Feline Cardiomyopathy

Chapter 8 Feline Cardiomyopathy



Richard D. Kienle



INTRODUCTION


The term cardiomyopathy literally means “heart muscle disease” and designates a disorder of the heart in which the primary abnormality lies within the muscle tissue (myocardium). Primary indicates the myocardial disease is not secondary to valvular disease, pericardial disease, coronary vascular disease, systemic or pulmonary hypertension, congenital abnormalities, or systemic disease. Most primary cardiomyopathies are of unknown etiology (idiopathic). A secondary cardiomyopathy is a disease that affects the myocardium secondary to infectious, toxic, metabolic, or other disease processes. The World Health Organization has categorized the types of cardiomyopathies and based the categorization scheme primarily on the dominant pathophysiology produced by the myocardial disease.



FELINE CARDIOMYOPATHIES



General Comments


• The majority of cardiomyopathies diagnosed in cats are idiopathic (primary). Only one etiology, taurine deficiency in dilated cardiomyopathy, has been identified for a feline cardiomyopathy.

• Intermediate (or intergrade) cardiomyopathy and restrictive cardiomyopathy are poorly defined clinical entities in cats. These “diagnoses” have been assigned to many feline patients with presumed primary myocardial disease that do not meet the criteria for making a diagnosis of hypertrophic or dilated cardiomyopathy.

• The widespread use of echocardiography in veterinary practice allows for more frequent and accurate recognition of myocardial disease in cats.


Key Point


In domestic cats, cardiomyopathies are the dominant form of cardiac disease.



Classification


• Cardiomyopathies are classified according to their morphologic appearance. Within each classification, a wide range of morphologic and clinical presentations may be seen. In some cats it may be difficult to comfortably place a cat’s myocardial disease into one of these categories. Also, since cardiomyopathies are so common in cats it is common for other forms of cardiac disease to be misidentified as one of the forms of cardiomyopathy.


Primary Cardiomyopathies


• Hypertrophic cardiomyopathy (HCM)

• Idiopathic dilated cardiomyopathy (DCM)

• Restrictive cardiomyopathy (RCM)

• Unclassified cardiomyopathies (UCM)

• Arrhythmogenic right ventricular cardiomyopathy


Key Point


In this chapter, I chose not to perpetuate the pretense that what have been called RCM and intermediate cardiomyopathy represent distinct and well-known disease processes for which substantiated recommendations regarding treatment and prognosis can be made. The term unclassified cardiomyopathy has been used as a reminder that the only conclusions that can be drawn about these cats’ hearts is that they have myocardial disease.



Specific/Secondary Cardiomyopathies


• Nutritional (taurine deficiency)

• Metabolic (hyperthyroidism, acromegaly)

• Infiltrative (neoplasia, amyloidosis)

• Inflammatory (toxins, immune reactions, infectious agents)

• Genetic (HCM, DCM)

• Toxic (doxorubicin, heavy metals)


Clinical Classification and Pathophysiology


• Abnormalities in myocardial function during systole or diastole can underlie or influence the clinical signs observed. Systolic dysfunction is present when the ability of the ventricle to eject blood is impaired and may result in signs of low output and possibly congestive heart failure (CHF). Diastolic dysfunction is present when the ability of the ventricle to relax is impaired and may result in signs of CHF.

Key Point


Ideally an understanding of the underlying etiology of a disease dictates specific therapy to reverse the condition; however, in most cases, treatment of cardiac disease is palliative. Therefore, when tailoring rational therapy for a patient with cardiac disease, the clinical status of the patient is the primary consideration.


• Right-sided CHF is present when elevated systemic venous, and therefore capillary, pressures resulting from cardiac disease manifest as ascites or peripheral edema.

• Left-sided CHF is present when elevated pulmonary venous, and therefore capillary, pressures resulting from cardiac disease manifest as pulmonary edema (and likely also as pleural effusion in cats).

• Biventricular CHF is present when elevated systemic and pulmonary venous, and therefore capillary, pressures manifest as any combination of the previously mentioned signs, or as pleural effusion.

• Low-output heart failure, or cardiogenic shock, is inadequate cardiac output, often a result of myocardial failure.

