Cardiomyopathy literally means disease of the heart muscle. The term is reserved for those cases where the cause of the disease is unknown.

Where the cause of the hypertrophy (enlargement) of the heart muscle is known, it is called secondary myocardial hypertrophy. There are a number of well recognised diseases which cause changes in the heart muscle, including taurine deficiency, which can lead to dilation of the heart, and hyperthyroidism, which is usually associated with hypertrophy of the heart muscle. Therapy in these cases is directed towards short term support of heart function while the primary disease is being treated.

Terminology to describe primary (without demonstrable underlying cause) cardiomyopathies in the cat is complex.

Most cases can be classified under one of three headings.

Dilated cardiomyopathy (DCM)

This condition is characterised by enlargement of the heart chambers and weakening of the heart muscle. When this occurs the heart is no longer able to pump the blood out of the heart in sufficient quantities to meet demand.

Hypertrophic cardiomyopathy (HCM)

Restrictive cardiomyopathy (RCM)

Both of these conditions are characterised by impaired relaxation of the heart muscle. In HCM inward thickening of the heart muscle results in a reduction in chamber volume thereby reducing the volume of blood that the heart can pump with each contraction. In RCM fibrosis (scarring) of the heart muscle results in a chamber that can no longer expand in the normal way.

Although the underlying disease process is important (because it affects the choice of drugs that can be used to treat the cat) the presentation of disease is similar. Most commonly cats will present in heart failure, but thromboembolic (blocking of a blood vessel by a blood clot) disease can also be responsible for the cat's clinical signs.

Congestive heart failure

Unlike dogs, coughing is not a major sign of heart failure in cats. Most frequently, breathlessness, lethargy and a loss of appetite are the first problems noticed. These signs can appear quite quickly, typically over a few hours/days. However, it is important to remember that the underlying heart disease has usually been present for a considerable period of time. Heart failure becomes obvious when the heart is no longer able to meet the demands for pumping blood around the body. In cats, the signs of heart failure arise due to an increase in venous pressures leading to fluid leaking into the lung tissue (pulmonary oedema) or around the lungs (pleural effusion). Both of these processes prevent the lungs from functioning normally, leading to the breathlessness and lethargy.

Thromboembolic disease

As the enlarged heart is no longer able to pump efficiently, stasis of the blood occurs resulting in activation of the clotting system. This results in the formation of an embolus (or clot) usually in the left atrium. Small pieces of the embolus can break off and travel through the circulation, becoming trapped in smaller arteries. The most common place for the embolus to be trapped is at the point at which the blood supply to each of the hind legs and the tail branches (aortic trifurcation). The resulting obstruction causes sudden loss of use of both hind legs and the tail,

which become cold, hard and painful. This sudden event can sometimes be mistaken for a cat that has been in a road traffic accident. Although some cats may recover full function of their limbs with appropriate treatment, their longer term outlook is often bleak due to the underlying heart disease and the possibility of other clots occurring.

Diseases affecting the heart

Hypertension (high blood pressure) is common, particularly in older cats and can result in damage to blood vessels. The blood vessels in the retina of the eye are particularly sensitive to hypertensive damage, which may result in retinal detachment and sudden onset blindness. Affected cats appear confused and disorientated, with widely dilated pupils. Retinal detachment is an emergency situation as blindness will become permanent unless the retina reattaches within a few days. In addition, the elevated blood pressure damages the glomerulus (the functional unit within the kidney), the heart (causing a thickening of the walls as the muscular wall is having to work harder pumping against a high resistance within the blood vessels) and the brain, causing small blood vessels to rupture. Drugs that lower blood pressure (hypotensive agents) are routinely used in the treatment of hypertension in cats. Other diseases, for example, chronic renal failure can also lead to hypertension, meaning that a number of different tests may be required to establish the underlying cause of the increase in blood pressure. Primary hypertension is rare in cats, unlike the situation in man.

Over recent years, the ability to measure blood pressure in cats has improved and an increasing number of practices have the necessary equipment.

