Cardiomyopathy is a term used to describe a group of diseases involving the heart muscle. Cardiomyopathy may be seen as a primary condition or secondary to other diseases. In cats, there are at least 3 classes of cardiomyopthies that have been described.
In cats, there are at least 3 classes of cardiomyopthies that have been described, namely hypertrophic, restrictive and dilated cardiomyopathy. A fourth cardiomyopathy, Arrhythmogenic Right Ventricular cardiomyopathy or ARVC has recently been described but it is very rare. In all cases, the heart disease may result in clinical signs of heart failure. These days, there is less emphasis on classifying the class of cardiomyopathy, as there seems to be a significant overlap on the type of heart muscle disease.
What are the clinical signs seen with cardiomyopathy in cats?
The broad range of clinical signs seen in cats with cardiomyopathy depends on the severity of the disease. In the early stages of the disease, the cat may be able to cope and hence show no overt signs of the disease. This situation is known as compensated heart disease. Often the cats will alter their activity levels to those that they can cope with. They may be sleeping more or playing less than the usual, all of which are subtle signs that are difficult to pick up on. This makes it difficult to diagnose cardiomyopathy until it is quite advanced. Occasionally, a heart murmur or alterations in heart rhythm may be detected during routine vet examination and these may be the only evidence of heart disease during the early stages of the disease.
The major long-term concerns with all types of cardiomyopathies are:
1. Development of congestive heart failure: Breathlessness (panting, open mouth breathing, increase respiratory effort and rate) and lethargy or weakness is the most frequently noticed signs of congestive heart failure. These clinical signs result from a failure of the heart to efficiently pump blood around the body resulting in pooling of the blood in the veins and excess fluid accumulation in the lungs, chest and abdomen. In some cases the cat may faint suddenly from arrhythmias (abnormal heart rhythm). Other clinical signs include hypothermia, pale mucous membranes (e.g. gums), and weak pulses due to a low blood volume resulting from the heart disease.
2. Thromboembolic disease: Altered flow of blood in the enlarged heart chambers predisposes to the formation of a blood clot within the chambers. The blood clot then becomes organised and is known as a thrombus. If parts of the thrombus become dislodged, they can travel in the bloodstream and become lodged in smaller blood vessels. These particles are called emboli and the most common place for them to lodge is at the bottom of the aorta, which is the biggest artery in the body. This result in obstruction to blood flow to the tail and back legs, which is usually very painful, and the back legs become paralysed and cold to touch. Cats with thromboembolic disease often become paralysed suddenly without any physical trauma to the cat. Thromboembolic disease is frequent and life-threatening complication of any cardiomyopathy. Approximately 50% of affected cats survive the crisis and those that survive typically show steady improvement in limb function usually within 24 to 72 hours after presentation. Surviving cats however are at risk of recurrence.
How is cardiomyopathy diagnosed?
Diagnosis of cardiomyopathy is made on the basis of clinical signs such as those of congestive heart failure, in addition to more specific tests such as chest x-rays and cardiac ultrasound scans (Echocardiography). Cardiac ultrasound scans are indispensable in distinguishing between the different types of cardiomyopathies. Electrocardiography (ECG) rarely provides useful information on heart conduction and size. Specific blood tests may be done in order to check that the cardiomyopathy is not secondary to some other disease (see below).
What causes cardiomyopathy?
There are many causes of cardiomyopathy, however, in older cats thyroid disease (hyperthyroidism) may cause a secondary cardiomyopathy. Other causes include hyperadrenocortism, aortic stenosis, anaemia, systemic hypertension and dietary deficiency of taurine, an essential nutrient. Taurine deficiency is often seen in cats fed a non-commercial diet but can also occur in cats with normal taurine intake. Particularly at-risk breeds of cats for taurine deficiency include the Burmese, Abyssinian, and Siamese.
More than often, the cause of cardiomyopathy is unknown. This is also known as idiopathic cardiomyopathy. However, theories on possible causes include genetic, viral, and immune mediated causes, inflammation of the heart muscle and abnormalities of energy production and metabolism.
Why is taurine important for cardiac function?
Taurine is an essential amino acid for cats. Cats at birth have a low concentration of the enzyme that is required for making taurine as a result cats have limited ability to make this amino acid necessary for normal structure and function of the heart and eye. Taurine deficiency leads to dilated cardiomyopathy (DCM).
Is the cardiomyopathy reversible with taurine supplementation?
Treatment with oral taurine supplementation may improve clinical signs, restores the heart muscle function, and improves survival when taurine deficiency is responsible for the heart condition. Echocardiographic evidence of any response may take 2-3 months. However, 38% of cats with taurine deficiency-induced DCM heart failure die in the first 30 days despite taurine supplementation. Cats that survive past 30 days have a more promising prognosis.
How is hyperthyroidism associated with heart disease?
