Dilated cardiomyopathy: Look for multiple disease entities

Q. Please review dilated cardiomyopathy in the dog.

A. Dr. Keith N. Strickland at the 77th Western Veterinary Conference in Las Vegas gave a lecture on canine dilated cardiomyopathy. Some relevant points in this lecture are provided below.

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Dilated cardiomyopathy (DCM) is an idiopathic disease of the myocardium that is characterized by progressive myo-cardial failure (pathologic decreases in contractility). The progressive myocardial failure results in enlargement of the cardiac chambers (sometimes one side worse than the other) in association with the compensatory mechanisms (renin-angiotensin-aldosterone system, sympathetic nervous system, and eccentric ventricular hypertrophy) that become activated in response to sustained cardiac dysfunction. This disease scenario is likely to be multiple diseases that are lumped into a group of idiopathic diseases that result in progressive myocardial failure, congestive heart failure (CHF), and occasionally, sudden death.

Although heartworm disease may be the second most common cardiac disease in some U.S. regions, DCM is the second most commonly encountered cardiac disease in dogs.

In general, DCM is seen in middle-age and older (median age is between 4-8 years) large- and giant-breed dogs. Males seem to be affected more commonly than females. Breeds that are predisposed for this condition include Doberman Pinscher, Irish Wolfhound, Great Dane, Newfoundland, German Shepherd, Old English Sheepdog, Boxer, St. Bernard and Afghan Hound. Medium-size breeds may also be affected and include the Portuguese Water Dog, American and English Cocker Spaniel and Dalmatian. Geographic variations in the prevalence of this disease may suggest genetic and/or environmental factors that may significantly increase the risk of developing this condition. Recently, a potential abnormality in the protein titin has been identified as a potential cause of DCM in Doberman Pinschers. DCM is uncommon in small-breed dogs. DCM occasionally is recognized in puppies as young has 6 weeks of age.

Pathophysiology

Marked dilation of the cardiac chambers is the most common finding at necropsy. The cause of this progressive myocardial failure is not known. In general, microscopic evaluation of the myo-cardium reveals moderate changes that are nonspecific. There are areas of myo-cyte degeneration, necrosis and fibrosis most pronounced in the subendocardial regions of the left ventricular free wall and papillary muscles. The cause of DCM in dogs is not known. There are many potential causes ranging from specific genetic mutations, biochemical abnormalities and nutritional deficiencies to immune-mediated causes.

The progression of the disease can be described as having three phases:

• Phase 1: Initiation of injury or defect,

• Phase 2: Physiologic and genetic response to myocardial dysfunction,

• Phase 3: Hemodynamic and clinical syndrome of congestive heart failure.

During Phase 2, cytokines (e.g., tumor necrosis factor-alpha and interleukins) and other neurohumoral substances (e.g., renin-angiotensin-aldosterone system, endothelin, antidiuretic hormone and the sympathetic nervous system) promote pathologic myocardial hypertrophy and ongoing loss of contractile mass.

Chamber dilatation results in secondary atrioventricular valvular regurgitation that further volume-overloads the failing ventricle. Dogs with symptomatic myocardial failure have elevated diastolic pressures resulting in venous congestion and edema formation. This clinical scenario is further complicated by the presence of arrhythmias. Atrial fibrillation is a common cause of decompensation in dogs with DCM. Atrial fibrillation results in a significant decrease in cardiac output and an increase in ventricular filling pressures. Ventricular arrhythmias may cause syncope in some dogs and sudden cardiac death in up to 25-30 percent. Ventricular arrhythmias can be particularly problematic in Boxers and Doberman Pinschers.

Clinical evaluation

The clinical evaluation of DCM starts with the signalment. It is uncommon to diagnose DCM in small-breed dogs. A history of syncope, exercise intolerance, coughing or tachypnea/dyspnea may be noted. Physical examination may reveal weak femoral pulses, cardiac arrhythmias with pulse deficits, soft cardiac murmurs and gallop rhythms. Gallop rhythms are best heard at the left apex with the low frequency part of the stethoscope with light pressure on the body wall. An S3 gallop rhythm suggests severe volume overload and elevated ventricular filling pressures. Presence of jugular pulses and ascites are suggestive of right-sided CHF.

The ECG may be normal or may demonstrate a variety of rhythm disturbances and chamber enlargement/conduction abnormality patterns.

Atrial fibrillation, characterized by a rapid, irregularly irregular narrow complex tachyarrhythmia with no P-waves, is common in dogs with DCM. Premature ventricular extrasystoles (VPCs) are common in dogs with DCM and may be associated with an increased risk of sudden death (depending on the severity and complexity of the arrhythmia). If left atrial enlargement is present, the P-wave may be wide (P-mitrale). Increased R-wave amplitude indicates left ventricular enlargement. A right axis shift may indicate a conduction abnormality or cardiomyopathy primarily effecting the right side of the heart.

Radiography

Thoracic radiographs are useful for detecting and assessing the presence of congestive heart failure. DCM can be associated with left-sided CHF (pulmonary edema), biventricular failure (combination of pleural effusion and pulmonary edema), or right-sided CHF (ascites). Radiographic CHF is usually associated with cardiomegaly and venous congestion. Advanced DCM is unlikely if cardiomegaly is not present, although Doberman Pinschers may appear to have less cardiomegaly than other breeds with severe DCM.

Echocardiographic evaluation of dogs with DCM reveals varying degrees of myocardial dysfunction as evidenced by a decreased fractional shortening, increased end-systolic volume index and reduced ejection fraction. Hemodynamically significant chronic myocardial failure results in chamber enlargement (eccentric hypertrophy); therefore, allowing for larger volumes without significant increases in diastolic pressures.

Atrioventricular valvular insufficiency is common in cases with moderate to severe chamber enlargement. The jet of mitral regurgitation is typically centrally located in this scenario as compared to the typical eccentric jet seen with degenerative valve disease.

Therapy

Therapy for DCM involves the use of drugs to relieve the signs of congestion, improve myocardial function, and blunt the compensatory mechanisms that continue to overload the failing heart. There is some suggestion that early administration of ACE inhibitors may delay the onset of clinical signs in dogs with occult DCM. Conventional therapy for symptomatic DCM involves the use of ACE inhibitors (enalapril or benazepril), diuretics (furosemide and spironolactone), and positive inotropic drugs (digoxin). Other therapies include the use of beta blockers (carvedilol), positive inotropic vasodilators (pimobendan), and nutritional supplementation (L-carnitine, taurine and omega-3 fatty acids). Although not readily available in the United States at this time (is available to veterinarians in Canada), pimobendan has been associated with significant increases in survival times for dogs with DCM. Unfortunately, the long-term prognosis for dogs with DCM is guarded with a greater than 90 percent mortality rate at one year after initial diagnosis.

Dr. Hoskins is owner of DocuTech Services. He is a diplomate of the American College of Veterinary Internal Medicine with specialities in small animal pediatrics. He can be reached at (225) 955-3252, fax: (214) 242-2200, or e-mail: jdhoskins@mindspring.com.