Cardiomyopathy is the name given to any disease affecting the heart muscle itself.
This is the most common form of heart disease seen in cats, and the most common cause of heart failure.
Disease of the heart valves (causing 'leaky' valves which prevent the heart functioning normally) are an important and common cause of heart disease in humans and in dogs, but this is rarely seen in cats.
Cardiomyopathies (disease of the heart muscle) are described according to the effect they have on the structure and function of the cardiac (heart) muscle. The main classification is to divide the disease into:
- Hypertrophic cardiomyopathy (HCM) – this is the most common form of heart disease in cats and occurs where there is an increase in the thickness of the muscular wall of the heart. This reduces the volume of blood within the heart and also prevents the heart muscle relaxing properly between contractions.
- Dilated cardiomyopathy (DCM) – this is where the muscular wall of the heart generally becomes thinner than usual, the heart enlarges, and the heart muscle cannot contract effectively.
- Restrictive cardiomyopathy (RCM) – here primarily there is fibrosis of the wall of the heart, making it stiff and inelastic, so preventing the heart chambers filling normally.
- Intermediate cardiomyopathy (ICM) – these cases have changes consistent with more than one type of disease – for example a mixture of both hypertrophy and dilatation present.
Although in the majority of cases of heart disease in cats the underlying cause is unknown, there are various potential recognised causes, and your vet may need to investigate some of these. Known potential underlying causes include:
- Cardiomyopathy secondary to other diseases
- Nutritional causes
- Taurine deficiency (causing dilated cardiomyopthy)
- Infiltrative of the heart muscle
- Lymphoma (a type of malignant tumour)
- Exposure to toxins
- Some drugs may have secondary effects on the heart
- Hereditary causes
- Genetic defects have been recognised in Maine Coon cats and other breeds that may contribute to development of cardiomyopathy
What happens in cardiomyopathy?
In cardiomyopathy, the underlying abnormality of the cardiac muscle leads to a compromise in cardiac function. The alteration in heart function depends on the type of cardiomyopathy though:
- With HCM and RCM - the disease mainly interferes with the ability of the heart muscle to relax properly between contractions. The relaxation phase between each cardiac contraction is called diastole, and if this does not occur properly, the heart cannot fill with blood so effectively. If severe, this will lead to heart failure, and a form known as 'diastolic heart failure'.
- With DM - the disease mainly affects the ability of the heart muscle to contract (called 'systole'). This will compromise the ability of the heart to pump (and eject) blood. In severe cases this too leads to heart failure - a form known as 'systolic heart failure'.
Early signs of heart disease
In the initial phase of disease, cats may show no signs at all and appear completely normal. In fact a number of cats with cardiomyopathy may never actually develop clinical disease. However, while in some cats progression of the underlying disease is slow, in others it can be quite rapid.
Some early signs of heart disease may be detectable during a clinical examination by your vet, prior to the onset of any overt signs. This is one of the reasons why every cat should be checked at least once a year by a vet (and ideally more often in older cats). Early warning signs that your vet might detect include:
- Presence of a heart murmur – this is an abnormal noise your vet can detect when listing to your cat's heart with a stethoscope and develops due to turbulence in the flow of blood through the heart.
- Presence of a gallop rhythm – during each cycle of heart contractions, normally you can hear two sounds when you listen to the heart with a stethoscope (these sounds are associated with closure of heart valves during contraction and relaxation of the heart). With significant heart disease, a third audible heart sound is sometimes detected and this is referred to as a 'gallop sound' or 'gallop rhythm'.
- Abnormalities in heart rate – with heart disease, the heart rate can sometimes significantly increase or decrease outside of the normal range for a cat, and sometimes there may be heart beats without any effective flow of blood (a heart beat but no pulse detectable in an artery (known as a 'pulse deficit')).
- Presence of cardiac rhythm disturbances – these are also referred to as cardiac dysrrhythmias. Normally, cats have a very regular heart rate, however with heart disease there can be interference in the normal electrical impulses that control heart contractions; this can lead to disturbances to the normal rhythm.
Many cats, especially those in the early stages of the disease, may only have changes in the cardiac muscle that are detected during ultrasound examination of the heart. These cats are clinically silent (or asymptomatic), although many will go on to develop signs later on.
If heart function is significantly impaired by cardiomyopathy, this will lead to heart failure (often called congestive heart failure), where there is compromise to blood flow through the heart and blood output from the heart.
Cats can sometimes develop clinical signs without prior warning, and some cats can deteriorate very rapidly. Some cats with heart disease show signs of collapse, or 'fainting'. However, this is relatively uncommon and usually associated with marked disturbances to the normal rhythm of the heart (which can lead to episodes where the brain is starved of oxygen through poor blood flow).
