Hypertrophic cardiomyopathy (HCM) is a form of heart disease that is characterized by concentric enlargement and thickening (hypertrophy), primarily of the walls of the left ventricle, which is one of the 4 chambers of the heart (left and right ventricles, left and right atrias). As the ventricular walls thicken, they become less flexible and increasingly rigid, causing increased resistance to filling and reducing the normal outflow of blood. HCM is common in cats and uncommon in dogs. However, it does occasionally occur in dogs.
Causes of Hypertrophic Cardiomyopathy
Most authorities believe that there is a hereditary component to hypertrophic cardiomyopathy (HCM), although the exact nature of the genetic abnormality is not known. Several different disease processes may together lead to HCM. Whatever the underlying cause, animals with hypertrophic cardiomyopathy have an inherent defect in certain heart muscle cells that are primarily located within the walls of the left ventricle. The walls of the ventricle thicken and become stiffer than normal. This allows blood to flow backwards into the left atrium and pulmonary vein rather than flowing forward into circulation. As a result, fluid builds up in and around the heart and lungs (pleural effusion and pulmonary edema). In severe cases, the blood becomes sluggish and can even become stagnant, increasing the risk of arterial thromboembolisms developing. In addition, the sluggish blood flow is unable to transport a sufficient amount of oxygen and other nutrients to bodily tissues.
Until the actual cause of hypertrophic cardiomyopathy is determined, there is no realistic way to prevent the disorder. In light of the suspected genetic component, dogs that have this condition probably should not be used for breeding, and their parents should be checked to determine whether they also have HCM.
Fortunately, hypertrophic cardiomyopathy is a fairly rare condition in domestic dogs. A different type of heart disease, known as dilated cardiomyopathy, is much more common in dogs.
How Hypertrophic Cardiomyopathy Affects Dogs
Hypertrophic cardiomyopathy (HCM), which is the most common heart disease seen in cats, is not frequently seen in domestic dogs. When it does occur in dogs, it may not be accompanied by any noticeable symptoms and may only be detected incidentally during a routine veterinary check-up. This suggests that the dog probably is not suffering substantial discomfort or distress due to its condition. Of course, some dogs are particularly stoic and may be feeling poorly without showing signs of their ailment.
Symptoms of Hypertrophic Cardiomyopathy
When dogs do develop clinical signs associated with hypertrophic cardiomyopathy, they typically include one or more of the following:
Increased respiratory rate (tachypnea; rapid breathing)
Difficulty breathing (dyspnea; respiratory distress)
Open mouth breathing (panting)
Lack of appetite (inappetence; anorexia)
Acute onset of hindlimb lameness (+/-pain)
Exercise intolerance (tires easily)
Dogs at Increased Risk
Hypertrophic cardiomyopathy is most frequently seen in young to middle-aged large breed dogs, but it still is an uncommon canine disorder. Males seem to be affected more often than females, although both genders are susceptible. Rottweilers, Dalmatians, German Shepherd Dogs, Boston Terriers and certain Pointers reportedly have an increased risk of developing HCM. The reasons for these apparent associations are not clear.
How Hypertrophic Cardiomyopathy is Diagnosed
Hypertrophic cardiomyopathy almost always involves stiffening and thickening of the walls of the left ventricle. To diagnose this disease, other potential causes of left ventricular thickening and rigidity must first be ruled out. This is called making a diagnosis on the basis of exclusion. The other conditions that can cause ventricular stiffness include hyperthyroidism, subaortic stenosis (SAS), acromegaly and systemic hypertension (high blood pressure). The best way to identify physical abnormalities in one or more heart chambers is through a technique known as echocardiography. An echocardiogram basically is an ultrasound examination of the heart and surrounding tissues. It is completely painless and noninvasive. However, mild to moderate sedation usually is necessary during the procedure, because the animal must lie still to ensure accurate results. In addition to ventricular thickening, an echocardiogram can also disclose outflow obstruction, abnormal blood turbulence, blood pooling (stagnation) and mitral valve regurgitation – all abnormalities that may be present in dogs with hypertrophic cardiomyopathy.
Auscultation of the heart (listening to heart sounds through a stethoscope) may reveal heart murmurs or other abnormal heart sounds in affected animals. An electrocardiogram (ECG) can also be helpful in the diagnosis of HCM. An ECG measures the rhythm, rate and electrical activity of the heart. The results of a urinalysis and routine blood work (a complete blood count and serum biochemistry profile) usually are unremarkable if the dog's only medical condition is hypertrophic cardimyopathy. Blood work to assess functional thyroid hormone levels (called a thyroid panel), together with measurement of systemic blood pressure, both are important to rule out other potential causes of ventricular thickening. Thoracic radiographs (chest X-rays) may or may not be helpful. Sometimes they can be useful to disclose whether the left atrium has enlarged from the backflow and pooling of blood from the left ventricle.
Some of the medications used to treat heart disease in domestic animals can have adverse side effects on the kidneys, liver or other organs. Regular monitoring of renal (kidney) function is important if affected animals are being treated for heart disease. Usually, this only involves a simple blood test and is relatively inexpensive.
The goals of treating hypertrophic cardiomyopathy (HCM) are to reduce any fluid build-up around the heart and lungs (pleural effusion and pulmonary edema), stabilize the dog's heart rhythms, relieve the dog's discomfort and restore a good quality of life. A number of different pharmaceuticals are available to help manage hypertrophic cardiomyopathy. Drugs known as beta blockers, ACE inhibitors, vasodilators and calcium channel blockers can help to reduce or stabilize a dog's heart rate and rhythm and prolong ventricular filling time. Other drugs, including diuretics, are available to help if signs of congestive heart failure are present. If the dog has an abnormal build-up of fluid in its chest, a procedure called thoracocentesis can be performed to remove some of that fluid and help normalize breathing. This involves inserting a sterile needle through the thoracic (chest) wall and drawing off the fluid through an attached syringe. Oxygen therapy, either in an oxygen cage or through a mask, may be appropriate in some cases. Intravenous fluids can be administered in the veterinary hospital if necessary to correct fluid or electrolyte imbalances.
Dogs with HCM should not be placed under undue stress. Their diet should be low in salt. Some experts recommend giving an anticoagulant drug – such as one human baby aspirin every 3 days - to try to prevent the development of arterial thromboembolisms. However, the efficacy of this protocol has not been proven in dogs, and owners should consult with their veterinarian before starting any type of drug therapy for their pet.
The prognosis for dogs with HCM is highly variable. In mild, nonprogressive cases with no outward clinical signs of disease, the outlook is fair to good. However, once the left ventricular wall becomes severely thickened, with accompanying left atrial enlargement, pleural effusion and pulmonary edema, the prognosis is guarded to poor. If the dog develops thromboembolisms, the chance of long-term survival is grave.