• High-output heart failure is CHF, left or right sided, resulting from excessive flow through a capillary bed.

• Myocardial failure is a reduction in myocardial contractility characterized by a reduced shortening fraction and an increased end-systolic dimension on the echocardiogram.


Key Point


It is important to realize that heart failure and myocardial failure represent heart disease, and that heart failure, either congestive or low-output, may, in some cases, be a result of myocardial failure; however, heart failure can be, and often is, present in the absence of myocardial failure. Similarly, myocardial failure may be present in association with or in the absence of heart failure (Figure 8-1).


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Figure 8-1 Venn diagram illustrates the various potential combinations of congestive heart (backward) failure, low-output (forward) heart failure, and myocardial failure (each represented by a circle) that may be detected in patients with heart disease (the box). As all the circles reside in the box, each represents a form of heart disease, and the overlapping portions of the circles illustrate how the conditions may coexist.



Signalment and Presenting Complaints



Key Point


The clinical presentation, physical exam findings, radiographic findings, and electrocardiographic (ECG) findings are similar for all forms of myocardial disease and generally cannot be used to differentiate among them. Echocardiography is necessary to determine the specific disorder that is present.


• The typical clinical presentation, physical exam findings, radiographic findings, and ECG findings will be discussed in this section. The echocardiographic findings will be discussed with the specific disease later.

• The most common historical clues in cats with myocardial disease include:
• Dyspnea/tachypnea

• Poor general condition, weakness, lethargy, or, rarely, exercise intolerance

• Anorexia

• Acute posterior paresis or paralysis

• Coughing and abdominal distention, common findings in dogs with cardiac disease, are rare findings in cats with cardiac disease.


Physical Examination


• Early detection of disease should be a primary goal. A thorough physical examination, with careful attention to auscultation, should be performed. Many patients present with acute onset of dyspnea, paresis, lethargy, or anorexia; however, the majority of patients is asymptomatic and will be identified after a murmur, gallop sound, or other abnormality is identified during a routine physical examination. The most common physical clues suggesting myocardial disease include:

• Systolic murmur (commonly heard along the sternal border). This murmur may relate to either mitral regurgitation or outflow tract obstruction or both.

• Gallop sound, At normally rapid heart rates these gallop sounds often represent a summation of the third and fourth sounds.
• Dysrhythmia

• Tachypnea/dyspnea

• Muffled or harsh lung sounds

• Hypothermia

• Jugular pulses/distention

• Acute paresis associated with pain in regions with evidence of reduced peripheral perfusion


Ancillary Tests


• Thoracic radiography and electrocardiography may direct or reinforce suspicion that a cardiac disorder is present. They may also further characterize the disorder and alert the clinician to secondary ramifications that may require attention; however, neither electrocardiography nor thoracic radiography provides adequate evidence for ruling out, confirming, or classifying feline cardiac disease. Contrast radiography or, preferably, echocardiography, is required to confirm or rule out and categorize myocardial disease.

Key Points


• A normal thoracic radiograph does not preclude the diagnosis of a cardiomyopathy. Many asymptomatic cats with mild changes and normal LA size will have a normal thoracic radiograph.

• Echocardiography, including Doppler echocardiography, is essential for noninvasive determination of a functional and anatomic diagnosis. Before assigning a diagnosis of cardiomyopathy based solely upon morphologic/functional appearance, a concerted effort should be made to rule out cardiac and extracardiac diseases that might mimic the echocardiography of primary myocardial diseases.

• Other diagnostic tests do not usually contribute to the diagnosis of myocardial disease but are important for determining the overall status of the patient, identifying concomitant disorders, and assessing the efficacy or untoward effects of therapy. When possible, routine biochemistries, urinalysis, and hemogram should be performed prior to pharmacologic intervention to establish baseline values for the patient and to rule out concurrent or secondary metabolic or hematologic disturbances.

• Chemical and cytologic evaluation of pleural fluid with respect to protein concentration and cellularity can help determine whether CHF underlies the production of pleural fluid. Cats with CHF can develop true chylous effusion.

