HCM Defined

Hypertrophic Cardiomyopathy
by Arnold Plotnick MS, DVM, ACVIM, ABVP


Getting to the heart of the matter can be tricky – especially when the heart is what’s the matter. Just ask Laura Clutz.

Laura was the proud owner of Rooney, Riley, and Michael, three young robust male cats. Michael and Riley, brothers from the same litter, were two years old. Rooney, the baby of the bunch, was four months younger. In July, all three cats were examined as part of their annual check-up.

Rooney passed his physical examination with flying colors. Michael and Riley, however, had more than their kinship in common. They both had periodontal disease that required professional dental scaling. More disconcerting, though, was the discovery that both had identical sounding heart murmurs.

Morning television news programs and pharmaceutical company advertising has made Americans keenly aware that heart disease is the number one cause of death in people in the United States. Most cat owners, however, probably don’t realize that cardiovascular diseases are also quite common in cats. In fact, disorders of the myocardium (heart muscle) are the major cause of heart failure in cats.

Michael and Riley’s heart murmurs were sending a message that something might be amiss in the heart department. Before these boys could be anesthetized for their dental cleaning, it would be necessary to prove that their hearts were healthy. Sadly, after a battery of tests that included cardiac ultrasound, the cause of Michael and Riley’s murmurs were revealed: both boys were suffering from a troublesome cardiac condition called hypertrophic cardiomyopathy (HCM).

Hypertrophic Cardiomyopathy

“Hypertrophic cardiomyopathy is the most commonly seen cardiovascular disease in cats”, says Dr. Jim Ross, board-certified cardiologist at Tufts School of Veterinary Medicine. “We see cats with the disease every day--either as follow-up cases, or new cases that are referred because the primary veterinarian heard a heart murmur or suspected heart failure.”

To understand the pathology of HCM, a brief lesson in heart anatomy is in order. The heart has four chambers that pump blood. The two chambers on the left side (the left atrium and left ventricle) are separated from the chambers on the right (the right atrium and right ventricle) by a dividing wall, called the septum. The atria are found near the top of the heart; the ventricles reside below the atria. The normal thickness of the walls of the feline heart have been measured and documented and are well-known to veterinary cardiologists. In cats suffering from HCM, the walls of the heart become progressively thicker and thicker. The thickening of the heart mainly affects the left ventricle, the chamber that pumps blood out through the aorta to the rest of the body. Very often, the lower part of the septum – the part that separates the left ventricle from the right ventricle – is also affected. When the left ventricle becomes thickened, the chamber becomes smaller and the muscle becomes very stiff, making it difficult to fill with blood. It also becomes difficult for the blood to be pumped out of the ventricle and through the aorta. Sometimes, a portion of the valve that separates the left atrium from the left ventricle gets sucked into the aorta’s outflow tract as the thickened ventricle tries to pump blood out of the aorta. This makes it even more difficult for blood to flow out of the aorta, and it also causes the valve to malfunction. As a result, the left atrium become stretched out and dilated. This can lead to increased pressure in the atrium. This increased pressure may be transmitted to the lungs, resulting in fluid retention in the lungs and eventual congestive heart failure. This phenomenon, in which one of the leaflets of the mitral valve is drawn into the aortic outflow tract, is called systolic anterior motion (SAM), and it occurs in about 2/3 of cats diagnosed with HCM.

Although there are specific conditions that can cause the heart muscle to become thicker (such as hyperthyroidism and high blood pressure), HCM is considered to be a primary disease because an identifiable disease process cannot be found. Excessive amounts of serum grown hormone and magnesium deficiency have been investigated as possible causes, but a cause-and-effect relationship could not be proven. For now, the cause remains unknown.

Clinical Signs

Cats of either sex can be affected, although males are typically affected more than females. Although the disease has been reported in cats ranging in age from 3 months to 17 years, most cases occur in middle age, usually between the ages of 4 and 8.

