FELINE HYPERTROPHIC CARDIOMYOPATHY
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 more than one of the above).
This article covers hypertrophic cardiomyopathy, often abbreviated “HCM.” HCM is the most common cardiomyopathy.
In cats, the average age at diagnosis is 6 years.
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:
Echocardiographic cross section of a heart with hypertrophic cardiomyopathy
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 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.
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? Numerous studies looking at cats with heart murmurs and with HCM have been conducted and are difficult to interpret for numerous factors (different criteria for the definition of HCM, circumstances under which murmurs were heard etc.) It is not clear what percentage of apparently healthy cats are hiding HCM. Not all cats with HCM will have murmurs and certainly not all cats with murmurs will have HCM (or any other heart disease).
Between 1/3 and 1/2 of cats diagnosed with HCM have no symptoms at the time of diagnosis.
In one study, approximately one apparently healthy cat in five had a heart murmur.
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
LVOTO – “Left Ventricular Outflow Tract Obstruction” The left side of the heart receives oxygenated blood from the lungs and pumps it to the body. The tract that conducts blood from the heart towards the body is called the “left ventricular outflow tract.” In HCM, sometimes, the leaves of the mitral valve (on the left) flap against the septum that separates the heart into right and left halves. Because the septum forms the left ventricular outflow tract, a blockage to blood flow can occur depending on the the flapping of the metal valve leaflets. This flapping is called “SAM”.
SAM – “Systolic Anterior Motion of the Mitral Valve”. When SAM is seen on an echocardiogram, this does not necessarily mean HCM is present but it is certainly highly suggestive that HCM is present. SAM causes obstruction of the left ventricular outflow tract which in turn causes a big pressure difference across the obstruction. 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.
CLOTS WITHIN THE HEART
Turbulent blood flow and left atrial enlargement can create a very serious situation in the form of a large blood clot in the heart. 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.) What therapy is appropriate is best is still unclear. Choice include low dose aspirin, or clopidogrel bisulfate (Plavix®). Studies are on going.
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. Research is on-going.
Genetic testing can be done at Washington State University
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: 1/24/11