FELINE HYPERTROPHIC CARDIOMYOPATHY

(Photocredit: Morguefile.com)

Heart disease can take many forms: abnormal rhythm of contraction, abnormal muscle tissue, valve disease or combinations of all of these as often disease in one heart tissue leads to abnormality in other heart tissues.

Cardiomyopathy literally means abnormality of the heart muscle and is the chief primary heart disease of the domestic cat. There are several types of heart muscle disease/cardiomyopathy: dilative (where the muscle becomes weak and flaccid), restrictive (where the muscle becomes tough/fibrous and thus inflexible), hypertrophic (where the muscle becomes enlarged and inflexible), arrythmogenic right ventricular (where the right ventricle becomes atrophied), and unclassified (which has features of more than one of the above). This article covers hypertrophic cardiomyopathy, often abbreviated “HCM.” HCM is the most common cardiomyopathy, accounting for 60% of all heart disease diagnosed in the cat.

Normal heart filling with and pumping out blood in its normal coordinated fashion. (Photocredit: Public Domain Graphic via Wikimedia Commons)

Normal heart with normal heart muscle thickness (Photocredit: Public Domain graphic via Wikimedia Commons)

Heart with hypertrophic cardiomyopathy. Note the thickened muscle walls. (Photocredit: Blausen.com staff. "Blausen gallery 2014". Wikiversity Journal of Medicine.)

In cats, the average age at diagnosis is 6 years.

Cats with heart disease are classified into two categories: cats with symptoms of heart disease and cats with no apparent symptoms. Recommendations are vastly different for each group and they will be discussed separately below. 

DIAGNOSIS: SYMPTOMATIC CATS

The thick, inflexible heart muscle that characterizes HCM leads to problems with the filling of the heart. The muscle is strong but is so thick that the normal blood volume the heart should be pumping cannot fit into the smaller chambers. This means less blood is ejected from the heart for each pump and the heart must pump faster to get the same amount of blood out.

Heart failure results when blood returning to the heart cannot be accepted by the small chambers. Blood volume backs up leading to pulmonary edema (fluid in the lung), pleural effusion (fluid in the chest but outside the lung), or abdominal effusion (fluid in the belly). The owner notices listlessness, poor appetite, possibly labored breathing or abdominal distension. Unlike the canine situation, cough is not usually a feature though it might be.

Observations such as these would generally bring the cat to the vet’s office for a check up. Physical examination findings might include an increased heart rate (though this is not consistent in cats), labored breathing/breathing with effort from the abdominal muscles, rapid breathing, even overt respiratory distress. A murmur may be heard with the stethoscope indicating turbulent blood flow or an extra heart sound, called a “gallop,” may be heard indicating the heart muscle is stiff.

From here, diagnostics depend how much distress the patient is experiencing; heart failure can be an emergency requiring hospitalization or the cat may be stable enough for home treatment. Complicating matters is the fact that there are other causes of the symptoms described above which might not be caused by heart disease. Tests which may be recommended include:

Echocardiography

Echocardiographic cross section of a heart with hypertrophic cardiomyopathy
(Photocredit: Kalumet via Wikimedia Commons)

This is the central part of the heart’s evaluation. Ultrasound is used to image the heart chambers, measure their thickness, observe the motion of the valves, check for presence of abnormal blood clots, even measure blood pressure gradients (depicted by the color changes seen above) and turbulence. It is through this type of testing that the diagnosis of HCM is confirmed. Often referral to a veterinary cardiologist is recommended for this type of evaluation.

Therapy is recommended depending on the presence or absence of heart failure, degree of outflow obstruction, efficiency of the valves, and ability of the heart to contract strongly.

Basic laboratory blood and urine testing are also generally done to get a sense of the patient's health beyond the heart and to get a baseline for future monitoring.

