Clinical Presentation

DCM typically is an adult-onset disease of large- and giant-breed dogs, with the greatest prevalence in Doberman pinschers, Irish wolfhounds, Great Danes, and Newfoundlands. Other breeds that may be overrepresented include Scottish deerhounds, dalmatians, German shepherds, Saint Bernards, Airedales, standard poodles, and Old English sheepdogs. The disease generally is rare in small- and medium-breed dogs with the exception of American and English cocker spaniels, and juvenile forms are recognized in Portuguese water dogs and the toy Manchester terrier. Although historically boxers have been listed among the breeds predisposed to DCM, they most likely share a unique and familial disease process more accurately classified as arrhythmogenic right ventricular cardiomyopathy (see Chapter 179). Most studies have demonstrated an increased prevalence in male dogs compared with females. Most cases are diagnosed in dogs aged 4 to 9 years, and the incidence increases with age. However, it may be diagnosed in dogs as young as 2 years (and infrequently in even younger dogs). The specific juvenile form of DCM affecting Portuguese water dogs typically develops within the first 6 months of life.

As noted earlier, DCM is characterized by a prolonged asymptomatic phase, referred to as preclinical or occult DCM, which may last for up to 2 to 4 years. Once clinical signs develop, the disease is referred to as overt DCM. Clinical manifestations are attributable to left ventricular dysfunction, heart rhythm disturbances, or both. Initial clinical signs can be subtle, such as exercise intolerance or weight loss, and may go unrecognized except in athletic or working dogs. Most commonly, DCM is recognized when clinical signs of CHF develop. Left-sided CHF signs usually predominate and include cough, tachypnea, and dyspnea; with right-sided or biventricular CHF, ascites and pleural effusion also may occur. Ventricular arrhythmias can cause syncope and even sudden death. These may precede any other signs, and sudden cardiac death is especially common in Doberman pinschers compared with other breeds.

There is an interest in identifying and often treating DCM in the preclinical or occult phase, and a number of screening programs have been suggested, especially for breeding animals. Both ambulatory electrocardiographic (ECG), or Holter, monitoring and echocardiographic methods have been used to identify early disease in animals at risk. Some suggest that members of predisposed breeds should be screened annually beginning at 2 years of age. Early work suggests that the cardiac biomarker N-terminal prohormone B-type natriuretic peptide (NT-proBNP) may have some ability to detect preclinical disease, but further studies are needed to define the precise usefulness of this screening modality (Wess et al, 2011). In general, the adult onset of the disease makes genetic counseling to promote breeding of unaffected animals a significant challenge.

Physical Examination

Cardiac auscultation may reveal a low-grade systolic left apical murmur caused by mitral regurgitation, an S₃ gallop caused by increased left ventricular filling pressure, or arrhythmias. Soft heart sounds sometimes are detected, related to impaired ventricular contractility even in the absence of overt effusions. Similarly, murmurs of mitral regurgitation often are softer than those encountered in primary degenerative valvular disease of dogs.

Weak femoral pulses from reduced ventricular ejection or pulse deficits associated with arrhythmias may be palpated. With left-sided CHF, increased bronchovesicular sounds or pulmonary crackles may be auscultated, but the lack of these does not rule out the presence of pulmonary edema. Ascites, hepatomegaly, jugular venous distension or pulsation, and muffled lung sounds caused by pleural effusion may be present with right-sided or biventricular CHF.

Echocardiography

Echocardiography is the test of choice for the diagnosis of DCM in both the occult and overt phases. The diagnosis is established by identifying primary left ventricular systolic dysfunction, often with evidence of cardiac remodeling. Comprehensive echocardiography with Doppler examination offers a combined assessment of disease severity, ventricular and secondary valvular dysfunction, and estimated ventricular filling pressures (indicating risk of CHF). Although echocardiography also is used as the main screening test for DCM, distinguishing between early DCM and systolic function at the lower end of normal variation is a challenge. Frequently, divergent results for left ventricular systolic function are obtained when different echocardiographic indices are used, and in these cases, serial examinations can be important for identifying progressive trends. Additionally, a normal echocardiogram does not rule out the future development of DCM; thus annual screening of dogs of predisposed breeds is recommended.

On two-dimensional or M-mode echocardiography, left ventricular, and sometimes right ventricular, chamber size is increased in both systole and diastole as assessed by left ventricular internal diameters at end systole and end diastole or by calculated end-systolic and end-diastolic left ventricular volumes. These measurements must be compared with established normal ranges based on body weight, for specific breeds when available, or indexed to body surface area. Recently, Wess and colleagues (2010) reported that in Doberman pinschers, an end-systolic and end-diastolic volume indexed to body surface area of more than 55 ml/m² and 95 ml/m², respectively, was superior to standard M-mode measurements for detection of occult DCM. Decreases in indices of left ventricular function, including fractional shortening and calculated ejection fraction, also are observed, but the diagnosis of DCM should not be based solely on a reduced fractional shortening or ejection fraction without assessment of the left ventricular chamber sizes in systole and diastole individually. Importantly, there is no single value for left ventricular shortening fraction that is both sensitive and specific for DCM in all breeds. Although the atria may be normal in size with occult DCM, the left atrium, and sometimes the right atrium, often is enlarged, and this is a consistent feature of overt DCM with CHF.

Doppler echocardiography often demonstrates a central jet of mitral regurgitation, which occurs because of mitral annular dilation and papillary muscle displacement. The mitral coaptation point often is displaced apically from the annulus in DCM (increased valve tenting). So-called secondary mitral regurgitation due to DCM sometimes can be differentiated from primary mitral valve degeneration with systolic dysfunction by imaging findings: a degenerative valve is typically thickened; one or both leaflets are likely to prolapse into the left atrium; and the jet of regurgitation typically is eccentric. Many dogs with DCM also develop Doppler imaging evidence of diastolic dysfunction, which may be demonstrated on transmitral flow. Experienced examiners generally look for evidence of impaired relaxation, indicating left ventricular dysfunction, but a restrictive transmitral filling pattern is more often associated with CHF. The latter finding is an important negative prognostic indicator. Recently several indices of ventricular filling pressure derived from Doppler and tissue Doppler imaging also have been shown to correlate with the presence and resolution of CHF in dogs with DCM (Schober et al, 2010, 2011). Other tissue Doppler imaging–derived indices such as tissue velocity, strain, and strain rate may be useful in assessing left ventricular systolic and diastolic function with perhaps more sensitivity than M-mode and two-dimensional indices, but there is not yet widespread acceptance of or data regarding these variables.