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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. They 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 increased 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.
There are various different possible causes of cardiac muscle
disease in the cat.
Infectious
• FIV
Metabolic
• Hyperthyroidism
• Hypertension — renal failure
• Acromegaly
Nutritional
• Taurine deficiency
• L-Carnitine deficiency
Infiltrative
• Lymphosarcoma
• Amyloidosis
Toxic
• Adriamycin
Hereditary
• Maine Coone cats
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 results in the
development of clinical signs of congestive heart failure. In cats the most
common presenting sign of heart failure is dyspnea (difficulty in breathing)
caused by either pleural effusion or pulmonary edema.
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 . 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 Enbolism
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 embolization is the distal aorta
although involvement of the forelimbs and cerebral circulation can occur. It
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 dyspnea.
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.
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