Cardiomyopathy is a disease of heart muscle that is associated with cardiac dysfunction. In the past, this term was reserved for idiopathic or primary heart muscle diseases. More recently, a task force of the World Health Organization (WHO) adopted the definition just stated. This was prompted by the growth of knowledge that has blurred the distinction between idiopathic myocardial disease and what were previously known as specific heart muscle diseases. The current WHO scheme classifies cardiomyopathies based on pathophysiology and cause, when it is known. The following morphopathologic designations are accepted: dilated cardiomyopathy, restrictive cardiomyopathy, hypertrophic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, and unclassified cardiomyopathy. Each of these basic types of heart muscle disease can be described as idiopathic or as a specific cardiomyopathy when the cause is known.

DCM is characterized by ventricular and atrial dilation that results from contractile dysfunction. Most often, left ventricular or biventricular dilation is present.

Loss of systolic myocardial function—a decrease in contractility—causes ventricular hypokinesis and initiates a series of events that lead to progressive ventricular dilation. Systolic myocardial dysfunction can result from loss of cardiomyocytes due to necrosis or from functional disorders of the contractile apparatus. However, the hemodynamic consequences of impaired systolic myocardial function are generally the same regardless of the cause of myocardial dysfunction.

When stroke volume declines as a result of systolic myocardial dysfunction (decreased contractility), the end-systolic ventricular volume increases. During diastole, this elevated end-systolic volume augments pulmonary venous return and this contributes to ventricular dilation. In addition, activation of the renin-angiotensin-aldosterone system (RAAS) occurs as a consequence of diminished cardiac output. One effect of RAAS activation is the retention of salt and water, which serves to expand the intravascular volume. This increase in intravascular volume further increases preload and contributes to progressive ventricular dilation. Elevated ventricular filling pressures together with atrioventricular valve incompetence resulting from distortion of the valvular apparatus associated with ventricular enlargement cause atrial dilation.

In a sense, DCM is an “end-stage heart disease” that represents the end result of virtually any insult to the myocardium. This insult could be a viral infection, a toxin, a metabolic derangement, or a nutritional deficiency. For example, taurine deficiency has been associated with the development of DCM in Cocker Spaniels. In some dogs, myocardial carnitine deficiency may play a role in the pathogenesis of DCM. In addition, antineoplastic agents such as doxorubicin can cause irreversible myocardial dysfunction. Attempts to define a precise genetic or viral cause of canine DCM have not yet been successful. However, the occurrence and distinctive clinical features of familial or breed-associated DCM suggest that there is a genetic basis for many forms of this disease. Although the results of pedigree analysis were not conclusive, it is probable that DCM affecting Great Danes is an X-linked recessive trait. A juvenile-onset form of DCM that has been observed in Portuguese Water Dogs seems to have an autosomal recessive mode of inheritance. Patterns of inheritance have not been reported for other forms of breed-associated DCM. However, the widespread occurrence of DCM in breeds such as the Irish Wolfhound, Newfoundland and Doberman Pinscher provide strong, indirect evidence for a genetic cause or at least, a genetic susceptibility to an as yet unidentified etiologic agent.

Large and giant-breed dogs, including Doberman Pinschers, Great Danes, and Boxers, are most commonly affected. There is a predisposition for males, and dogs with DCM are often middle-aged or older. A 5-year-old male Doberman Pinscher is a typical signalment for DCM.

Although some clinical features are common to all canine patients with DCM, the presentation of DCM in specific breeds is somewhat distinctive. Interestingly, these breed-associated differences in clinical presentation might provide further support for the contention that DCM in purebred dogs has a genetic basis, since it suggests that the disorder is different in genetically distinct populations. This occurrence of breed-specific clinical features might not be anticipated if DCM were in fact a homogeneous disorder that affected large dogs.

Cardiomyopathy in Boxers is characterized by a high incidence of ventricular tachyarrhythmias and sudden death. Harpster classified the manner of presentation of Boxers with cardiomyopathy. Boxers in category 1 have ventricular arrhythmias but do not have associated clinical signs. In category 2 the syndrome manifests as syncope that is presumably related to ventricular tachyarrhythmia. Category 3 is characterized by congestive heart failure (CHF) due to systolic myocardial dysfunction. Progression from one category to the next does not appear to be inevitable, and ventricular tachyarrhythmia in the absence of overt myocardial dysfunction is common in Boxers. The arrhythmogenic form of cardiomyopathy in Boxers has a genetic basis; recent data suggest that it is an autosomal dominant trait, at least in some cases. Because of a presumed genetic basis as well as the electrocardiographic and pathologic characteristics of some Boxers with this disorder, it has been suggested that Boxer cardiomyopathy might be more appropriately described as a form of arrhythmogenic right ventricular cardiomyopathy. Dilated cardiomyopathy as typically observed in other breeds—that is, congestive heart failure due to systolic myocardial dysfunction—is less common in Boxers than are ventricular arrhythmias and might represent a separate and distinct disorder.

There are some similarities between the characteristics of cardiomyopathy in boxers and that in Doberman Pinschers. The incidence of ventricular tachyarrhythmia in affected Dobermans is high, as is the incidence of sudden cardiac death. CHF in Doberman Pinschers with DCM is often associated with a short and rapidly progressive course. The subclinical phase of DCM in Doberman Pinschers is long and, perhaps in contrast to the situation with respect to Boxers, arrhythmias and congestive heart failure appear to be manifestations of a single, progressive disorder. On the basis of pathologic studies, the disorder affects the left ventricle almost exclusively. Probably for this reason, signs of biventricular failure—pleural effusion and ascites in association with pulmonary edema—are relatively uncommon in this breed.

