We are all familiar with the pathophysiology—myocardial disease results in a progressive decrease in myocardial contractility and ventricular dilation. The cause in many cases has not been determined and is, thus, considered to be idiopathic.
In a presentation on dilated cardiomyopathy (DCM) at the CVC in San Diego, Barret Bulmer, DVM, MS, DACVIM (cardiology), said that although the likely cause of this form of cardiomyopathy is genetically mediated, there are some conditions that will produce a secondary form of cardiomyopathy that must be ruled out before you can diagnose DCM. And you must differentiate other cardiac diseases that can manifest similar clinical signs. This article reviews the current diagnostic tests for confirming the disease and shares breed differences that may influence your clinical outcome.
Deliberate on those differentials
Since DCM is a diagnosis of exclusion, Dr. Bulmer says you must rule out the following conditions:
• Heartworm disease
• Pericardial disease
• Degenerative valvular disease
• Taurine insufficiency
• Doxorubicin toxicosis
• Primary arrhythmia (e.g. persistent supraventricular tachycardia) or congenital conduction abnormality (accessory pathway) that contributes to a DCM-like state
Heartworm disease, pericardial disease and degenerative valvular disease can produce heart failure similar to DCM, says Dr. Bulmer. Taurine insufficiency has been seen in cocker spaniels, Newfoundlands and some Doberman pinschers, and there is a potential for a response to supplementation.1 A persistent primary tachyarrhythmia can produce a DCM phenotype. If a primary tachyarrhythmia is the driving force behind myocardial dysfunction, control or resolution of the arrhythmia may help the heart revert back to normal.
What breeds, what signs?
Great Danes, Doberman pinschers, Irish wolfhounds, bullmastiffs, boxers and cocker spaniels are the breeds that most commonly develop DCM. Dr. Bulmer says the classic pathologic change for DCM in large-breed dogs includes a wavy fiber form or attenuation of normal myocardial fibers. This results in cardiac dilation and poor systolic function. Boxers and some Doberman pinschers have a fibrofatty form with adipose and/or fibrous tissue infiltration into the myocardium at a microscopic level that may affect the clinical outcome.
According to Dr. Bulmer, the history and clinical signs will vary since these dogs don’t have classic auscultatory findings. It is difficult to truly suspect the disease before a dog comes into the hospital with clinical signs. He says the initial clinical presentation will vary as well. Some dogs will present with weakness or exercise intolerance. Some dogs present with left-sided heart failure exhibited by coughing, dyspnea or tachypnea. Others will present with abdominal distention and perceived weight gain due to right-sided heart failure. Some boxers or Doberman pinschers will have syncopal episodes, and some may die suddenly without any preceding clinical signs. Other breeds will present with anorexia and weight loss from cardiac cachexia and end-stage cardiac disease.
DCM is different from valvular disease in that these dogs don’t have a loud murmur before they develop clinical signs. There may be a gallop sound prior to an actual murmur, says Dr. Bulmer. A gallop is a triple cadence, low-frequency, extra sound and not an arrhythmia. With DCM, the gallop is usually an S3 sound associated with dilation or eccentric hypertrophy. As the ventricles dilate, they may distort the mitral and/or tricuspid valve annulus, thereby producing mitral or tricuspid insufficiency. A grade II to III/VI systolic left apical murmur can be auscultated in some large- or giant-breed dogs with DCM. In Doberman pinschers with occult DCM but not in heart failure, common auscultatory findings can include arrhythmias (most commonly ventricular premature complexes [VPCs]), a gallop sound, and/or a murmur.
Dig into the diagnostics
Dr. Bulmer says the top three diagnostic tests for DCM are:
• An electrocardiogram (ECG)
An ECG is a requirement because of the high prevalence of arrhythmias in this disease process. Dr. Bulmer says radiographs are also critical as they are the only way to identify left-sided heart failure and assess pulmonary vasculature. Cardiac dilation related to eccentric hypertrophy is also often visible on radiographs. “In the appropriate context, echocardiography is often a conformational test for DCM,” says Dr. Bulmer, “although it does enable exclusion of other forms of cardiac disease that may negatively influence prognosis or warrant further therapeutic considerations.”
Dr. Bulmer says to look for these on an ECG: Left atrial enlargement may cause wide P waves. Left ventricular enlargement may cause tall R waves and wide QRS complexes. The ECG is more important for detection of arrhythmias. Doberman pinschers and boxers will classically have VPCs. Giant breeds, such as Great Danes, bullmastiffs and Irish wolfhounds, more commonly present with atrial fibrillation.
“Assessment of the frequency and severity of VPCs often requires a Holter monitor, as does evaluation of therapeutic efficacy of commonly used antiarrhythmics,” says Dr. Bulmer. Atrial fibrillation is sustained, so a Holter monitor is not warranted for diagnosis, although in some instances it can be useful for evaluation of heart rate control for management of atrial fibrillation.
