CAUSES & CONTROL of Hypertrophic Cardiomyopathy in cats

Cardiomyopathy is a disease condition in which the heart muscle is primarily affected and it may subsequently result in other malfunctions of the heart. Let’s find out the causes, symptoms and diagnosis of Hypertrophic Cardiomyopathy (HCM) in cats. 

Hypertrophy refers to thickening. Hence, Hypertrophic Cardiomyopathy (HCM) can be defined as a disease in which there is thickening or enlargement of the heart muscle.

Who can suffer from it?
The exact cause of HCM is often unknown but it does appear to be an inherited disorder in some feline breeds, including Maine Coons, American Short Hairs, British Short Hairs and Persians. Hypertrophic Cardiomyopathy tends to affect cats of one to five years of age. However, it has been detected in kittens as young as three months and in cats as old as ten years.

How it affects the cat?
In cats which develop Hypertrophic Cardiomyopathy, the muscle of the ventricle of the heart becomes thickened. In most cases, the left ventricle is affected to a greater extent than the right ventricle. The severity of the condition depends on how thick the muscle wall ultimately gets. Some kitties develop only minor thickening; others develop a much more significant problem.

As HCM progresses, the actual structure of the heart changes and heart function is affected. Thickened muscle walls become less flexible, and the left ventricle can no longer relax or stretch efficiently to fill with blood. In fact, the heart is actually weakened as the affected wall of the heart becomes less elastic and the heart chambers get smaller. This is the most common cause of heart disease in cats and the most frequent cause of spontaneous death in indoor adult cats.
Even though in HCM the muscle of the affected heart is thickened, the ventricle lumen or chamber inside of the ventricle remains the normal size or sometimes even smaller than normal. As a result the cardiac output i.e., the amount of blood that the heart pumps with every contraction or systolic stroke will gradually decrease and as a compensatory effect, the heart rate increases, resulting in tachycardia.

Causes for feline HCM
Though the exact cause for feline HCM is not clearly understood, these are the
general causes:
Primary causes: The primary feline HCM is commonly noticed in certain breeds of cats in which the disease is inherited and can be passed from the parents to the kittens. These breeds include Maine Coon, American Shorthair, British Shorthair, Ragdoll and Persian. There are DNA tests that help identify at least some of the gene mutations which have been found to contribute to the development of HCM in cats. HCM is sometimes also seen in
mixed breed cats.

Secondary causes: Thickening or enlargement of myocardium or cardiac muscle similar to that of HCM may also occur secondary to few systemic diseases. The most common disease in which HCM like myocardial thickening occurs is hyperthyroidism. Feline hyperthyroidism is a disorder in which there will be elevated levels of thyroxine and other associated thyroid hormones in the blood stream. These high levels of thyroid hormone can act on the heart to cause Hypertrophic Cardiomyopathy. Many other systemic disorders such as renal disorders, that are associated with hypertension or elevated blood pressure, also lead to thickening or enlargement of heart muscle over a period of time, as the myocardium heart must work harder to push the blood through the system. Another secondary cause for feline HCM is aortic stenosis, a structural abnormality of the cardiovascular system.
Many veterinary cardiologists feel that thickening of the ventricles of the heart resulting from these secondary influences should not be termed hypertrophic cardiomyopathy. However, the resulting symptoms are often very similar to those seen with HCM.

Symptoms of HCM

• Gradual decline in physical activity: The affected cat becomes dull and depressed, shows less interest towards play and other physical activities.
• Disinclined to move: In severe conditions the affected cat will not move even for regular loo and potty.
• Less interest towards food: Gradual inappetence resulting in poor physical condition.
• Poor physical exercise: If forced, there are more chances of collapse. Pet parent can notice that their cat fails to climb stairs or even show less interest to get onto sofa or bed, which were her favourite places to sit and relax.
• Cough is another specific sign: Cough is usually more severe in night compared to day and is ignited by exercise.
• Few cats also show signs of fluid accumulation at belly and/or limbs: Fluid accumulation may also be seen in the thorax that may result in severe respiratory distress and dyspnoea at rest which becomes more severe during exercise.

Diagnosis of HCM
Early signs of HCM are vague and indefinite. Increased heart rate and a murmur are common signs. Other than possibly hearing a heart murmur, it is unusual to detect heart disease before signs of heart failure. In severely affected ones, the first and only sign may be sudden death. Based on these clinical signs, the veterinary cardiologist can suspect the cardiac disorder and can be diagnosed based on auscultation, radiography, thyroid profile and electrocardiography. However, confirmatory diagnosis is only possible by means of echocardiography. Echocardiograms, especially with Doppler technology, are excellent diagnostic tools. Once diagnosed treatment should be
started immediately and may need to be continued for life.

Treatment of HCM
Treatment of Hypertrophic Cardiomyopathy in cats involves different types of medications designed to make it easier for the heart to achieve its pumping function and also to remove excess fluid from the lungs and other body tissues. Cats with HCM require drugs that relax the heart and increase its efficiency. The specific drug chosen depends on the stage of illness and presence or absence of complicating factors, such as arrhythmia. Drug choices include diuretics, calcium channel blockers, beta blockers and ACE inhibitors. Finally, restricting the cat’s activity reduces the strain on the heart, and hence, prevents complications.

(Dr K Satish Kumar is Associate Professor and University Head of Medicine, Sri PV Narasimha Rao Telangana State University for Veterinary, Animal and Fishery Sciences, Hyderabad, Telangana while Dr D Srikala is Assistant Professor, Veterinary Medicine, SVVU, Tirupati, Andhra Pradesh).