Hypertrophic cardiomyopathy (HCM) in cats

First of all, if you are reading this post about hypertrophic cardiomyopathy (HCM) in cats, it is likely because you just got a diagnosis from your veterinarian. If that is true, I want you to know, from one cat lover to another, that I know a little bit about your pain and worry. The absolute worst thing about loving a cat is facing illness with them.

The second thing I want to tell you, before I tell you anything else about this disease, is that you did not cause your cat to have HCM. And just as importantly, there was nothing you could do to prevent your cat from having HCM. You are doing exactly the right thing at this very moment: you are educating yourself about this disease and you are giving your cat the veterinary care he needs to manage HCM as well as possible.

And the third thing I want to say is that veterinarians have known about hypertrophic cardiomyopathy in cats for more than 50 years, and they still don’t fully understand how or why cats get HCM, and, although there are many helpful treatments, they cannot cure it.

HCM in cats is still difficult to detect, difficult to treat, and the course of the disease is unpredictable. Learn as much as you can about HCM now so you can understand the treatments your veterinarian is recommending, and feel comfortable asking questions as they arise for you.

How does the feline heart function?

It’s helpful to understand the basics of how a normal, healthy heart functions to better understand HCM.

The heart is a muscle and it has four little rooms, called chambers, inside of it. There are two rooms at the top called atria (which is plural for “atrium”), and there are two rooms at the bottom called ventricles. We refer to the chambers by the side of the heart they are on, so there are left and right atria, and left and right ventricles.

The feline heart is designed much like a human heart and it works like a human heart. (In fact, humans get HCM, too. It’s estimated that 1 in every 500 adults have it.[1])

Blood is constantly moving around the body, thanks to the heart and the highway of veins and arteries. Its purpose is to bring oxygen from the cat's lungs to all the cells in his body. Once the oxygen gets used up by the cells, the blood returns to the heart and lungs to get more.

This is how blood flows: veins take the “used up” blood from the body and send it to the right atrium of the heart. The right atrium squeezes the blood into the right ventricle. The right ventricle squeezes the blood into the lungs to get refreshed with oxygen.

The refreshed blood flows into the left atrium. The left atrium squeezes it into the left ventricle, and then the left ventricle squeezes the blood through a valve in the aorta – the largest artery in the body. The aorta is the superhighway that takes the blood that is now refreshed with oxygen, all around the body.

The cycle is continuous, with blood flowing to and from the heart with every one of your cat’s heartbeats.

What exactly is hypertrophic cardiomyopathy, or HCM?

HCM is the most common form of heart disease in cats[2] and it’s also one of the most common causes of death in cats.[3]

The name of the disease itself explains exactly what HCM is: hypertropic means “excessive development” and cardiomyopathy means “disease of the heart muscle.”

HCM is an overgrowth, or a thickening, of the heart muscle.

It almost sounds like a good thing, right? Normally, we think of bigger muscles as being better, but in this case, it isn’t. Thicker walls in the little heart rooms mean that there is less space to put blood. Thicker walls are also less flexible. Stiff heart walls don’t squeeze very efficiently.

HCM mostly affects the left ventricle of the heart – the primary pumping chamber that sends all that oxygenated blood out through the aorta. The disease not only makes pumping harder for this chamber, but it makes relaxing in between pumps harder, too.

Sometimes, the result is that the heart pumps more rapidly, using up too much oxygen. Without enough oxygen, heart cells can die. This can lead to arrythmias in which the heart beats too fast, too slowly, or with an irregular rhythm.[4]

But that is not the only problem. This disease can also affect the valve between the left atrium and the left ventricle. Little flaps – they call them “leaflets”– in the valve get sucked into a place they’re not supposed to be, called the aortic outflow tract.

When the valve isn’t working right, it puts pressure on the left atrium, which gets stretched out. An overstretched left atrium strains the lungs, which causes them to fill with fluid. When heart weakness causes fluid to build up in the lungs, we say that a cat is suffering from congestive heart failure.

About 2/3 of cats who are diagnosed with HCM also have this valve malfunction and a stretched-out left atrium, and with it, the risk of developing congestive heart failure.[5]

And finally, when the heart isn’t pumping well, and valves are malfunctioning, blood can back up into the other chambers of the heart and into the lungs. Blood that is flowing in the wrong direction, or sitting around too long, can form clots[6], which create a whole new set of dangers that we’ll talk about in a moment.

Which cats are at risk for HCM?

Almost any cat can get hypertrophic cardiomyopathy. Male cats and female cats can be affected. Young cats, old cats and every age of cat in between can be affected. This terrible disease has been reported in cats as young as three months and as old as 17 years.

