CARDIOMYOPATHY is the name given to any disease process where the abnormality responsible for the development of clinical signs resides within the heart muscle. It may be a primary abnormality or secondary to other disease processes.

Cardiomyopathies are described according to the effect they have on the morphology and function of the cardiac muscle. The main classification is to divide the disease into:

1 Hypertrophic. This can by symmetrical and there is a sub-category of hypertrophic obstructive cardiomyopathy. The muscle thickness is inc r eased and the ventricular lumen is reduced in diameter. Ventricular contraction is normal but relaxation is compromised.

2 Dilated. Here the ventricular lumen is increased in diameter and the wall thickness is normal or decreased relative to the lumen size. Muscle contraction is compromised.

3 Restrictive. In this condition the lumen and wall size may be relatively normal but fibrosis of the wall leads to increased wall stiffness. Thus the ventricle is inelastic and unable to fill normally.

4 Intermediate. In these cases there are changes consistent with both hypertrophy and dilatation present.

Aetiology  

There are various different possible causes of cardiac muscle disease that are recognised in various different species.

Infectious

• Parvovirus in dogs

• Trypanosomiasis

• ?FIV

Metabolic

• Hyperthyroidism

• Hypertension — renal failure etc

• Acromegaly

Nutritional

• Taurine deficiency

• L-Carnitine deficiency

Infiltrative

• Lymphosarcoma

• Amyloidosis

Toxic

• Adriamycin

Hereditary

• Maine Coone cats

• Various forms of cardiomyopathy in humans

Heredity  

Mode of inheritance in Maine Coone cats appears to be autosomal dominant with modifying genes. The affected cats are heterozygotes. Homozygotes may be lethal as there appears to be a high incidence of stillbirth. There is strong evidence to suggest a familial or hereditary pattern in some other breeds.

Pathophysiology  

The abnormal function of the cardiac muscle leads to compromise of cardiac function. In dilated cardiomyopathy the compromise is predominantly systolic, ie, affecting the ability of the heart to contract and eject blood.

In hypertrophic and restrictive cardiomyopathy the compromise of the heart is predominantly diastolic resulting in an inability to relax and receive blood.

Both types of disease result in a drop in cardiac output which will result in the initiation of compensatory mechanism leading to the development of clinical signs of congestive heart failure. In cats the most common presenting sign of heart failure is dyspnoea (difficulty in breathing) caused by either pleural effusion or pulmonary oedema.

The other sign which can occur with some frequency in cats, and may be the first indicator of cardiac disease, is the development of feline aortic thromboembolism (FATE). Thromboembolism occurs as a consequence of the stasis of blood within distended cardiac chambers. The cardiac atria enlarge as heart disease progresses and the resistance to ventriculate filling increases. Blood pools within the enlarged atria and clots form. These clots then fragment and obstruct the arterial circulation. The most common site for clots to form is in the left atrium. The fragments then tend to spread to the aorta where, as the aorta tapers, they wedge and obstruct flow to the back legs. This can occur in up to 40 per cent of cats with hypertrophic cardiomyopathy.

Clinical manifestations   

The clinical signs shown by cats are very varied and the disease progresses through various phases. In the initial phase of the disease the cat may well appear completely normal. Cats at this stage of the disease are rarely identified but studies of the development of the disease have demonstrated the presence of change within the myocardium wall before the onset of clinical signs. Some clinical signs may be detectable in cats on physical examination prior to the onset of physical signs. Such early warning signs might include:

The development of heart murmur — Distortion of the normal ventricular and valvular architecture and partial obstruction to the outflow of blood from the heart can result in turbulent blood flow within the heart. This will result in the development of a murmur. Murmurs due to partial obstruction to the outflow of the ventricle may be quite variable in intensity. They are likely to be more audible when the heart rate is higher or there is greater sympathetic stimulation of the heart.

The development of a gallop rhythm — A gallop rhythm occurs when there is a third audible heart sound. This third sound indicates that the ventricle is indispensable and is found in individuals with diastolic failure.

The development of a cardiac rhythm disturbance — Cardiac rhythm disturbances can accompany any cardiac disease.

Many cats with detectable changes in the heart muscle on an ultrasound examination will have no evidence of abnormality on a complete physical examination. Thus in the early stages of the disease quite significant abnormality can be present but it may be clinically silent.

Onset of signs in cats   

When cats develop clinical signs these may occur without prior warning and the cats can deteriorate very rapidly. The clinical signs that become apparent are either those of respiratory difficulty because of congestive heart failure or signs of aortic thrombosis. More rarely cats may show signs of collapse.

There are various reasons why the onset of signs may be very rapid in cats.

• Owners are rarely aware of the ability of their cat to exercise. Cats are not encouraged to exercise by their owners and therefore a deterioration in exercise tolerance is rarely noted. Cats are expected to lie around all day doing nothing.

• The compromise of diastolic function in cats makes them very susceptible to the effects of stress. An increase in heart rate will further compromise diastolic function. Diastole is the period between contractions and is compromised significantly by an increase in heart rate.

Time spent in diastole = 60 -(HRxQT interval)

Therefore any stress resulting in an increase in heart rate compromises diastole and exacerbates signs. The paradox is that as the cat becomes anxious at the onset of signs of heart failure the heart rate increases and the signs get worse. This is why some cat owners appear to get into a vicious cycle of stress — resulting in deterioration — resulting in more stress.