Diagnosis of cardiomyopathy

Heart disease can often be suspected on the basis of the presenting clinical signs as well as the age and breed of the cat. A precise diagnosis depends on more specific tests. Radiographs (X-rays) of the chest and electrical recordings of heart activity (ECG) are commonly used. Ultrasound examination (echocardiography) is usually required to make a definitive diagnosis as it is the best way of assessing heart size and function. Ultrasound will also detect the presence of secondary problems such as thromboembolism or obstruction of the outflow of blood (which may occur due to extreme hypertrophy of the heart muscle). Depending on the presentation of the case, other tests may also be valuable (such as a general blood screen, ocular examination or measurement of thyroid hormone levels), which can be useful when looking for primary disease problems causing secondary changes of the heart muscle.

What causes cardiomyopathy?

At present, the cause of cardiomyopathy in most cases is unknown. However, a number of factors are suspected to play a role, such as previous viral disease and/or genetics. Cardiomyopathy seems to be more common in young and middle aged male cats. In people, the majority of cases of HCM are secondary to a genetic mutation; over 120 different mutations have been identified. Recently, mutations which can lead to HCM have been identified in the Maine Coon and Ragdoll breeds; however, it is well recognised that there are many other breed predispositions and there are thought to be many more, as yet unidentified mutations, present in the feline population.

How is cardiomyopathy treated?

Treatment will, to some extent, depend on the presenting signs, severity and type of cardiomyopathy. Where an underlying cause can be identified and corrected, the secondary cardiac dysfunction may resolve. Where no underlying cause is found, treatment is aimed at management of heart rate and hypertension symptoms.

Sadly, the cardiomyopathy itself is usually progressive, although progression may be slow. Treatment is focused on:

• Reduction of circulating volume to remove fluid from within (pulmonary oedema) or around (pleural effusion) the lungs. This is usually achieved by use of a diuretic drug (most commonly frusemide). Pleural fluid can also be drained directly by placing a needle or catheter through the chest wall and removing the fluid.
• Supporting heart function; the drugs used will depend on the cause of the cardiomyopathy. Most commonly used drugs would include:

  •ACE (angiotensin converting enzyme) inhibitor, which causes vasodilation reducing the heart's work load.

  •Calcium channel blockers, which will help the heart muscle relax so there is more room for blood to fill the ventricle (pumping chamber).

  •Beta-blockers, which are sometimes used to slow the heart rate down in cases where the heart rate is excessively fast so there is no time for chamber filling to occur.

• Reduction of the risk of thromboembolism; this may be achieved by the use of one or more of the following: low doses of aspirin to inhibit platelet function; clopidogrel, another platelet inhibitor; or a form of heparin, which decreases blood clotting. It is important that the dose rate and interval is followed as it is easy to overdose a cat, with fatal consequences.
• In cats in which hypertension is the major problem, amlodipine is used to reduce the blood pressure. Whereas cats with hyperthyroidism should have the overactive thyroid treated.

What is the long term outlook for cats with cardiomyopathy?

The outlook for cats is very variable and depends on the type and severity of the disease, whether or not congestive heart failure develops and whether the heart disease is a primary problem, or if it is secondary to another disease such as hypertension or hyperthyroidism. Some cases will remain stable for years. In general, cats with thromboembolic disease, and those with heart failure which does not respond well to treatment, have a poorer outlook.

What about diet?

Except in cases where taurine deficiency is suspected, no specific dietary change is usually required, although obese cats usually benefit from a controlled weight loss programme. Highly salty foods, which include some cat treats, are best avoided. This is because they can promote sodium retention, which increases the volume of fluid and therefore the volume of blood that the heart needs to pump. In some circumstances low salt diets may be recommended.

Testing for HCM?

Genetic tests are available for particular mutations which are recognised in the Maine Coon and Ragdoll breeds. However, not all cats with HCM will demonstrate these mutations. Therefore, a negative gene test does not rule out HCM (even in one of the aforementioned breeds). At present, the most accurate method for diagnosing HCM is the use of cardiac ultrasound (echocardiography).

Is there a screening scheme available to detect cardiomyopathy in cats?

Yes, there is a Feline Advisory Bureau scheme available which is aimed at detecting the presence or absence of hypertrophic cardiomyopathy (HCM). It is run in association with the Veterinary Cardiovascular Society (VCS).

For further information about the Scheme, please click here...

Further information on HCM in cats can also be gained from: Susan Little's information on HCM for Cat Breeders: http://www.catvet.homestead.com

Updated November 2008