Breed-specific variations of cardiomyopathy in dogs

Cardiomyopathy in dogs encompasses a wide spectrum of diseases, ranging from systolic and diastolic dysfunction to various arrhythmias.

Carl Sammarco

There are many sources to review the pathophysiology of cardiomyopathy. This article will discuss the various common breed-specific variations of cardiomyopathies.

Dilated cardiomyopathy can occur in any breed, although it is seen more frequently in large-breed dogs. According to the Veterinary Medical Data Base at Purdue University, breeds affected (number of cases in the United States from January 1986 to December 1991) included: Doberman Pinscher (603), Boxer (131), Great Dane (122), Labrador Retriever (73), American Cocker Spaniel (53), Golden Retriever (42), Irish Wolfhound (38), Saint Bernard (29), Springer Spaniel (25), Newfoundland (22), English Sheepdog (18), Afghan Hound (15), Scottish Deerhound (7) and English Cocker Spaniel (5). Pertinent information about some of the most commonly affected breeds will be discussed.

Doberman

Doberman Pinschers are the most common breed to develop dilated cardio-myopathy and it is suspected to be inherited by an autosomal dominant pattern. Male Doberman Pinschers generally present with heart failure at a younger age than females, but the overall prevalence of the disease is evenly distributed between males and females. The primary cause of their cardiomyopathy is unknown, but many secondary changes in the muscle have been found, including low cardiac muscle L-carnitine, low myoglobin and abnormal mitochondrial function. L-carnitine levels can be low in the muscle with normal plasma concentrations. Oral supplementation produces poor to no response. The disease is most commonly diagnosed from 7 to 10 years of age and dogs diagnosed at 2 years of age and under appear to have an accelerated progression. Progression of disease in dogs diagnosed older than 6 years tends to be prolonged. Dobermans can be identified in the occult stage with electrocardiogram (ECG), echocardiography and Holter monitoring.

The majority of Dobermans with dilated cardiomyopathy will present with signs of poor systolic function (Figure 1). Many will have various forms of atrial and ventricular arrhythmias. If atrial or ventricular tachycardias are present, they may add to symptomatolgy and require primary antiarrhythmic therapy. If just periodic atrial premature or ventricular premature beats are present, anti-arrhythmic therapy may not be needed. A small subset of Dobermans may present with ventricular arrhythmias, particularly ventricular tachycardia, as the primary problem. In these dogs, mild left ventricular abnormalities may be present with a normal left atrium. They present with weakness or syncope due to their arrhythmia and may not be in congestive heart failure. These dogs will need primary anti-arrhythmic therapy and their myocardial disease may progress over time.

Figure 1: Echocardiogram of Doberman with dilated cardiomyopathy. This M-mode was taken from the right parasternal, short axis position. It shows a dilated left ventricle with poor systolic motion. The underlying rhythm is irregularly irregular. The dog is in atrial fibrillation. The ECG can be seen at the bottom of the M-mode in blue.

Therapy for dilated cardiomyopathy with congestive failure is the same for all breeds. Dogs in congestive failure typically are treated with furosemide, an ACE inhibitor and pimobendan. Other therapy such as spironolactone and beta blockers, including carvedilol, theoretically are warranted, but clinical studies that prove these medications will improve survival are lacking. Spironlactone may help prevent potassium depletion in cases with low potassium or in cases where hydrocholorthiazide is added to furosemide to control congestive failure. Beta-blockers may help control rate, especially if atrial fibrillation is present.

Occult DCM usually refers to an apparently healthy Doberman Pinscher with evidence for heart disease on ECG, Holter or echocardiogram. Parameters for the diagnosis of occult DCM (oDCM) in Dobermans have been established. A finding of ventricular premature complexes (VPCs) on a two-minute ECG in a Doberman free of systemic disease, the presence of more then 50 VPCs on a 24-hour Holter recording, or the finding by echocardiogram of left ventricular internal dimension in diastole greater than 4.6 cm and greater then 3.8 cm in systole is suggestive of oDCM. Dobermans diagnosed with oDCM may take 18 to 24 months before developing clinical signs of heart disease.

Therapy for occult DCM is still being evaluated, but early information shows that ACE inhibitors may help prolong the occult period. The addition of beta blockers also may help prolong the occult period, but further data for oDCM in Dobermans is needed.

Boxer

In recent years, cardiomyopathy in Boxers has been termed arrhythmogenic right ventricular cardiomyopathy (ARVC). It is believed that ARVC is inherited by an autosomal dominant trait.

The gene responsible for the defect may code for a protein that stabilizes the cardiac ryandine receptor.

ARVC refers to fatty and fibrofatty infiltration of the right ventricular muscle that can lead to arrhythmias. The most common is ventricular ectopy, including frequent ventricular premature beats or ventricular tachycardia (Figure 2. Some Boxers may present with supraventricular arrhythmias as well. The fatty changes can lead to dilation of the right heart and right heart failure. It is unclear if the classic DCM with poor left-ventricular function is a subset of this disease or a separate disease. Less then 25 percent of Boxers that have ventricular ectopy on a two-minute ECG or a Holter will demonstrate poor left-ventricular function or classic DCM.

Figure 2: This is an ECG from a Boxer that presented with syncope. The salvos of ventricular tachycardia are the large, wide, rapid complexes throughout this strip.