Studies performed in Doberman Pinschers demonstrate that DCM is a slowly progressive, insidious disorder. A long subclinical phase precedes the development of overt myocardial dysfunction; in some dogs, sudden death interrupts this progression. Working independently, O’Grady and Calvert have provided criteria that define subclinical or occult DCM in Doberman Pinschers. Quantitative echocardiographic findings that predict the development of overt myocardial dysfunction or sudden death have been reported. In some Doberman Pinschers, electrocardiographic abnormalities precede the development of myocardial dysfunction. The finding of more than 100 premature ventricular contractions (PVCs) in a 24-hour record of ambulatory electrocardiographic (Holter) monitoring also predicts the development of overt DCM in Dobermans.

Irish Wolfhounds, Great Danes, and Newfoundland dogs with DCM may have pleural effusions as a manifestation of congestive failure more commonly than do other breeds. The prevalence of atrial fibrillation tends to be high in these breeds, although the prevalence might simply reflect their large stature; in general, the propensity to develop and sustain atrial fibrillation depends on cardiac size.

The Cocker Spaniel is the only small-breed dog that commonly has DCM. The syndrome in Cocker Spaniels is further distinguished by low plasma taurine concentrations in affected dogs. Some Cocker Spaniels with DCM also have myocardial carnitine deficiency.

Taurine is an amino acid; the precise function of this nutrient relative to cardiovascular function is not known. However, an association between nutritional taurine deficiency and feline dilated cardiomyopathy was discovered in 1987. Subsequent supplementation of commercial cat foods resulted in a radical decrease in the prevalence of feline dilated cardiomyopathy. Taurine is not an essential amino acid in the dog, and decreased intake alone does not explain low plasma taurine levels in this species; a metabolic defect, perhaps in the ability to synthesize taurine, might explain the low plasma levels in affected dogs.

The Multicenter Spaniel Trial (MUST) was a double-blind clinical trial that compared the effect of nutritional supplementation with taurine and carnitine to placebo in Cocker Spaniels with DCM. All Cocker Spaniels enrolled in the study had low plasma taurine levels. Improvement in echocardiographic parameters was observed in animals that were randomly assigned to receive taurine and carnitine. The investigators were unblinded after 4 months; taurine and carnitine supplementation was then initiated in animals that had received placebo. Ultimately, it was possible to withdraw cardiovascular drugs from all animals that received taurine and carnitine. The improvement in myocardial function was apparently sustained and many of the dogs included in the study ultimately died of extracardiac disease. The importance of carnitine deficiency was not determined. However, it can be stated that taurine deficiency is prevalent in Cocker Spaniels with DCM and that nutritional supplementation with taurine and carnitine is likely to result in substantial clinical improvement. When DCM is observed in Cocker Spaniels, it is advisable to evaluate the plasma taurine level. In dogs in whom this value is low, nutritional supplementation with taurine and possibly carnitine is recommended. If the plasma taurine level cannot be obtained, or a delay in obtaining the result is anticipated, empirical supplementation is suggested.

Clinical findings in a group of dogs with dilated cardiomyopathy and blood or plasma taurine deficiency were recently reported. All of these dogs had been fed a diet that consisted partly of lamb meal, rice, or both. Improved cardiac function was documented after nutritional taurine supplementation. Interestingly, the majority of dogs in this case series were of breeds not known to be predisposed to DCM. Based on this, evaluation of blood or plasma taurine concentration should be considered whenever DCM is observed in atypical breeds.

Animals with DCM are usually presented for evaluation of clinical signs related to CHF. The history and physical examination may reveal dyspnea, cough, abdominal distention due to the presence of ascites, and syncope. In addition, the owner of a dog with DCM may observe exercise intolerance, weight loss, depression, and lack of appetite.

Animals with DCM often have CHF at the time of presentation. Consequently, tachycardia is commonly present and an arrhythmia may be evident on auscultation. Often, but not invariably, there is a murmur due to functional AV valve incompetence that results from distortion of the AV valve apparatus. The murmur is systolic, usually soft, and heard best over the left cardiac apex.

In some affected dogs, audibility of the third heart sound results in a gallop rhythm. Rapid deceleration of early diastolic transmitral flow is the hemodynamic event associated with the occurrence of an S₃ gallop. The rapid deceleration of early diastolic flow is probably related to the presence of a large end-systolic volume, high atrial pressures, and reduced ventricular compliance. In the presence of an audible third heart sound, pulmonary crackles suggest pulmonary edema.

The cardiac silhouette is usually enlarged and most often there is radiographic evidence of left atrial enlargement. When the left atrium is enlarged, pulmonary opacities indicate the presence of pulmonary edema and CHF (Fig. 10-1). It should be noted that on plain chest radiographs the resolution of specific cardiac chambers is limited and the radiographic appearance of DCM is variable. Some affected Doberman pinschers, for example, have minimal radiographic evidence of cardiac enlargement. In some affected dogs of this breed there may be only straightening of the caudal border of the cardiac silhouette, indicating left atrial enlargement, and alveolar pulmonary infiltrates of edema.