Third Street
Veterinary Hospital, PC
McMinnville, Oregon 97128
Office phone: (503) 472-9418
Cardiomyopathy
in Cats
Cardiomyopathy literally means "disease of the heart
muscle." This is a disease that
occurs in purebred and non-purebred cats of any age. Males and females are equally affected. There are three distinct forms and a
variation of one of them.
Each will be discussed separately.
The cat's heart is comprised of four chambers: the right and
left atria (singular: atrium) (the top chambers) and the right and left
ventricles (the bottom chambers). The
right side of the heart receives oxygen-depleted blood from the body and pumps
it to the lungs so it can receive oxygen.
The left side of the heart receives oxygenated blood from the lungs and
pumps it to the body. Each chamber has a valve at its opening and another valve
at its exit. Each entry valve opens to
allow blood to enter its chamber, and then it closes. At that precise moment,
the exit valve opens and the walls of the chamber contract pushing blood out.
Cardiomyopathy
principally involves the left side of the heart, the side that pumps
oxygen-rich blood to the body. The left
ventricle is involved first, and then the right ventricle often is affected.
Hypertrophic cardiomyopathy (HCM) causes the walls of the
left ventricle to become thicker than normal. They are normally 4-5 mm thick;
however, if HCM is present they may become over 10 mm thick. As they thicken they also become stiffer and
less able to contract with the force needed to send blood to the distant parts
of the body. Most of the thickening that occurs in the ventricular walls is
directed inward causing the left ventricular chamber to be smaller. This limits the amount of blood that can
enter the left ventricle and the amount of blood that can be pumped to the
body. In addition, it puts great stress
on the mitral valve, the valve between the left atrium and the left ventricle.
After a few weeks the valve weakens and begins to leak, creating a murmur. This
murmur will often be present several weeks to months before the cat goes into
heart failure and may be detected during a routine physical examination.
HCM is also a disease of humans, and 50% of the time it is
an inherited disorder. There is
mounting evidence that HCM in cats is also a genetic disease, especially since
it is often seen in cats less than 2 years of age. It is suggested that littermates of HCM cats be tested for the
disease. The diagnosis of HCM is based on several tests. Hearing a murmur or an abnormal heart rhythm
is a signal that other tests should be performed. An electrocardiogram (EKG) may show abnormalities that also
suggest this disease. Radiographs (x-rays) of the heart usually reveal
characteristic changes in the shape of the heart, especially in the latter
stages when the left atrium enlarges.
However, many cats in the early stages of the disease will have normal
radiographs. An echocardiogram
(ultrasound study of the heart) is the most precise means of diagnosing this
disease. Ultrasound waves are directed
into the heart and analyzed by a computer permitting precise measurements of
the thickness of the heart's walls and the size of the chambers. We can also observe the mitral valve to
access its function. Treatment is determined by the stage of the disease at the
time of diagnosis. Cats in advanced
disease are in congestive heart failure and treated with several drugs to keep
the cat from dying. Diltiazem is used
to relax the walls of the left ventricle so more blood can enter and so it can
contract with more force. Propranolol
is used to slow the heart rate when it is beating too fast. Furosemide is a diuretic that stimulates the
kidneys to remove fluid that may have collected in the lungs (pulmonary
edema). Aspirin is used to prevent
blood clots that often form around the
mitral valve.
Eventually, they dislodge from the valve and cause obstruction of
arteries in various parts of the body.
Enalapril is a new drug used for this disease. One study has shown that its use may result in
thinning of the left ventricular walls and even return to
normal thickness in some cats. However,
this drug takes 2-4 months to cause these results. If HCM is detected before
the onset of congestive heart failure, enalapril and aspirin may be the only drugs
that are needed. However, because the
disease appears to be genetically caused, it is thought that enalapril probably
should be given for the rest of the cat's life to prevent recurrence.
The prognosis is largely dependent on when the disease is diagnosed
and how successful treatment is in correcting congestive heart failure. If it is diagnosed early or if congestive
heart failure is controlled, many of these cats live for several years.
Dilated cardiomyopathy (DCM) results in thinning of the
walls of the left ventricle. Instead of
being 4-5 mm thick, they may become only 1-2 mm thick. As they become thinner they also stretch and
become unable to contract properly.
This results in enlargement (dilation) of the left ventricle. Even though more blood can enter the
ventricle, the walls become so weak that they cannot pump the blood with enough
force to send it to the distant parts of the body. The end result is congestive heart failure. In the 1980's, DCM
was almost as common as HCM. However,
it was discovered that a deficiency of the amino acid taurine was the cause of
this disease in many cats. Cat food
manufacturers were notified that cats need more taurine than was previously
thought, and they responded by increasing its level in commercial cat
foods. Consequently, DCM has almost
disappeared. However, it is still seen
in cats that eat primarily dog food or table food and occasionally in cats that
eat commercial cat food. The latter
causes us to realize that taurine deficiency is not the only cause of this
disease, although other causes are not known at this time.
The diagnosis of DCM is based on several tests. An electrocardiogram (EKG) may show
abnormalities that suggest this disease, although most of these cats have
normal EKG's. Radiographs (x-rays) of
the heart often reveal characteristic changes in the shape of the heart,
especially in the latter stages when the left ventricle is markedly enlarged. However, many cats in the early stages of
the disease will have normal radiographs.