• Plasma and whole blood taurine concentrations should be measured in all cats with echocardiographically documented myocardial failure (see Taurine Deficiency–Induced Myocardial Failure).

• Electrocardiography often shows:
• Intraventricular conduction abnormalities (left bundle branch block, left anterior fascicular block pattern, pre-excitation syndrome)

• Increased amplitude of R waves

• Ventricular arrhythmias are common but are generally mild with relatively infrequent single premature ventricular contractions.

• Occasionally cats will have more complex arrhythmias. Some cats with severe LA enlargement will develop atrial fibrillation.

• Thoracic radiography is most useful for detecting gross cardiac enlargement and clinical sequelae to cardiac dysfunction (e.g., pulmonary venous congestion, pulmonary edema, enlarged great veins, pleural effusion) (Figures 8-2 and 8-3). Restraint for radiographic procedures can be life threatening to dyspneic cats. Extreme caution should be taken before proceeding with radiography. The author often delays radiography until after stabilizing the patient (Figure 8-4). Diagnostic and potentially therapeutic thoracocentesis should precede radiography in dyspneic cats.

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Figure 8-2 A, Lateral thoracic radiograph from a cat with dilated cardiomyopathy demonstrates severe generalized cardiomegaly. B, Dorsoventral thoracic radiograph from the same cat as in A.


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Figure 8-3 Lateral thoracic radiograph from a cat with hypertrophic cardiomyopathy demonstrates marked left atrial enlargement, pulmonary venous engorgement, and pulmonary edema.


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Figure 8-4 Algorithm outlining choices and decisions encountered in the management of life-threatening dyspnea in the cat. The most important point illustrated is not to proceed with stressful diagnostic or therapeutic procedures until the patient is stable. Stressed cats die.



Therapy


• Therapy should be based upon the clinical and functional classification of the disease process in the individual patient and not by following a standard approach based solely upon the diagnosis.

• With only two exceptions, the indications for and benefits of therapeutic intervention in asymptomatic cats with myocardial disease are controversial. These exceptions are:
• Myocardial failure secondary to taurine deficiency

• Thyrotoxic heart disease

• Dyspneic cats are easily stressed and may acutely deteriorate and die if stressful diagnostic or therapeutic interventions are initiated too early. An algorithm for management of the dyspneic cat is presented in Figure 8-4.

• All patients with evidence of significant and life-threatening CHF (pulmonary edema, pleural effusion) require immediate therapy (i.e., appropriate combinations of pleurocentesis, diuretics, oxygen).
• Furosemide is the diuretic of choice in cats. Furosemide should be administered intravenously (1 to 2 mg/kg every 1 to 2 hours as needed) or intramuscularly (1 to 2 mg/kg every 2 hours as needed) depending upon the stress level of the cat. Dosing must be dramatically reduced once the respiratory rate begins to reduce. Generally, aggressive diuretic therapy is continued until the respiratory rate is below 40 breaths per minute.

• Not all cats respond well to being placed in an oxygen cage. Carefully observe patients after placing in a closed oxygen cage and opt for a quiet, unoxygenated environment if the patient appears more distressed in the oxygen cage.

• Tranquilization with an agent such as acepromazine may be indicated to calm distressed patients.

• The use of topical nitroglycerin as a preload reducing agent in acute and chronic situations is recommended by some, but evidence of efficacy is lacking.

• Cats with significant pleural effusion will benefit most from immediate pleurocentesis. Patients with significant pericardial effusion and cardiac tamponade require pericardiocentesis and should not receive diuretics prior to pericardiocentesis.

• Maintenance therapy is generally aimed at minimizing signs and prevent acute crisis. Lower doses of furosemide (6.25 mg twice a day to 12.5 mg three times a day PO) are indicated for chronic maintenance control of CHF. Angiotensin-converting enzyme (ACE) inhibitors (enalapril 0.25 to 0.50 mg/kg PO every 24 to 48 hours or benazepril 0.25 to 0.50 mg/kg PO every 24 hours) are also quite effective in cats with CHF. Antiarrhythmic drugs may be indicated to control significant arrhythmias. Inotropic agents may be indicated in patients with myocardial failure or low-output heart failure.