Syncope is a common clinical presentation for the disease, but some dogs may present just with weakness or lethargy. If poor systolic function and secondary atrial enlargement are present, then the predominant symptoms may be due to congestive heart failure. Many dogs may be asymptomatic and evaluation is initiated based on an arrhythmia detected on routine exam or as part of screening for the disease by concerned owners or breeders.

Diagnosis is presumptive, based on changes found on ECG or Holter in the absence of systemic disease. In asymptomatic dogs with ventricular premature complexes (VPCs) on a two-minute ECG, a 24-hour Holter is needed to confirm diagnosis and to help determine if therapy is indicated. In large-breed dogs, there usually are less than 50 VPCs in a 24-hour period. For Boxers, more then 300 VPCs and/or ventricular tachycardia are considered abnormal. If there are between 100 and 300 VPCs with no triplets or pairs of VPCs, then diagnosis is equivocal and a repeat Holter will be needed to confirm or deny the presence of disease. Less then 100 VPCs/24 hours is considered normal for a Boxer.

Therapy is reserved for symptomatic patients with evidence of frequent ventricular salvos (three to six beats of ventricular ectopy) or ventricular tachycardia. Therapies for dogs with arrhythmias alone include sotalol at 1-4 mg/kg PO BID or combination of mexiletine at 5 to 8 mg/kg PO TID with atenolol at 0.25 to 1.0 mg/kg. These medications can be effective in reducing or preventing syncope, but they may not prevent sudden death. Therapy may be initiated in apparently asymptomatic dogs that have frequent VPCs (>3,000 but no pairs, triplets or runs) on a 24-hour Holter. These dogs may have higher energy level with therapy, despite appearing asymptomatic prior to treatment.

Repeat Holter evaluation three to four weeks after starting therapy is recommended. Prognosis is guarded, although many dogs live for years with therapy.

Great Dane

Great Danes typically present with the more classic dilative form of cardiomyopathy. Atrial fibrillation (AF) is a common secondary change in Great Danes with DCM. Great Danes are prone to developing lone atrial fibrillation (LAF) or primary atrial fibrillation (Figure 3). Lone atrial fibrillation occurs without atrial enlargement. Dogs heavier than 80-90 lbs. can develop lone atrial fibrillation. Atrial fibrillation requires short atrial refractory time plus enough tissue area to perpetuate the arrhythmia. Vagal tone shortens atrial refractory time and is naturally high in dogs. These factors make giant-breed dogs susceptible to atrial fibrillation with normal-sized hearts. It is important to distinguish AF in dogs with enlarged hearts versus LAF since both prognosis and therapy differ. Dogs with LAF may require some rate-control therapy with digoxin, a beta-blocker (atenolol, sotalol), diltiazem or a combination of these; enalapril and furosemide may not be needed. Dogs with LAF may live years with or without rate control.

Figure 3: This is an ECG from a Great Dane with atrial fibrillation. This dog was not on medication and had a normal-size heart with good systolic function on echocardiogram. The dog has recently recovered from pneumonia and is now normal. This is an example of lone atrial fibrillation.

American Cocker Spaniel

The more common acquired cardiac disease in American Cocker Spaniels (ACS) is endocardiosis and mitral regurgitation, but they are prone to developing dilated cardiomyopathy. Differentiating an ACS with DCM from one with longstanding mitral regurgitation and secondary myocardial failure can be challenging and usually requires echocardiography. Presenting signs may be similar, but accurate diagnosis is important because some ACS with DCM will respond to taurine and carnitine supplementation. It may take a month to see a clinical response and three to four months to see a change in the echocardiogram parameters. Taurine is used at 500 mg BID and L-Carnitine at 1 gram BID. Other cardiac therapy (furosemide, ACE inhibitors and pimobendan) should be used during this period if needed to control CHF.

Wolfhound

Dilated cardiomyopathy in the Irish Wolfhound is thought to have an auto-somal dominant pattern of inheritance. Taurine levels were evaluated in a group of Irish Wolfhounds in Europe, but the results were equivocal for cause and relationship. Wolfhounds are prone to LAF.

Newfoundland

Dilated cardiomyopathy in the Newfoundland is also suspected to have an autosomal dominant pattern of inheritance. The cause of DCM in Newfoundlands is unknown.

Dalmatians

This breed is prone to both endocardiosis and DCM. There has been a case study showing nine cases with DCM; eight of which were on a low-protein diet. The relationship between diet, taurine levels and heart disease remains unclear. The prescription diet is now supplemented with taurine.

Portuguese Water Dog

Portuguese Water Dogs (PWD) develop a severe juvenile form of cardiomyopathy. Affected puppies die between 1 and 7 months of age due to congestive heart failure secondary to dilated cardiomyopathy. The cause is unknown. There is a genetic marker that can be identified using whole blood. Details for submitting blood can be found at: www.pwdca.org/health. JDCM of PWDs is not responsive to medical management and fulminant congestive heart failure develops rapidly.

Golden Retriever

Golden Retrievers are prone to developing DCM, but it is not a common disorder in this breed. There have been two reports of taurine-deficient DCM in this breed. The etiology remains unknown. Measuring taurine in a dog with DCM is advisable in American Cocker Spaniels, Golden Retrievers, atypical breeds, mixed breeds, dogs being fed a lamb and rice diet or protein restricted diet that is not supplemented with taurine, dogs with cystinuria or a dog that is on a home-made diet.

Dr. Sammarco joined Red Bank Veterinary Hospital, Tinton Falls, N.J., in 2001. He previously served as a lecturer/assistant clinical professor at the University of Pennsylvania, where he completed a residency in cardiology in 1994.