An echocardiogram (ultrasound study of the heart) is the most precise
means of diagnosing this disease.
Ultrasound waves are directed into the heart and analyzed by a computer
permitting precise measurements of the thickness of the heart's walls and the
size of the chambers. Treatment consists of drugs to control heart
failure. Digoxin is usually used to
cause an increase in the force of the contractions of the walls of the left
ventricle. Furosemide is a diuretic
that stimulates the kidneys to remove fluid that may have collected in the
lungs (pulmonary edema). Aspirin is used to prevent blood clots that often form
around the mitral valve. Eventually,
they dislodge from the valve and cause obstruction of arteries in various parts
of the body. Enalapril has shown some
promise in treating this disease even though the mechanism of action is not
understood at this time. Unfortunately, we do not have a drug that reverses the
thinning of the left ventricular walls.
Therefore, the long-term prognosis is not very good in many of these
cats.
Restrictive cardiomyopathy (RCM) is an uncommon variation of
HCM. The walls of the left ventricle
are thickened in some areas and not in others; they also become very stiff. One distinct difference is the presence of
inflammation in the walls of the left ventricle. This finding raises the question of some type of infection as a
possible cause of this disease. RCM progresses to congestive heart failure,
just as the other types. A murmur is
common, and the left atrium often dilates.
Treatment is much the same as HCM, but the results are often not as
good. Enalapril has not been used on
enough of these cats to determine its effect.
Thyrotoxic cardiomyopathy is a very mild form of
cardiomyopathy compared to the others.
This disease is the result of another disease of cats,
hyperthyroidism. Hyperthyroidism results
in thyroid gland enlargement and overproduction of thyroid hormone. Excess thyroid hormone stimulates the heart
resulting in mild thickening of the left ventricular walls. However, in this disease the force of heart
contractions is usually greater than normal and these cats often have high blood
pressure. Thyrotoxic cardiomyopathy resolves when hyperthyroidism is treated;
therefore, it has a very good prognosis if proper treatment for its underlying
disease occurs.
When the heart is functioning normally, blood flows through
it in a laminar (straight path) fashion.
Each of the forms of cardiomyopathy cause abnormal flow patterns,
especially around the mitral valve.
This often results in blood clot formation on the valve. These clots get larger and larger until
parts of them break off and are carried with the blood into the aorta and other
major arteries. Most commonly, a clot
will be carried down the aorta toward the rear legs. When the aorta splits to go to the rear legs, each new artery is
smaller than the aorta, much like a tree that branches. The clot then is too large to continue down
either artery so it suddenly stops, usually in such a way that blood flow is
prohibited to the main artery of each rear leg. Suddenly, the cat is in severe pain and cannot use either rear
leg. Within a few minutes to hours the
feet become cold and the pads become bluish.
This creates a severe crisis for the cat. It cannot walk on its rear legs, and it is in a great deal of
pain. The clot that obstructs these
arteries is commonly called a saddle thrombus.
Fortunately, there are other arteries that go to the rear
legs. Although they are not as large,
they are able to supply enough blood to the legs to keep the tissues from
dying, in most cases. Over the next
5-10 days, these smaller arteries send new branches to the blood-deprived parts
of the legs. During that time, various drugs are used to make the cat more
comfortable, and massage and physical therapy are performed to support the
muscles until they become functional again.
The vast majority of cats will regain their ability to walk.
Surgery has been performed on some of these paralyzed cats
to remove the blood clot. Sometimes the
surgery is very successful, and the cat resumes walking almost
immediately. However, the underlying
disease that causes the blood clot to form is cardiomyopathy, so many of these
cats will die during surgery because of heart failure.
Blood clots may obstruct other arteries. If one enters the renal (kidney) artery,
kidney failure will result. If one goes
to a front leg, lameness (but usually not paralysis) will occur. If one goes to the brain, a seizure or
stroke is likely.
We do not have a drug that will dissolve the blood clots on
the mitral valve or in the smaller arteries.
However, aspirin is used to prevent their formation. The average 10-pound cat should receive an
81 mg aspirin 2-3 times per week (every 2-3 days). Eighty-one milligram aspirin is sold as "children's
aspirin" or "low-dose adult aspirin." Children's aspirin (formerly known as baby aspirin) is usually
chewable and orange flavored, a taste that is generally objectionable to
cats. Low-dose adult aspirin is often
enteric (hard) coated and more easily swallowed; that works best for most cats.
You may have heard that aspirin should never be given to a
cat. Obviously, that is not a true
statement. However, cats are able to
break down aspirin very slowly. One
dose remains in the cat's body for 48-72 hours. Therefore, as long as the dose
is correct and the dosing interval long enough, it is very safe.
DO
NOT SUBSTITUTE ACETAMINOPHEN (TYLENOL) FOR ASPIRIN. ACETAMINOPHEN IS VERY TOXIC TO CATS.
Warfarin is another drug that may also be used to prevent
blood clots. It appears to be somewhat
more effective than aspirin in some cats, but overdosing results in severe
bleeding. Therefore, its use requires
very careful monitoring of the cat's blood clotting ability.