Dilated cardiomyopathy (DCM) is a primary disease of heart tissue that is characterized by cardiac enlargement, usually of one or both of the ventricles, which are two of the four chambers of the heart. Dogs with DCM have abnormal heart muscle fibers, which prevents their hearts from contracting and pumping blood properly. This, in turn, causes one or more of the chambers of the heart to stretch and enlarge, and at the same time causes the walls of those chambers to become abnormally thin. These changes further impair the heart's ability to pump blood normally and adversely affect the overall output or amount of blood that enters circulation with each heart contraction. These abnormalities are called "reduced cardiac contractility" and "reduced cardiac output," respectively. All organs and tissues can be adversely affected by reduced cardiac contractility and output, because of the decreased delivery of well-oxygenated blood to all parts of the body. The lungs and kidneys are especially vulnerable. DCM is often accompanied by congestive heart failure (CHF), heart rhythm abnormalities (arrhythmias), or both. In fact, DCM is one of the most common causes of congestive heart failure in large and giant breed dogs.
Causes of Dilated Cardiomyopathy
The causes of dilated cardiomyopathy (DCM) are not known. The medical term for this is "idiopathic". A number of different things have been suggested as possible contributors to the disorder. These include malnutrition (dietary deficiencies in taurine [a sulfur-containing essential fatty acid] and/or carnitine [a co-enzyme of fatty acid oxydation), immune-mediated abnormalities, viral infection, protozoal infection, microvascular hyperreactivity and genetic disorders involving specific myocardial (heart muscle) structural or contractile proteins. Hypothyroidism and myocarditis have also been associated with DCM in domestic dogs. In rare cases, DCM is accompanied by bacterial infection or exposure to some toxic substance.
There is no reported way to prevent dilated cardiomyopathy from developing in dogs. Given the suspected hereditary component, affected dogs certainly should not be used as part of a responsible breeding program. Many authorities suggest that the parents of dogs with DCM not be bred again – at least not to each other. Some experts recommend that even unaffected littermates of affected animals should not be bred.
DCM is usually seen in middle-aged to older dogs, with the exception of Portuguese Water Dogs, which tend to develop a juvenile version of the disease that becomes apparent very early-on, typically by several weeks to several months of age. DCM is the most common cause of congestive heart failure in large and giant breed dogs. However, it is fairly uncommon in toy and small breeds.
How Dilated Cardiomyopathy Affects Dogs
Many dogs with dilated cardiomyopathy (DCM) seem to be unaffected by their serious heart condition, or at least they often show no outward signs of discomfort or distress. However, when DCM becomes clinical – which means that observable signs of the condition exist – affected animals usually have respiratory difficulties which we can only assume are uncomfortable at best, and extremely painful and frightening at worst.
Symptoms of Dilated Cardiomyopathy
As mentioned above, many dogs with DCM do not show any noticeable symptoms of their disease. When this is the case, they are said to have preclinical dilated cardiomyopathy. When symptoms of the condition do occur, they typically are the same as those associated with congestive heart failure and cardiac arrhythmias (irregular heart beats). These include one or more of the following:
Difficulty breathing (dyspnea; respiratory distress)
Rapid breathing (tachypnea)
Coughing (may or not have blood in the sputum; coughing is especially common at night)
Abnormal heart sounds (murmurs)
Abnormal heart rhythms (arrhythmias)
Abnormal breath/lung sounds (from fluid build-up around the heart and lungs; pleural effusion; pulmonary edema)
Lack of appetite (inappetence; anorexia)
Weight loss (often rapid and marked)
Exercise intolerance (tires easily)
Distended/swollen abdomen (ascites)
Fainting (syncope; temporary suspension of consciousness due to cerebral anemia - lack of blood supply to the brain)
Dogs at Increased Risk
Middle-aged large and giant breed dogs have an increased risk of developing dilated cardiomyopathy. Boxers, Doberman Pinschers, Golden Retrievers, Scottish Deerhounds, Irish Wolfhounds, Newfoundlands, Great Danes, Saint Bernards, Burmese Mountain Dogs, Afghan Hounds, Old English Sheepdogs, Dalmatians, Portuguese Water Dogs, Springer Spaniels, Schnauzers and Cocker Spaniels are predisposed to this disorder, which seems to be more common in males.