• The judicious use of intravenous fluids may infrequently be indicated in patients with signs of low-output heart failure, primarily in situations where the patient has stopped taking in oral fluids, has received excessive treatment with diuretics, or has concurrent renal dysfunction and there is concern about maintaining adequate renal perfusion.

• Specific therapies designed to alter the natural history of disease should be instituted concurrently and may, in some cases, in time, eliminate the need for drugs to control heart failure (see specific conditions discussed later).

• All cats with myocardial failure should be supplemented with taurine (see section on Taurine Deficiency–Induced Myocardial Failure) until proven to be not taurine deficient and not taurine responsive.

• Several strategies to prevent an initial thromboembolic event or to avoid recurrence of aortic thromboembolism in cats with cardiomyopathy have been devised and recommended. None of these strategies has been evaluated by controlled studies.
• Low-dose aspirin (25 mg/kg PO every 2 to 3 days) is the most widely employed prophylactic measure. Although aspirin is known to exert antithrombotic effects, there is no objective evidence of its efficacy for the prophylaxis of systemic aortic thromboembolism in cats. Recurrence of thromboembolic events in aspirin-treated cats were as high as 75% in one study.

• Lovenox (enoxaparin), a low molecular weight heparin, has shown promise in anecdotal clinical settings. No large clinical trials have been completed. The most commonly used dose is 1 mg/kg SQ every 12 to 24 hours.

• Left untreated, the outcome of arterial occlusion will depend upon the extent of occlusion and time to spontaneous reperfusion, either via the primary vessel or the collateral circulation. Cats may lose the affected leg(s) because of ischemic necrosis, die of toxemia, remain paralyzed from peripheral nerve damage, or regain full or partial function of the leg. Overall, response to presently available conservative or aggressive clinical intervention has been poor.

• Therapeutic options include:
• Surgical removal of emboli

• Catheter embolectomy

• Medical therapies—most are untested and unproven

• Anticoagulation with heparin (220 units/kg IV followed 3 hours later by maintenance dose of 66 to 200 U/kg SQ four times a day) is used to prevent further thrombosis. Adjust dose to maintain the activated partial thromboplastin time at or slightly above the upper limit of the normal reference range.

• Vasodilation with acepromazine (0.2 to 0.4 mg/kg SQ three times a day) or hydralazine (0.5 to 0.8 mg/kg PO three times a day) is used to promote collateral blood flow.

• Streptokinase and urokinase are significantly less expensive than newer fibrinolytic agents (e.g., tissue plasminogen activator), but little clinical experience has been reported. Tissue plasminogen activator: Though clinically effective thrombolysis has been documented in the cat, expense, morbidity associated with rapid reperfusion, and inability to prevent recurrence make this option impractical in most cases.


Prognosis


• Inadequate information is available to make broad generalizations regarding prognosis for cats with myocardial diseases. Although echocardiography provides the basis for diagnosis, clinical and radiographic data should be strongly considered for prognostication.

• Cats with severe myocardial disease and no evidence of heart failure may survive for long periods of time. Conversely, cats with much less severe echocardiographic evidence of disease presenting with significant and difficult to control signs of heart failure may survive for very short periods under the best of situations.

• The long-term prognosis for cats with thromboembolic disease is grave because mortality associated with individual episodes is high, and recurrence is common despite prophylaxis.


Pathology


• Gross examination of the heart provides useful anatomic information and should be performed when possible to confirm antemortem findings.

• In most cases histopathologic evaluation adds little useful information and, unless readily available at low cost, is not recommended unless specific indications are present (see the section Specific Diseases).


HYPERTROPHIC CARDIOMYOPATHY


HCM is a disease of the ventricular (primarily left ventricular [LV]) myocardium characterized by mild to severe thickening (concentric hypertrophy) of the papillary muscles and ventricular walls. The word primary in this context means that the hypertrophy is due to an inherent problem in the myocardium and is not secondary to a pressure overload or to hormonal stimulation.