DIAGNOSIS: ASYMPTOMATIC CATS AND MURMURS

A heart murmur is simply the sound generated by turbulent blood flow. While turbulent blood flow can come from heart disease in cats, it can also come from anxiety or even compression of the chest from the stethoscope. This makes interpretation of a heart murmur in an apparently healthy cat difficult. Does the cat have latent heart disease or no disease at all? Further complicating matters is the fact that not all cats with HCM even have murmurs. So lots of cats with heart disease will have no symptoms and no discernable heart murmur and lots of cats with no heart disease will have audible murmurs.

NT-ProBNP(blood test)
This is a recently developed blood test that measures a protein that is present when heart muscle has been inappropriately stretched. The idea is that an abnormal amount of this protein can indicate that moderate or even severe heart disease is present. The test can be sent out to a laboratory to obtain a numeric value or a simple positive/negative test can be run in a matter of minutes in the vet's office. It has become a helpful tool in screening of asymptomatic cats, with or without murmurs, though it is only reliable in detecting moderate to severe heart muscle disease. More subtle cases will still be undetectable.

Between 1/3 and 1/2 of cats diagnosed with HCM have no symptoms at the time of diagnosis.

In one study, approximately one healthy cat in five had a heart murmur despite having no heart disease.

Obviously, it would be great to identify cats with HCM prior to the development of clinical signs so as to intervene and prevent or delay problems or at least be able to inform the owner what to watch for. Unfortunately, no therapy has been shown to definitively alter the course of the disease prior to the onset of heart failure. Despite this, therapy is commonly recommended to help the stiff heart muscle relax so as to allow for more efficient chamber filling. Therapy in cats without heart failure is fraught with controversy as we do not know if it is helping or not, despite theory that it ought to.

Retrospective studies of asymptomatic patients suggest
survival times of greater than 3 years and it is not clear
which drugs if any are associated with the best results.

STRANGE ACRONYMS: "LVOTO" AND "SAM"

These are terms you may hear your cat's cardiologist use. LVOTO stands for "Left Ventricular Outflow Tract Obstruction." The left ventricular outflow tract is the tract or path taken by the blood being pumped out of the left side of the heart. The tract consists of the left ventricle wall on one side and the inter ventricular septum on the other side. (In the diagram shown, the left ventricular outflow tract is the "cavity" in the center of the heart and to the left of the mitral valve.) In HCM, the leaflets of the adjacent mitral valve can actually flap against the septum when the ventricle is pumping. This action obstructs the path the blood is trying to take and is called SAM or "Systolic Anterior Motion" of the mitral valve. SAM and LVOTO go hand in hand. When SAM causes obstruction of the left ventricular outflow tract the result is a big pressure difference across the obstruction which ultimately leads to even more muscle hypertrophy. SAM also causes the mitral valve (the valve on the left side of the heart separating the receiving chamber from the pumping chamber) to leak (“mitral insufficiency”). This all sounds like it would bode especially poorly but despite what seems to make sense and despite what is seen in the human version of HCM, the significance of SAM remains debated. SAM is not something a cat either has or does not have; it is something that can come and go depending on what the patient is doing or feeling. Both LVOTO and SAM are common notations on an echocardiographic report for a patient with hypertrophic cardiomyopathy and if they are found medication (usually atenolol) is generally prescribed to reduce the severity of SAM.

Heart with hypertrophic cardiomyopathy. Note the thickened muscle walls. (Photocredit: Blausen.com staff. "Blausen gallery 2014". Wikiversity Journal of Medicine.)

CLOTS WITHIN THE HEART

Arrow shows a large clot in the left atrium of a cat with a saddle thrombus.
(Image courtesy of Veterinary Specialists of the Valley and the North Hollywood Animal Care Center)

The heart's left atrium must fill the inflexible narrowed left ventricle with an effective amount of blood. Because of the difficulty of this task and the back pressure involved, the left atrium in HCM becomes enlarged and may retain blood that it was unable to pump forward. This abnormal flow pattern promotes the formation of a large clot in the left atrium. The large blood clot spits off smaller blood clots which shoot down the aorta (the largest artery of the body) and lodge in unfortunate locations. Classically, such a clot lodges where the descending aorta splits to supply blood to the rear legs. This cuts off the circulation to the rear legs creating a potentially fatal and painful situation whereby the legs become unusable, and the feet become cold. Many cats can recover from this crisis so the situation is not entirely hopeless in and of itself; however, even if the cat is able to regain circulation to the rear legs, the clot in the heart remains as does the HCM and the situation can recur. This syndrome is known by the acronym “FATE” which stands for Feline Aortic Thromboembolism. The clot itself is often called a “saddle thrombus” because of its shape.