• Thrombosis may be the first sign of any problems and this is very acute in onset.

Clinical signs associated with congestive heart failure

The most common sign that cats will show is breathlessness. There is an increased depth and rate of respiration. This occurs as a consequence of one of two possible causes. These cats either have a pleural effusion (fluid around the lungs) or pulmonary oedema (fluid in the lungs). Both of these will compromise the ability of the cat to ventilate and exchange gases effectively within the lungs. In combination with this cats may have a cold periphery and have pale mucous membranes suggestive of a poor peripheral circulation. There may also be signs of cyanosis.

Cough is rare in cats with heart disease and is more likely to be associated with airway disease such as feline allergic bronchitis.

Clinical signs associated with FATE

Signs of embolism depend upon the artery which has been obstructed by the embolus. The signs are associated with a cessation of normal blood supply to the region and lead to loss of normal function, pain and necrosis. The most common site of embolisation is the distal aorta although involvement of the forelimbs and cerebral circulation can occur. FATE is associated with loss of function of the hindlimbs. The cats are unable to stand and walk. On palpation the hindlimbs are cold and there are no detectable pulses in the femoral arteries. As the underlying heart disease is usually fairly severe and the cats become very stressed by the episode of thrombosis there are often concurrent signs of heart failure such as dyspnoea. If the cats survive the original episode of thrombosis then signs of neuromuscular abnormalities may persist after the restoration of blood flow. The skin and muscle of the hindlimbs may undergo necrosis.

Differentiation of forms of cardiomyopathy  

There are various diagnostic tests available to assist the diagnosis of feline cardiac disease.

ECG This is useful for the detection of cardiac rhythm disturbances. It is limited in value for the determination of the type of heart disease present. Some conduction disturbances and hypertrophy patterns commonly accompany cardiomyopathies but are not specific for the different forms.

Radiography This is very useful for the determination of changes in the overall shape and size of the heart. It is also invaluable for the detection of pulmonary and pleural fluid. The presence of pleural and pulmonary fluid establish the existence of heart failure and serial radiography enables you to monitor the efficacy of treatment. Radiography will not establish the internal architecture of the ventricle and therefore cannot conclusively distinguish between the various forms of cardiomyopathy.

Echocardiography Echocardiography enables the internal dimensions of the heart to be measured. It also enables the function of the heart to be evaluated, ie, the ability to contract. This is the only test which can definitively distinguish between the different morphological types of myocardial disease in cats. Doppler echocardiography is required to enable the measurement of blood flow within the heart. This is the only technique which can conclusively demonstrate the presence of an obstructive cardiomyopathy.

Differential diagnosis   

In cats where evidence of myocardial disease is discovered, the possibility of an underlying disease should be considered, particularly in an older cat. Common underlying diseases are hyperthyroidism and hypertension due to chronic renal disease. For this reason it is worthwhile checking blood pressure, a biochemistry profile and a total T4 in cats with acquired heart disease.

Treatment   

There are various aims of treatment which can be achieved to varying degrees with myocardial disease in cats.

Cure —The only myocardial failure in cats which is known to be curable is the dilated cardiomyopathy secondary to taurine deficiency. In other cases of myocardial disease cure is unlikely to be a realistic aim of treatment.

Treat underlying disease —Where an underlying disease is found then the most effective method of management of the cardiac abnormality is to manage the underlying disease. Therefore management of hypertension or hyperthyroidism may lead to resolution of the cardiac abnormalities. Where there are concurrent signs of heart failure these will need to be managed as well

Improve myocardial function — Determined by the type of disease present.

Improve systolic function in hypertrophic cardiomyopathy with Diltiazem or Beta-blockade.

Prevent progression — It may be possible to slow the progression of the underlying myocardial abnormality with various treatments.

Diltiazem

ACE inhibitors

Beta-blockers

Treat signs of congestion — Diuretics and vasodilators are appropriate for this use in cats. Cats may well be more sensitive to the effects of over-diuresis than dogs due to the diastolic nature of their heart failure.

Thrombolytic therapy for FATE — No improvement in survival has been demonstrated with thrombolysis or surgical embolectomy. Problems arise with reperfusion. Therapy has been attempted with Streptokinase and Tissue Plasminogen Activator.

Prevent thrombosis — Where significant atrial enlargement is identified anti-thrombotic treatment has been advocated.

Unfortunately few of the treatments have undergone rigorous scientific scrutiny and therefore it is difficult to advise one treatment over and above another. Clinical trials are currently planned to compare the efficacy of various different therapies for hypertrophic cardiomyopathy.

Outcome  

A number of cats die acutely as a consequence of the acute congestive heart failure or thrombosis. Where the initial episode is controlled and appropriate therapy instituted many cats can live for months or, in some cases years, on continued therapy.

Conclusions   

As the progression of clinical signs (once they occur) can be rapid and fatal, the early detection of signs of heart disease and the early diagnosis of a cardiac condition can allow appropriate preventive therapy to be instituted which may delay the onset of clinical signs and prevent the development of heart failure. More rigorous evaluation of therapy is required before we can really know at what stage therapy should be introduced and which therapy is the most beneficial. Evidence from human and canine patients suggests that in some cases introduction of treatment before the onset of clinical signs may delay the progression of the underlying disease and prolong life.