General Comments


• When any other disease process that may lead to concentric hypertrophy is present, the diagnosis of HCM is excluded. Secondary disorders typically produce symmetric concentric hypertrophy and typically produce a maximum increase in wall thickness of 50% or less, even with severe disease. If severe or asymmetric concentric hypertrophy is present in a patient with one of these disorders, then concomitant HCM should be considered.

• HCM is the most commonly diagnosed cardiac disease in cats and its prevalence appears to be increasing; however, echocardiographic screening for the disease has also become more prevalent over the past ten years so increased awareness and ease of diagnosis may be a major contributing factor to this increase.

• In most cases the etiology is unknown (idiopathic). The disease is known to be inherited in some breeds of cats. The first “family” of cats with an inherited form of HCM was identified in Maine coon cats in 1992 and reported in 1999. The disease is inherited as a simple autosomal dominant trait in this breed and 100% expressed in experimental cats housed in a colony. A family of American shorthair cats, primarily with systolic anterior motion (SAM) of the mitral valve, but with other evidence of HCM as well has also been identified. The disease in this breed also appears to be inherited as an autosomal dominant trait. In addition to these breeds, there is anecdotal evidence of HCM being inherited in numerous other breeds, including Persian, British shorthair, Norwegian forest, Ragdoll, Turkish van, and Scottish fold cats, along with others.

• HCM is most likely inherited when it is identified in a specific breed; however, HCM is most commonly identified in domestic (mixed-breed) cats. Whether the disease is inherited in these cats, is due to a de novo mutation in these cats, or is associated with a completely different disease process is unknown although suspicion of inheritance has been reported in mixed breed cats.


Clinical Classification and Pathophysiology


• HCM is characterized by enlarged papillary muscles and a thick LV myocardium with a normal to small LV chamber. These changes may be mild, moderate, or severe and may be symmetrical or asymmetric. When it is severe, the concentric hypertrophy by itself increases chamber stiffness. In addition, blood flow and especially blood flow reserve to severely thickened myocardium is compromised, which may cause myocardial ischemia, cell death, and replacement fibrosis. Increased concentrations of circulating neurohormones may also stimulate interstitial fibrosis. These also increase chamber stiffness and are probably the primary reasons for the marked diastolic dysfunction seen in this disease. The stiff chamber causes an increase in diastolic intraventricular and LA pressures, LA enlargement, and may lead to CHF. The concentric hypertrophy may also result in a decrease in afterload because of the increase in wall thickness which may result in a decrease in end-systolic volume, often to zero (cavity obliteration).

• Abnormal papillary muscle orientation and other unexplained factors commonly produce SAM of the mitral valve. In one survey of 46 cats, SAM was present in 67% of cases. Cats with HCM and SAM are commonly said to have the obstructive type of HCM or to have hypertrophic obstructive cardiomyopathy. SAM of the mitral valve produces a dynamic subaortic stenosis that increases systolic intraventricular pressure in mid to late systole. The dynamic subaortic stenosis increases the velocity of blood flow through the subaortic region and often produces turbulence. Simultaneously, when the septal leaflet is pulled toward the interventricular septum, this leaves a gap in the mitral valve creating mitral regurgitation which is typically only mild to moderate in degree. These abnormalities are by far the most common cause of the heart murmur heard in cats with HCM. The degree to which SAM and mitral insufficiency contribute to the development of left-heart failure in cats with HCM is unknown and deserves further study.

• Sudden death may occur in any individual and may be unrelated to disease severity. The incidence of sudden death in cats with HCM is unknown.

• Pleural effusion is common in cats with heart failure. It can be a modified transudate, pseudochylous, or true chylous in nature. The exact pathophysiology of pleural effusion secondary to heart failure is unknown. The most likely possibility is that feline visceral pleural veins drain into the pulmonary veins such that elevated pulmonary vein pressure causes the formation of pleural effusion.

• Thrombi in cats with HCM may form in the left atrium or left auricle. LA thrombi commonly break loose (become emboli) and are carried by blood flow most commonly to the terminal aorta where they lodge. These thromboemboli occlude aortic blood flow and elaborate vasoactive substances that constrict collateral vessels. The net result is cessation of blood flow to the caudal legs resulting in acute paresis/paralysis and pain.