Echocardiography in dogs with DCM will show dilation and poor systolic function often in all four chambers, says Dr. Bulmer. It is possible to see a small volume of pericardial fluid in dogs with heart failure. To assess severity of DCM on an echocardiogram, Dr. Bulmer says you will need to focus on left ventricular dimensions, systolic function and estimated left atrial pressures. As the dog transitions into congestive heart failure, you will need to monitor the lungs with radiographs, with the goal of keeping the lungs clear of fluid. Radiographs are the best diagnostic tool at this stage, says Dr. Bulmer.
There are other diagnostics tests of importance. Dr. Bulmer says it is important to monitor renal values and electrolytes, specifically potassium. These laboratory values are important as many of the medications used to treat congestive heart failure require adequate kidney function, and they can negatively impact the renal values and electrolytes.
Genetic testing is available for a deletion mutation with the striatin gene.2 When the test for the mutation is negative, it doesn’t mean the dog won’t develop the disease, says Dr. Bulmer. But boxers, particularly with the homozygous genotype, have a high likelihood to develop phenotypic expression of DCM.3
“It’s my parents’ fault!”: Breed-specific presentations
Classic dilated cardiomyopathy. This form involves Great Danes, Irish wolfhounds and bullmastiffs—the giant breeds. Dr. Bulmer says that if they present with clinical signs, they often have atrial fibrillation. Right-sided heart failure is more predominant than left-sided heart failure. These dogs may battle weight loss and inappetence in some cases more than other breeds. In Dr. Bulmer’s experience, they have more favorable prognosis with treatment and often live for about a year even when considered to be in congestive heart failure. These breeds are much more likely to respond to treatment than Doberman pinschers or boxers.
Doberman DCM. Doberman pinschers with DCM have a relatively high prevalence of arrhythmia and sudden death. Affected dogs typically present with left-sided heart failure. Initially they present with a good body condition score and little muscle wasting, but they can exhibit weight loss and muscle wasting over time, even with treatment, says Dr. Bulmer. Typical survival time with treatment for Doberman pinschers with congestive heart failure is often four to six months. Although younger dogs may have the disease, Dr. Bulmer advises screening Doberman pinschers starting at 4 years of age with echocardiography and a Holter monitor to establish a baseline for future monitoring.
Boxer DCM. Boxer cardiomyopathy is more often called arrhythmogenic right ventricular cardiomyopathy (ARVC). While some boxers with ARVC exhibit the DCM phenotype with development of congestive heart failure, others may have normal cardiac dimensions and systolic function yet have severe ventricular arrhythmias that contribute to a risk of syncope or sudden death. “In many instances both echocardiography and Holter monitoring are required to screen and manage boxers at risk for or with known heart disease,” says Dr. Bulmer “Even asymptomatic boxers can have VPCs. Similarly VPCs can be related to diseases other than ARVC including infectious disease, metabolic disease or neoplasia.”
A final important consideration is that a syncopal boxer doesn’t always constitute VPCs or ventricular tachycardia as a cause for the syncope, says Dr. Bulmer. “Some Boxers may have vasovagal syncope, hemodynamically important bradyarrhythmias, or central nervous system, metabolic, or musculoskeletal disease. Therefore other differentials may need to be excluded.”
Cocker spaniel DCM. As a breed, cocker spaniels are more likely to have valvular heart disease than DCM. But for those that do have DCM, they often respond well to treatment, says Dr. Bulmer.4
Inducing a change of heart
Treatment of DCM is directed toward reducing the clinical signs of heart failure and improving survival time. “Although in most dogs these goals can be achieved for a period of time, there is typically no cure, and most dogs with DCM and congestive heart failure will ultimately succumb to their disease,” says Dr. Bulmer. Diuretics aid in the control of pulmonary edema, angiotensin-converting enzyme inhibitors reduce activation of the renin-angiotensin-aldosterone system to try to augment survival, and positive inotropes are used to improve systolic function. Antiarrhythmic drugs may also be required to manage ventricular or supraventricular arrhythmias. “Care must be used in both the decision to use antiarrhythmics along with which antiarrhythmic to prescribe because of their negative inotropic properties,” says Dr. Bulmer.
The breed differences outlined here can influence what further diagnostic tests are needed and the prognosis for the patient. But, in summary, early detection of the disease through the various diagnostic tests discussed is the best means to prolong a good quality of life for our patients with DCM.
1. Kittleson MD, Keene B, Pion PD, et al. Results of the multicenter spaniel trial (MUST): taurine- and carnitine-responsive dilated cardiomyopathy in American cocker spaniels with decreased plasma taurine concentration. J Vet Intern Med 1997;11(4):204-211.
2. Meurs KM, Mauceli E, Lahmers S, et al. Genome-wide association identifies a deletion in the 3’ untranslated region of striatin in a canine model of arrhythmogenic right ventricular cardiomyopathy. Hum Genet 2010;128:315-324.
3. Meurs KM, Stern JA, Sisson DD, et al. Association of dilated cardiomyopathy with the striatin mutation genotype in boxer dogs. J Vet Intern Med 2013;27:1437-1440.
4. Fuentes VL, Corcoran B, French A, et al. A double-blind, randomized, placebo-controlled study of pimobendan in dogs with dilated cardiomyopathy. J Vet Intern Med 2002;16:255-261.