How Dilated Cardiomyopathy is Diagnosed
Dilated cardiomyopathy (DCM) often goes undiagnosed, even for many years. Unfortunately, it frequently is only discovered when an affected dog suddenly dies for no apparent reason. Sometimes, DCM is an incidental finding during an otherwise routine physical examination, or when a veterinarian is taking chest radiographs (X-rays) for some other reason and notices an abnormally large cardiac silhouette. The attending veterinarian may hear abnormal heart or lung sounds when listening to the chest of a dog with dilated cardiomyopathy during an annual veterinary examination, depending upon the severity of the particular dog's disease. When a veterinarian (or a human doctor, for that matter) listens to heart and lung sounds through a stethoscope, the process is referred to as "auscultation."
The results of a urinalysis and routine blood work (a complete blood count and serum biochemistry profile) typically are unremarkable in dogs with DCM, unless kidney function has been significantly compromised. Thoracic radiographs (chest X-rays) may or may not disclose an enlarged heart or fluid build-up around the heart and lungs – referred to as pleural effusion and pulmonary edema - which are consistent with congestive heart failure. Frequently, the left ventricle and atrium are the most obviously enlarged heart chambers visible on chest radiographs, especially in the early stages of disease. Doberman Pinschers in particular usually have marked enlargement of their left atrium with patchy, diffuse radiographic evidence of pulmonary edema. Liver enlargement (hepatomegaly) may be identified on radiographs taken of the abdomen.
The gold standard for diagnosing DCM is echocardiography, which essentially is a highly sophisticated ultrasound of the heart. This is a completely noninvasive procedure, but it does usually require some degree of sedation so that the animal remains quiet and still. This technology is usually not available in a general veterinary practice; specialized referral centers and veterinary teaching hospitals usually have veterinary cardiologists on staff with the training and equipment necessary to perform an echocardiogram. An echocardiogram can reveal ventricular and atrial enlargement (dilation), as well as abnormal regurgitation of blood through the valves that separate the four heart chambers. An electrocardiogram can also identify erratic heart rhythms (arrhythmias). Atrial fibrillation and ventricular tachycardia are extremely common arrhythmias in Doberman Pinschers. Boxers more commonly have isolated ventricular arrhythmias that can be seen using echocardiography.
In most cases, canine dilated cardiomyopathy is a progressive disease that worsens with time. It is not particularly difficult to diagnose. Unfortunately, it cannot be cured and almost always is fatal.
All but the most severely affected dogs with dilated cardiomyopathy (DCM) usually can be treated on an outpatient basis, meaning that they rarely require hospitalization. The therapeutic goals for treating DCM are to resolve the signs of any associated congestive heart failure, alleviate any build-up of fluid in the lungs, chest and/or abdomen, improve the force of heart muscle contractions, control life-threatening irregular heart rhythms, relieve the dog's discomfort and improve the dog's overall quality of life. Unfortunately, cure of DCM is not possible.
Dogs in acute respiratory distress can be given oxygen in the veterinary clinic either in an oxygen cage or through a mask, depending upon their size. They should be kept in a warm, quiet environment and administered intravenous fluids to correct any electrolyte or fluid imbalances, if necessary. A number of diuretic medications can be prescribed to relieve pleural effusion and/or pulmonary edema if they are present. Diuretics are medications that increase urine excretion. Fluid can also be drained from the thoracic (chest) cavity using a procedure called thoracocentesis. This is a somewhat invasive procedure that involves inserting a sterile needle into the chest cavity and basically suctioning off excess fluid through an attanched syringe. Many drugs, including vasodilators, ACE inhibitors, beta blockers and calcium channel blockers, are available to help stabilize and regulate heart contractility, activity and function. Owners of dogs with DCM may be referred to a specialized board-certified veterinary cardiologist for specific treatment protocols and follow-up.
Affected dogs usually should be allowed to select their own level of activity and exercise throughout the day. They should be fed a diet that is low in sodium, although extreme sodium restriction usually is not necessary if the dog responds well to pharmaceutical management. The treating veterinarian can recommend a good commercially available diet. Prescription diets are also available.
Unfortunately, dilated cardiomyopathy is almost always fatal within 6 to 24 months after the date of diagnosis. Many affected animals live with the condition without ever showing any apparent symptoms, and then simply drop dead without warning. Doberman Pinschers usually have the gravest prognosis, living on average less than 6 months following diagnosis of their disease. Dogs with severe heart rhythm abnormalities, such as atrial fibrillation and ventricular tachycardia, often have a very short survival time and may die suddenly without any prior warning.