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Hypertrophic cardiomyopathy (HCM) is the most common heart
disease in cats. It develops and progress insidously over
time and its early signs may be subtle or nonexistent.
Certain breeds, including Maine Coons, Ragdolls en British Shorthairs, appear to have an increased
risk for development of HCM.
A cat that seems healthy may appear to become very ill
quickly, or even die suddenly.
Hypertrophic cardiomyopathy is a heart muscle disease in
which the muscular walls of the left ventricle become
abnormally thickened. Internal cardiac structures, such as
the papillary muscles, also thicken. HCM is diagnosed when
it is determined that the muscle thickening is not caused
by another illnes such as hyperthyroidism, systemic hypertension
or any of several other more rare diseases.
HCM is seen in cats at almost any age. Familial occurences
heve been observed in several breeds, and in thes instances
cats are most commonly diagnoses when they are less than
five years of age. While specific feline genetic mutations
have not been isolated yet, the pattern of inheritance in
families of cats strongly indicates an autosomal dominat
mode of inheritance, as is the case with humans.
To date, no viral or dietary causes of HCM have been
identified in humans or animals.
IVS = Interventricular Septum
LA = Left Atrium
MV = Mitral Valve
PA = Pulmonary Artery
TV = Tricuspid Valve
As HCM develops and progresses, it alters the structure of
the heart and impairs its function in several ways. While
the hallmark of feline HCM is thickened left ventricular
walls, early HCM is usually also characterized by enlarged
papillary muscles and an effect called systolic anterior
motion (SAM) of the mitral valve. This abnormal motion of
the mitral valve may partially obstruct the outflow of
blood from the left ventricle to the aorta.
The abnormal mitral valve position also allows blood to
flow backward, or regurgitate, into the left atrium. This
distortion in the mitral valve’s position causes a systolic
heart murmur that often becomes lauder with excitement or
softens as a cat relaxes.
With HCM, the thickened left ventricular walls become
stiff, which prevents the ventricle from relaxing
sufficiently to fill with blood at a normal pressure from
the left atrium. The increased pressure in the relaxed left
ventricle increases pressure in the left atrium, which, in
turn, enlarges. This increased pressure also backs up into
the capillaries into the lung tissue, causing pulmonary
edema, and into the ches cavity, causing pleural effusion.
These abnormalities are called ‘congested heart failure.
They make it difficult for the cat to breathe.
Blood flow in the enlarged left atrium slows, which can
cause blood clots to form in this heart chamber. A blood
clot can dislodge and be forced by blood flow into the
body’s circulatory system, where it eventually becomes
lodged and blocks the flow of blood to tissue beyond that
point. A classic example of this is rear leg paralysis,
which is caused when a clot lodges in the descending aorta
at the point where it branches to go to each of the back
legs. This situation is commonly reffered to as a saddle
Outward signs of HCM may include a barely noticeable
increase in breathing rate to marked signs of respiratory
difficulty, rear leg paralysis, or the sudden death of a
cat that seemed healthy only months earlier.
Early signs can be so slight that even an observant owner
may not notice them. Many cats with HCM develop a heart
murmur, a gallop rhythm, and/or a rapid heart rate, which
the cat’s veterinarian may detect during a routine
examination. An echocardiogram (= an ultrasound of the
heart) with color Doppler imaging is the most conclusive
means of diagnosing HCM. While electrocardiograms and
X-rays may provide the veterinarian with additional useful
information, they cannot be used to arrive at an
unequivocal diagnosis. HCM may also be diagnosed at
veterinary autopsie. The clinician must consider several
factors, such as the thickness of the left ventricular
walls, the weight of the heart, and the size of the left
atrium. The heart often contracts after death, causing it
to thicken. Consequently, an increase in thickness alone
cannot be used to diagnose HCM. A normal cat heart should
weigh less than 20 grams.
At present there is, unfortunately, no cure for HCM. The
veterinarian may, however, prescribe one or more
medications in an effort to reduce the signs of heart
failure and to help the heart function as efficiently as
possible. Treatment options and the specific drugs
prescribed will vary depending on the cat’s clinical signs
and the heart’s functional impairments. Calcium channel
blockers and beta blockers are drugs that are commonly used
in cats with HCM, either when they are in heart failure or
before heart failure occurs. Beta blockers are particularly
effective at reducing SAM. Cats in heart failure need a
diuretic and an angiotensin converting enzyme (ACE)
inhibitor. Fluid withdrawal from the chest is required if
fluid is present in the chest cavity. Aspirin or other
agents may be given in an attempt to prevent clot
formation. A cat that is suffering from respiratory
distress, acute heart failure, or limb paralysis requires
veterinary care as quickly as possible. There is nothing
you can do at home to relieve your cat’s distress or pain.
If you have questions about your cat’s treatment, please
discuss them with your veterinarian. Never attempt to
medicate your cat without veterinary instruction and
It is difficult to predict the course of HCM; the disease
may progress rapidly or it may stabilize for a long period
of time. The good news is that a cat with mild to moderate
HCm may enjoy an essentially normal life for a number of
years. The bad news is that the long-term outlook for a cat
that is suffering from severe heart failure or a saddle
thromboembolus is often guarded to poort despite everyone’s
Other Forms of
refers to Hypertrophic Cardiomyopathy (sometimes known as
Idiopathic HCM). Another type of cardiomyopathy is Dilated
Cardiomyopathy, which is related to insufficient taurine in
the diet. Fortunately, all cat food manufacturers
supplement their foods with taurine, so this disease is
A third condition, Restrictive/Intermediate Cardiomyopathy,
mimics HCM in history and examination, but can be
differentiated with echocardiography. Unfortunately, its
prognosis is poor.