Heart disease, a very common health concern for aging dogs, is a relatively rare problem in cats. While dogs with advanced heart disease exhibit obvious symptoms, the most common form of feline heart disease, cardiomyopathy, is often a silent killer of cats. Heart disease in general is usually a heritable condition, and cats who acquire Hypertrophic Cardiomyopathy (HCM) are thought to have a genetic predisposition. Breeds known to be at an increased risk for developing HCM include Maine Coon cats, Oriental breeds, Ragdolls, and Bengals. Nonetheless, and probably due to the fact that purebred cats constitute a very small percentage of housecats, the Domestic Short Hair (mongrel) is the most commonly diagnosed breed of cat with HCM.
The term Hypertrophic Cardiomyopathy simply means “disease of a thickened heart muscle”. In dogs, Dilatative Cardiomyopathy (DCM) causes the overall size of the heart to increase as the chambers increase in volume due to thinning of the heart muscle. HCM, which only very rarely affects dogs, instead causes the cat’s heart muscle to become thickened while the chamber volume decreases. On a chest x-ray, DCM is apparent, but HCM may show little or no visible enlargement of the cardiac silhouette.
Furthermore, while dogs will develop a cough and other prominent symptoms as the heart disease progresses, this is seldom true for cats. Unfortunately, the first symptom seen in cats that develop HCM may be collapse and sudden death. Canine heart disease eventually progresses into congestive heart failure. Symptoms are caused by fluid leaking into the lungs or abdominal cavity due to heart muscle insufficiency. Hypertrophic Cardiomyopathy can develop into congestive heart disease as well, but cats often succumb to its earlier effects. As the HCM worsens, an abnormal heart rhythm can result in sudden death. Equally dire, the cat may suffer a thromboembolism, a blood clot that dislodges from the heart and occludes blood flow into the rear limbs.
Lacking overt symptoms, cats are more easily discovered if a heart murmur is heard with a stethoscope during a routine wellness exam. Heart murmurs do not occur in all cases of HCM. Mild symptoms may go unnoticed by the cats owner and not be reported to the veterinarian. Slightly decreased appetite, weight loss, or mildly increased respiratory effort are all easy symptoms to miss.
There is unfortunately no simple test available to screen for HCM in cats*. A Doppler echocardiogram (ultrasound of the heart muscle and blood flow through its vasculature) is the best diagnostic test. This allows the veterinarian, usually a specialist in cardiac health, to visualize and measure the heart walls and observe for abnormal heart valve function.
*A blood test has now become available for purebred Maine Coon cats that can determine if they carry the genetic mutation responsible for HCM. It is not diagnostic for other purebreds or mixed breeds. Nor does a positive test result confirm active heart disease; rather, it helps the veterinarian to be on alert for developing Hypertrophic Cardiomyopathy.
HCM may also be secondary to common endocrine disorders like hyperthyroidism (elevated thyroid levels) and hypertension (high blood pressure). A blood-chemistry panel and blood pressure measurement will be performed to rule out these underlying causes. While x-rays are not useful to measure the heart muscle thickness, they are used to rule out lung congestion and solid masses (cancer). Electrocardiogram (ECG) may help identify heart arrhythmias which may indicate developing problems.
Treating any underlying disease process leading to HCM may partially or completely resolve the associated heart muscle changes. Unfortunately, there is no medication or therapy that will reverse the effects of heritable HCM. The goal of treatment is to prevent or reduce the outcome of the disease. For instance, if congestive heart failure exists, diuretics and ACE inhibitors can be prescribed to remove fluid from the lungs, reduce blood pressure, and improve cardiac output. Blood thinners that reduce the blood’s clotting ability may be given to avoid the formation of a thrombus (platelet clot). These must be used judiciously to avoid side effects like gastrointestinal ulceration and bleeding. The potential benefit of using blood thinners is still unproven. Antacids may be given prophylactically to prevent gastrointestinal ulceration.
The prognosis for cats with HCM is unpredictable. Some cats enjoy a very good quality of life and long survival time after diagnosis, and others will succumb to HCM rapidly and without warning. The more advanced the heart disease becomes, the poorer the outcome. If congestive heart failure or a thromboembolism occurs, the prognosis is guarded to very poor.
The only way to prevent Hypertrophic Cardiomyopathy is to eradicate the genetic mutation from familial lines. This means cats with a history of heart disease should never be used for breeding.