Unlike dogs, cats are not often 'exercised' (eg, taken for walks on a lead) so it is often much more difficult to detect that they have reduced exercise ability – often an early sign of heart disease. Cats are likely just to spend a little more time resting or sleeping and this may not be very obvious. Detecting early disease (without an examination by your vet) is often especially difficult as cats are good at hiding signs of disease. Additionally, there may be no obvious signs until a 'critical point' is reached due to advancement of the disease or due to stress (which may result in sudden or rapid development of quite marked signs).
In cats, the most commonly seen sign of heart failure is the development of difficult breathing (dyspnoea) and/or more rapid breathing (tachypnoea). This is generally caused by either a build up of fluid in the chest cavity around the lungs (called a pleural effusion), or due to a build up of fluid within the lungs themselves (called pulmonary oedema).
Along with breathing difficulties, cats may have cold extremities (eg, ears and paws), and may have pale mucous membranes (gums and eyes) suggesting poor circulation. Occasionally the mucous membranes of the mouth and eyes, and even the skin, may show signs of cyanosis (a bluish colour). Coughing is rarely seen in cats with heart disease, although it is quite common in dogs. If coughing is seen in cats, it is more likely to be caused by a disease of the airways (such as bronchitis).
Feline aortic thromboembolism (FATE)
Another sign which can occur in cats, and may sometimes be the first indicator of underlying heart disease, is the development of what is known as 'feline aortic thromboembolism' or FATE. A thrombus (blood clot) may develop within one of the heart chambers (usually left atrium) in a cat with cardiomyopathy. This occurs mainly because the blood is not flowing normally through the heart. The thrombus, or clot, is initially attached to the wall of the heart, but may become dislodged and be carried into the blood leaving the heart. A thrombus that moves into the blood circulation is called an embolus, hence the term 'thromboembolism'. Once in the circulation, these emboli can lodge in small arteries and obstruct the flow of blood to regions of the body. Although this can happen at a number of different sites, it more commonly occurs towards the end of the major artery that leaves the heart (the aorta) as it divides to supply blood to the back legs. This complication is seen most commonly with HCM, and will cause a sudden onset of paralysis to one or both back legs, with severe pain and considerable distress.
Differentiation of forms of cardiomyopathy
Various diagnostic tests can be done to assist the diagnosis of heart disease in cats.
- Electrocardiogram (ECG) – this is an electrical trace of the heart activity. It can be very useful for the detection of cardiac rhythm disturbances, but has more limited use beyond that.
- Radiography (X-rays) – are helpful for showing changes in the overall shape and size of the heart, and for detecting a build up of fluid (pulmonary oedema or pleural effusion). Repeating radiographs may also allow monitoring of the efficacy of any treatment.
- Heart ultrasound (echocardiography) – is very helpful as it allows a view of the internal dimensions of the heart, the wall thickness, and the contractility of the heart to be assessed. It can also show where a heart murmur is originating from. This is the only test which can readily distinguish between different types of heart disease in cats. Although a small area of skin usually needs to be shaved to perform ultrasound, the procedure is not uncomfortable or painful and so can be performed in most cats without any sedation or anaesthetic.
- Tests for underlying disease – may need to be performed in some cases, including blood tests and blood pressure measurement.
The underlying cause of cardiomyopathy is only rarely treated, however if it develops secondary to taurine deficiency in the diet (which can be a cause of DCM), or secondary to conditions such as hypertension (high blood pressure) or hyperthyroidism (overactive thyroid gland), then treating the underlying disease may improve cardiac function.
Where heart failure develops, various drug treatments may be available to help improve and manage the condition. These may include drugs such as:
- Beta-blockers such as atenolol or propranolol, which slow down the heart rate and reduce the oxygen demand on the heart.
- Diltiazem – this drug is known as a 'calcium-channel blocker', and reduces both heart rate and the strength of heart contractions. It reduces the oxygen demand of the heart and may help the heart muscle to relax between contractions
- ACE-inhibitors (angiotensin-converting enzyme inhibitors, e.g., benazepril, ramipril, enalapril) or ARBs (angiotensin receptor blockers, e.g., telmisartan) – help block the activation of the renin-angiotensin-aldosterone system (RAAS) – a hormone system stimulated in cats with heart disease. Their use may help in the management of heart failure and possibly also in earlier stages of heart disease.
- Pimobendan – is an example of a calcium channel sensitiser. It increases the strength of the contraction of the heart and also acts to dilate blood vessels which may help the flow of blood. It has been used in some cats with congestive heart failure.
- Diuretics (such as frusemide/furosemide) – these are extremely valuable once signs of congestive heart failure develop, to help remove the fluid build up in or around the lungs. Dosage can be adjusted over quite a wide range to achieve the desired result.
Unfortunately the true effectiveness of many drugs in treating heart disease in cats is unknown, and more clinical trials are needed. Different drugs also act in different ways, and so may be helpful in different situations. In general, diuretics are the most useful drugs in managing signs of congestive heart failure,. With early diagnosis of heart disease, treatment may help to slow or delay its progression and help to maintain a good quality of life.