Hyperthyroidism is a disease of the thyroid gland usually associated with old age. It results in high circulating levels of the thyroid hormone, which results in an increased outflow of blood and subsequent increase workload for the heart. To compensate for the increase work the heart has to do to pump the increased volume of blood, the heart muscles thicken resulting in thickened heart chambers. A thickened heart chamber is less able to relax and fill properly causing increased blood pressures during filling of the heart and subsequently leading to congestive heart failure.
What is Hypertrophic cardiomyopathy?
Hypertrophic cardiomyopathy, otherwise known as HCM, is the most common heart disease of the cat. It is characterised by unexplained significantly thickened left heart wall. The cause is unknown although it has been suggested that there is some genetic defect in the muscle fibers of the heart in cats with HCM.
The average age of affected cats is 6 years although it can affect cats from 8 months to 16 years. The Domestic Shorthair, Persian, Domestic Longhair and the Maine Coon are thought to have increased incidence of the disease.
What is Restrictive cardiomyopathy?
Restrictive cardiomyopathy, also known as RCM, intermediate or unclassified cardiomyopathy (UCM), is the next most common cause of cardiomyopathy in the cat after hypertrophic cardiomyopathy. RCM or UCM is a disease where there is increased stiffness in the heart chambers to expand during filling of the heart. This results in abnormal filling of the heart on the left side and increased end-diastolic pressures.
The majority of cats affected with RCM are aged (9-15 years). The cause is unknown but in most cases, the heart of affected cats exhibit extensive fibrous deposition in the heart muscle. RCM is thought to be the result of a wide range of inflammatory, and possibly genetic influences, however it is not heritable.
What is Dilated cardiomyopathy?
Dilated cardiomyopathy or DCM is a far less common heart condition in cats, accounting for around 5% of all heart conditions reported in the cat. Cats with DCM have large heart chambers with normal wall thickness. Most cases of feline DCM are primary or idiopathic (i.e. unknown cause). A diagnosis of DCM is made only after other causes of heart failure, such as nutritional or taurine deficiency, congenital heart disease, toxic, ischemic, or metabolic-induced heart failure, have been ruled out.
How is cardiomyopathy treated?
In cases where an underlying cause of the heart disease is identified, then treatment of this may result in improvement or reversal of the heart disease. Hyperthyroidism is the most treatable cause of cardiomyopathy since complete resolution of the heart disease is possible if treated early. In cases where no cause is identified, (referred to as idiopathic cardiomyopathy), and in cases where the disease remains following treatment for the underlying cause, then medication may be needed.
Treatment varies according to each case but may include the following drugs:
1. Diuretics: are the most common diuretic used to remove excess fluid in the lungs. Diuretics work by increasing renal excretion of water.
2. Beta blockers work by reducing heart rate, blood pressure, heart contractility and therefore myocardial oxygen requirements where these parameters are excessive.
3. Calcium channel blockers are thought to help the heart muscle to relax and hence help more effective filling of the heart. Its indirect effects include reducing blood pressure by dilating the blood vessels and decreasing heart contractility and heart rate, thereby decreasing myocardial oxygen demand.
4. ACE inhibitors and ARB's (ACE blockers) are often used in treatment of chronic congestive heart failure in cats, regardless of cause and is often used in combination with diuretics. Such drugs are said to reduce the severity of the heart disease, reduce fibrosis of the heart muscle therefore improving heart relaxation. They may also reduce hypertrophy of the left heart chamber and decrease the risk of myocardial infarcts.
5. Anti-thrombotic drugs designed to reduce the chances of arterial thromboembolism. These drugs work by inhibiting the formation of thrombus. Unfortunately, blood clot are one of the first signs of underlying heart disease. Once these occur, some cats will make a full reocvery, but the logn term outlook for most patients is not good.
6. New positive inotropic drugs work as calcium sensitiser heightening the strength of the heart contractions. It also results in dilation of the arteries and veins, thereby reducing systemic blood pressure. The only catch is that Vetmedin is currently only registered for dogs for treatment of DCM and use in cats is strictly off-label. There are a sub-set of cats with thickened heart muscle that these drugs may be of use.
The long-term outlook for a cat with cardiomyopathy is extremely variable depending on the cause and severity of the disease. Cats with idiopathic cardiomyopathy may remain stable for years and lead near-normal lives.
Does a cat with cardiomyopathy need a special diet?
Other than in cases of taurine deficiency, no specific diet is recommended in cats with no clinical signs although excessively salty foods should be avoided since they will predispose to fluid retention. A sodium-restricted diet is recommended once overt congestive heart failure has developed. The commercial cat diets that are available on the market are usually adequate.
Fatty acid (Omega 3) supplementation, such as those found fish oils, may help decrease anorexia and reduce the oxidative damage to the heart caused by various free radicals and cytokines that may be increased in congestive heart failures. Other nutriceutical supplementation such as Coenzyme Q10 and L-carnitine are said to be beneficial and have been used as conjunctive therapies in congestive heart failure in humans.