Signalment and Presenting Complaints


• In cats the disease has been observed as young as 6 months of age and as old as 16 years of age. In Maine coon cats that are going to develop severe disease, HCM most commonly develops to its most severe stage in males by around 2 years of age. Females are more variable but most frequently develop to an end-stage of wall thickening by three years of age. The average age of onset and rate of progression in mixed breed cats is unknown.

• Males are affected more commonly than females.

• Cats with HCM may be completely asymptomatic, may be presented to a veterinarian with subtle signs of heart failure, may have moderate to severe heart failure, or may be presented because of thromboembolic disease. Sudden death is also a common “presentation.”

• Asymptomatic cats can have mild to severe LV thickening; however, those with severe thickening usually go on to develop heart failure. Cats with severe disease that appear to have no clinical signs may show subtle signs of heart failure (e.g., tachypnea) that may be detected by an observant owner. The respiratory rate is often increased in these cats at rest and they may become more tachypneic or even dyspneic if stressed. Stressed cats with severe HCM may recover quickly following stress or may go on to develop fulminant heart failure. Cats with mild to moderate thickening may never develop clinical signs referable to their disease and may live normal lives. In others, the LV wall may progressively thicken and complications may develop when they are older.

• Cats with severe HCM and moderate to severe heart failure are usually presented to a veterinarian because of respiratory abnormalities (tachypnea and/or dyspnea) due to pulmonary edema, pleural effusion, or both.

• Cats with HCM may develop systemic thromboembolism.

• Cats with HCM may die suddenly, often with no prior clinical signs referable to heart disease or failure. The cause of the sudden death in these cats is unknown.

• In humans, sudden death appears to be either due to an arrhythmia, acute worsening of the outflow tract obstruction associated with stress or exercise, or a large thrombus occluding flow in the left.

• The incidence of sudden death in feline HCM is probably under represented in the veterinary literature because cats that die suddenly are not presented or reported to veterinarians.


Echocardiography


• Cats with severe HCM have papillary muscle hypertrophy, markedly thickened LV walls (7 to 10 mm), and usually an enlarged left atrium (Figure 8-5). The hypertrophy can be global, affecting all areas of the LV wall or can be more regional or segmental. Segmental forms can have the entire or a region of the interventricular septum or free wall primarily affected, the apex primarily affected, or the papillary muscles (and often the adjacent free wall) primarily affected. Papillary muscle hypertrophy may be the only manifestation of the disease.

• Because of these forms, HCM is a diagnosis that should be made by examining several different two-dimensional echocardiographic views and measuring wall thicknesses in diastole from the thickest region or regions on the two-dimensional images. M-mode echocardiography may miss regional thickening unless it is guided by the two-dimensional view. The LV end-diastolic or end-systolic dimension may be normal or decreased and end-systolic cavity obliteration may occur. An enlarged left atrium indicates increased LV end-diastolic pressure. Occasionally, a thrombus is imaged in the LA or its appendage.

• SAM of the mitral valve may be identified by two-dimensional or, more commonly, by M-mode examination (Figure 8-6). Color flow Doppler echocardiography can be used to demonstrate the hemodynamic abnormalities associated with SAM. Two turbulent jets originating from the LV outflow tract are seen—one regurgitating back into the left atrium and the other projecting into the aorta. Spectral Doppler can be used to determine the pressure gradient across the region of dynamic subaortic stenosis produced by the SAM. The pressure gradient roughly correlates with the severity of the SAM although it can be quite labile, changing with the cat’s level of excitement. SAM is not present in all cats with HCM. The majority of cats with severe HCM have SAM. However, on the other end of the spectrum, some cats develop SAM before they have any evidence of wall thickening, when their papillary muscles are thickened or long. Although the basilar region of the interventricular septum is often thickened in diastole in cats with HCM, the basilar LV outflow tract does not need to be narrowed for SAM to occur.

Related posts:

  1. Emergency Management and Critical Care
  2. Anesthesia of the Cardiac Patient
  3. Cardiac Surgery
  4. Radiology of the Heart

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Aug 15, 2016 | Posted by in SMALL ANIMAL | Comments Off on Feline Cardiomyopathy

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