The clinical signs of HCM can vary greatly from cat to cat. Some cats present with very non-specific signs, such as lethargy, inappetence, weight loss, hiding, and reluctance to socialize with the owner and with other cats. Coughing occurs occasionally, but it fairly uncommon, especially when compared to dogs with heart disease. In most cases a cat has no clinical symptoms, however, some abnormality is detected when the stethoscope is placed on the cat’s chest. “Most of our referral (non-emergency) cases that come to our cardiology service are referred because a murmur and/or a gallop rhythm is detected during the cat’s annual physical exam”, confirms Dr. Ross. Such was the case with Michael and Riley. “I had no idea anything was wrong with either of my cats” recalls Laura Clutz. “They were both acting perfectly normal”.

Michael and Riley’s case is typical. In fact, 55% of cats with HCM present with no symptoms at all. Unfortunately, a fair number cats are discovered to have HCM when they present to the veterinarian already in congestive heart failure. An even more upsetting (and dire) scenario is the discovery that a cat has HCM when it presents as an emergency with sudden painful hind limb paralysis. This devastating complication of HCM occurs when a blood clot (also known as a “thrombus”) forms within the left atrium, and a small piece of this clot (called an “embolus”) breaks off from the main clot and travels down the aorta, lodging at the end of the aorta, rapidly cutting off the blood supply to the legs. This condition is known as arterial thromboembolism (ATE). In the next issue of Catnip, subscribers can read about this devastating complication of HCM, and the possible new treatments on the horizon.

Despite a lot of research, the cause of the HCM remains unknown, although there is substantial evidence that the disease is inherited in at least 2 breeds: the Maine Coon and the American Shorthair. In fact, researchers have recently identified the gene mutation responsible for causing HCM in Maine Coons. It is the first time that a spontaneous genetic mutation has been reported to cause any type of heart disease in a cat or a dog. This discovery paves the way for the development of a screening test that will identify Maine coon cats carrying this genetic mutation, so they can be identified before they are bred. Until such a test becomes available, however, if HCM is identified in any cat it is advisable not to breed the affected cat, and to carefully screen closely related family members. The majority of cases, however, are domestic shorthaired cats without any family history of disease.

Making the diagnosis

X-rays, an electrocardiogram (EKG) and echocardiography (cardiac ultrasound) are the common diagnostic tests performed on animals with suspected heart disorders. For cats with HCM, x-rays tend to be of limited usefulness. X-rays tend to be normal in the early stages of the disease. As the disease progresses, however, the left ventricle and left atrium may appear enlarged on an x-ray. Classically, a valentine-shaped heart silhouette is seen. If the cat goes into heart failure, pulmonary edema (fluid in the lungs) and/or pleural effusion (fluid in the chest cavity) may be visible on the x-rays.

An EKG can provide the veterinarian with useful information, as disturbances in the electrical conduction system of the heart occur in about 30% of cats with HCM. Although EKG abnormalities are common, changes are often non-specific, and can even be normal in cats with HCM.

Ultimately, a definitive diagnosis of HCM is achieved by means of echocardiography (cardiac ultrasound). Ultrasound allows for evaluation and measurement of numerous parameters, including the size of the chambers, the thickness of the heart muscle, the function of the valves, how well the heart is contracting, how efficiently the blood is flowing through the heart, and whether or not there is a blood clot in the left atrium.

Echocardiography revealed that Michael and Riley both had thickening of their heart muscle. Riley’s left ventricle was a bit thicker than Michael’s, but the rest of his chambers were normal or only mildly dilated. Michael’s left ventricle wall and septum were a little less thickened than Riley’s, however, the thickening was seriously affecting the ability of the ventricle to push blood out of the aorta. Michael also had SAM - a portion of his mitral valve was also being drawn into the aortic outflow tract – further taxing the ventricle’s ability to pump.


Therapy is mostly palliative, as there is no way to reduce the thickness of the heart muscle. The goals of managing cats with HCM are to improve the ability of the ventricle to fill, prevent or delay the onset of congestive heart failure, and prevent such complications as arterial thromboembolism.