Currently, preventive therapy is still being worked out. The idea is to anti-coagulate the cat’s blood prior to the formation of the large clot. It is currently believed that cats with very large left atria are at risk and if “smoke” is seen on the echocardiogram this should be considered an additional risk factor. (Obviously, there is no actual smoke in the cat’s heart but whisps of turbulence can be seen and appear similar to curls of smoke rising into the air.) In the past, low doses of aspirin were used as preventive blood thinners but more recently a new drug called clopidogrel bisulfate, appears to yield superior results.

For more information about FATE, click here.

SUDDEN DEATH

Hypertrophic cardiomyopathy can cause congestive heart failure, aortic thromboembolistm (FATE), no symptoms at all or sudden death may be the only symptom. Many cats appear perfectly normal until they are found dead having had no symptoms of any kind to suggest they were diseased. Undetected HCM is classically responsible for anesthetic death during elective surgery procedures in apparently healthy cats. What exactly happens to these cats is unknown but a sudden fatal arrhythmia is suspected meaning that the abnormal muscle does not conduct electrical impulses properly and loses the coordinated electrical rhythm needed to produce effective blood pumping. Death can occur in minutes. We do not know how to prevent sudden death even in cats who are known to have HCM, though screening with the NT ProBNP test (see above) has made for a non-invasive screening test for heart disease in asymptomatic cats. Despite, improvements in how to detect HCM, it is still not clear how to prevent sudden arrhythmia (or heart failure, as discussed above) before it happens. For now, one must accept these possibilities once the diagnosis has been made.

HEREDITY

MAINE COON CATS AND RAGDOLLS 

In humans, HCM has a genetic basis so it should not be surprising that a genetic basis has been identified in cats as well. The Maine coon cat and Ragdoll are breeds where mutations have been identified creating the abnormal muscle of HCM (different mutations in each breed). There are likely many potential genetic mutations that can create HCM. In the Maine Coon Cat, the heart muscle mutation responsible is estimated to reside in 15% of the population.

Genetic testing can be done at the Genetics Lab at the University of California at Davis:

www.vgl.ucdavis.edu/services/cat/MaineCoonHCM.php
(for Maine Coon Cats)

www.vgl.ucdavis.edu/services/cat/HCM.php
(for Ragdolls)

It is also worth noting there is a predisposition towards males over females for the development of HCM.

HYPERTHYROIDISM AND THE HYPERDYNAMIC HEART

We have discussed the classification of cardiomyopathy on a structural basis but it is important to realize that heart disease can be caused by diseases in other organ systems. The so-called “hyperdynamic” heart can be indistinguishable from the heart with primary cardiomyopathy but the exact same changes in the heart muscle may be reversible if an underlying cause can be determined. In other words, the condition cannot truly be called “cardiomyopathy” if there is an identifiable underlying cause for it.

The chief cause of the hyperdynamic heart is hyperthyroidism, a very common hormone imbalance usually of geriatric cats. Unlike cardiomyopathy patients, cats with hyperthyroidism are likely to find that their heart muscle changes resolve in time when their metabolic state is corrected.

Other causes conditions that might lead to heart disease similar to that of hypertrophic cardiomyopathy include: acromegaly, arterial hypertension, and even subaortic stenosis (chiefly a canine problem). If further testing for a particular patient is warranted, your veterinarian will make recommendations based on what is reasonably likely.

Page posted: 1/24/2011
Page last revised: 6/3/2022