rhythm and arterial pulse.
• Signs of elevated systemic venous pressure, including hepatomegaly, pleural effusion, and elevated jugular venous pressure, are common. This may indicate RV dysfunction from RV involvement, pulmonary hypertension, or simply the generalized response of fluid retention in compensation for chronic left heart failure.
• The electrocardiogram is frequently abnormal showing cardiomegaly patterns, axis deviations, or conduction disturbances along with atrial or ventricular ectopy.
• Thoracic radiography is often impressive and characterized by LA dilation and cardiac elongation that is typical of LV enlargement.
• Some cats manifest astounding biatrial enlargement. Pericardial effusion may further enhance cardiac silhouette size.
• Echocardiography demonstrates a number of possible abnormalities. The most characteristic feature of RCM viewed by echocardiography is marked LA or biatrial dilation, mildly reduced shortening fraction, and irregular or hyperechoic ventricular walls, ± bands spanning the LV. Often there is marked wall thinning typical of myocardial infarction. Infarction or severe myocardial fibrosis often creates a segmental, hypocontractile wall.
• Clinical laboratory tests of cats with RCM are not specific and most abnormalities are attributable to CHF, diuretic therapy, or thromboembolism as described above for HCM. A plasma or whole blood taurine should be measured in cats with decreased LV ejection fraction. Analysis of pleural effusates indicates a transudate, modified transudate, or chyle. The predominant cells present are macrophages, mesothelial cells, and small lymphocytes unless there is chylothorax, in which case, well-preserved neutrophils may be more numerous in response to the irritation.
DILATED CARDIOMYOPATHY IN CATS
• The primary post-mortem lesions of primary DCM are left-sided or four-chamber dilatation, accompanied by necropsy findings of CHF. There is no demonstrable congenital, coronary, or valvular heart disease. Histological findings include myocyte loss with prominent interstitial fibrosis and variable degrees of hypertrophy, myocytolysis, and inflammation (which is typically minimal).
• Left-sided or biventricular CHF, often complicated by systemic thromboembolism, are the typical consequences of this disorder. These conditions are readily identified by careful clinical examination and results of thoracic radiography.
• Classic physical examination features of DCM include soft heart sounds (from reduced contractility or pleural effusion); gallop rhythm with or without a systolic murmur; hypokinetic arterial pulses; dull left apical impulse; and clinical signs of profound CHF. Exceptional cases are seen prior to onset of CHF.
• Cats with DCM are very likely to present in “cardiogenic shock” with sinus or junctional bradycardia, marked hypothermia, and severe hypotension (systolic ABP <70).
• Ophthalmic examination may indicate hyperreflective lesions of retinal degeneration adjacent to the optic disk (if the cause is taurine deficiency).
• The principle functional disturbance of DCM as shown by echocardiography is marked reduction of myocardial contractility as measured by shortening or ejection fractions. Morphologically, a dilated, thin-walled, hypokinetic left ventricle is observed often with dilatation of the other cardiac chambers. Ventricular filling also is impaired related to abnormal relaxation of the muscle and pronounced cardiac dilatation, which reduces diastolic compliance. There is often MR and TR by Doppler studies.
OTHER FELINE MYOCARDIAL DISEASES
A number of other diseases that affect the myocardium of the cat are briefly considered below.
• Myocarditis—Nonsuppurative myocarditis occurs sporadically in cats. The cause is unknown and definitive diagnosis requires microscopic examination of myocardium. Since myocardial biopsy is difficult in cats, and enzyme or plasma troponin-I elevations are nonspecific for inflammation, the diagnosis is usually tentative or reserved for the necropsy table. Some cats with myocarditis are presented for ventricular arrhythmias, while others develop fulminant heart failure, RCM (chronic myocarditis, healing phase), or thromboembolism.
• Right Ventricular Cardiomyopathy—A cardiomyopathy involving the RV has been reported in cats. The clinical findings can be explained by right-sided myocardial failure and severe right ventricular cardiac dilatation. There may be atrial standstill or apparent atrial fibrillation. A murmur of tricuspid regurgitation may be evident. Ventricular ectopic rhythms are common. Pleural effusion and ascites can develop from CHF and sudden death may occur. Diagnosis depends on echocardiography and exclusion of other predominately right-sided diseases such as atrial septal defect, tricuspid valve malformation, and pulmonary hypertension.
• Hyperthyroidism—Clinical findings of hyperthyroid heart disease are common in older cats with functional thyroid tumors. Thyrotoxicosis causes cardiac hypertrophy related to a hypermetabolic state, peripheral vasodilation, and increased demands for cardiac output. In addition, increased sympathetic activity and elevated thyroid hormone concentrations may stimulate myocardial hypertrophy.
• In chronic cases of hyperthyroidism, the left ventricle becomes hypertrophied. Concurrent systemic hypertension may contribute to this hypertrophy.
• Constitutional signs such as weight loss and abnormal behavior are typical. A thyroid “slip” is generally identified during physical examination.
• Cardiovascular findings can include sinus tachycardia (that can approach 300 per minute); premature atrial or ventricular complexes; hyperkinetic (bounding) arterial pulses; gallop sounds; and systolic murmurs (either functional ejection murmurs or murmurs stemming from mitral or tricuspid regurgitation).
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