Several drugs are available to the veterinary practitioner. Medications are prescribed to slow down the heart, so that there is adequate time for the stiff ventricles to fill, and to relax the heart muscle, again to facilitate filling of the stiffened chambers. Which drug or drugs are prescribed depends on the ultrasound findings, whether or not the cat has concurrent congestive heart failure, and the personal preference and experience of the veterinarian or veterinary cardiologist. Dr. Ross weighs in with his treatment philosophy: “If the cat has no symptoms, and relatively mild left atrial enlargement, we begin treatment with either a beta-blocker such as atenolol, or a calcium channel blocker such as diltiazem. Both can slow the heart rate, improve ventricular relaxation, reduce myocardial oxygen consumption, and reduce abnormal rhythms. Some cats respond better to beta blockers, others to calcium channel blockers – it’s impossible to predict how an individual cat will respond. We start with one drug, and follow-up to see how they are doing after several weeks of therapy, and make adjustments as necessary.”

For cats with HCM that are already in congestive heart failure, Dr. Ross says that more aggressive therapy is necessary. “If cats present in heart failure, oxygen therapy, cage rest, and nitroglycerine ointment may be necessary to get the cat stabilized.”, he says. Once the cat is stabilized, other medications may be required. Cats that are in heart failure and have fluid accumulation in their lungs often benefit from having diuretics administered. Another category of drugs, called angiotensin converting enzyme (ACE) inhibitors has been shown, in some studies, to be useful in managing HCM. Some veterinarians use ACE inhibitors only when the cat has developed congestive heart failure. Others believe the drug to be beneficial in cats with HCM regardless as to whether or not they have CHF. To reduce the chance of a thrombus forming within the heart, many cats are given medications that reduce the blood’s ability to clot, such as aspirin or heparin.


The prognosis for HCM varies. Hypertrophic cardiomyopathy can progress rapidly in some cats, while in others, the condition remains relatively static for years. Many cats will have slowly progressive disease that ultimately leads to congestive heart failure. A significant number of cats with HCM will be fine for a while, only to develop rear limb paralysis due to ATE. Although the end of the aorta, where it branches off to supply the rear legs is the most common site for an embolus to lodge, other arteries can be affected, including those that supply the kidneys or the front legs. When the heart muscle becomes thickened, the coronary arteries have trouble supplying enough blood to the heart muscle. The areas of inadequate blood supply may serve as a site where abnormal rhythms are generated. These abnormal heart rhythms can lead to fainting, or in some instances, sudden death.

Cats who have no clinical signs and whose only evidence of disease is limited to physical exam findings such as a heart murmur or a gallop rhythm have a better survival rate than cats that present to veterinarians already in congestive heart failure or with rear limb paralysis due to a blood clot. Two large studies have looked at survival times for cats diagnosed with HCM. Both studies reach the same conclusion: cats that develop a blood clot do the worst; those that survive their initial 24 hours have a median survival time of 2 to 6 months. Cats with congestive heart failure fare somewhat better, surviving for 3 to 18 months. Those with no symptoms survive for 3 to 5 years after the diagnosis.

Sadly, Michael’s HCM progressed rapidly. The increased force needed to pump blood out of Michael’s partially obstructed aorta put too much of a strain on his left ventricle. Three months after Michael’s initial diagnosis, despite medical therapy, Michael’s heart began to fail, and he died of congestive heart failure. His brother Riley continues to do well on medication. “Each cat with HCM is unique in its disposition, environment, ability to administer and tolerate medication, and concurrent medical problems in other body systems. You need to work with your veterinarian to provide the best hospitalization and home care that fits your situation”, says Dr. Ross.

Sidebar: possible outcomes of HCM

• No symptoms - Cat remains asymptomatic for years, does well on medication
• Heart failure - Cat’s condition gradually progresses, cat develops congestive heart failure
• Arterial thromboembolism (ATE) – A thrombus (blood clot) forms within the heart, and an embolus (a piece of the clot) breaks off, lodges in a branch of the aorta, causes sudden hind limb paralysis
• Fatal arrhythmia – a disturbance in the electrical conduction system of the heart develops, leading to an abnormal rhythm and sudden death

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