That being said, most cases of HCM occur in middle-aged cats: those between four and eight years of age.[7] And while HCM affects male and female cats equally, males tend to get more severe disease at a younger age.[8]

Although scientists have not yet clearly identified the cause of HCM, it is believed to be an inherited disorder.[9] Several heart-gene mutations have been found that seem to be associated with the disease, which suggests that genetics plays an important role.[10]

HCM has been more deeply studied in certain cat breeds. A specific genetic defect in a muscle-contracting protein, called myosin binding protein C, has been identified in Ragdolls and Maine Coon cats with HCM.[11]

But even if your Maine Coon or a Ragdoll tests negative for this genetic defect, it doesn’t mean that your cat will never develop HCM, as other genetic mutations may be at play.[12] In humans, more than one genetic defect has been associated with HCM, and this is likely also true for cats.[13]

HCM seems to be more prevalent in these cat breeds:

• Maine Coon
• Ragdoll
• British Shorthair
• Sphynx
• Chartreaux
• Persian[14]
• Devon Rex
• Burmese
• Himalayan
• American Shorthair[15]
• Bengals
• Norwegian Forest Cat
• Turkish Van[16]

Knowing that your cat is one of the breeds that may be at risk for HCM is a good thing. Don’t bury your head in the sand and hope for the best. Stay on top of this potential problem. Early diagnoses and treatment can help stave off certain symptoms of HCM and improve your cat’s quality of life for as long as possible.

What are the symptoms of HCM in cats?

One of the problems with HCM is that the symptoms that a cat may have vary, and vary in severity, possibly because different genetic mutations are at play.

Some – actually, most – cats do not even appear ill; in fact, 55% cats with HCM have no symptoms at all.[17] The first time you hear that your cat may have hypertrophic cardiomyopathy might be at a vet visit when your vet mentions that she hears a heart murmur or irregular heartbeat during a normal physical exam.

But some cat owners see the signs before their vets do. Here’s what a cat owner might notice, especially if the disease is advancing to congestive heart failure:

• Weight loss
• Hiding
• A reluctance to socialize with the guardian, or other cats
• Coughing (uncommon)
• Loss of appetite
• Lethargy
• Labored, or rapid breathing
• Open-mouth breathing
• Collapsing

Certain additional symptoms arise when a cat develops a blood clot in the in the left atrium, as described above. If a blood clot forms, a piece can break off and travel through the body.

If the blood clot travels down the aorta and gets stuck, it could cut off the blood supply to the cat’s hind legs. If this happens, it could cause severe pain in one or both back legs, or, in extreme cases, paralyze the hind limbs.[18]

Sudden death is also a terrible possibility that is associated with HCM, although this “symptom” is, fortunately, relatively rare.

How is HCM in cats diagnosed?

Quite often, the first time you hear that your cat may have HCM is when your vet hears a heart murmur or irregularity during a regular office visit.

But a heart murmur doesn’t necessarily mean your cat has HCM. There are other potential causes of unexpected heart sounds, including high blood pressure and hyperthyroidism. In fact, 25% of otherwise healthy cats may have a murmur, but only a quarter of those cats will actually have HCM.[19] Your vet will check your cat’s blood pressure and blood hormone levels to rule these causes out.

And just as importantly, not all cats with HCM will have a heart murmur. So, how is HCM definitively diagnosed?

Let’s take one step back first. Is there a way to know if your cat is even at risk of developing HCM? There actually are new genetic tests to see if your cat has an increased risk of HCM. But just because your cat has a particular genetic mutation doesn’t mean she will definitely develop this disease. The test is helpful because it allows you to keep a watchful eye over your cat throughout his life. Should he develop HCM, you’ll be more likely to catch it early.

In terms of diagnosing HCM, there is a blood test, called the NT-proBNP test, that measures the hormones that can show if a cat’s heart is stretching or straining too much. This test is best at identifying cats who have no symptoms but are actually suffering with moderate to severe disease. Unfortunately, the test can produce false positives – meaning that the test will say that your cat has HCM even if he doesn’t.[20]

One of the best ways to identify HCM is through echocardiography – also known as ultrasound.

Ultrasound uses sound waves to create an image of the heart. A vet who is trained in this method, or a veterinary cardiologist, can see and measure the thickened walls of the left ventricle. Normal heart-wall thickness is well known and documented; your vet will be looking for left-ventricle walls that are more than 6mm thick.[21]

Note that ultrasound is a painless, non-invasive procedure. Many cats do not even require sedation for an ultrasound. Some wigglier or fussier cats may require only a small amount of sedation.[22]

Your veterinarian might also use an electrocardiogram (EKG or ECG) to evaluate an irregular heart rhythm.

 X-rays are not a very useful diagnostic tool in the early stages of this disease. As the disease progresses, however, it may be possible to see an enlarged left ventricle and atrium, fluid in the lungs (called “pulmonary edema”), or fluid in the chest (called “pleural effusion”) on an X-ray.[23]

How is HCM in cats treated?

Unfortunately, there is no known cure for HCM in cats. There is simply no way to reduce the thickness of the heart muscle. The goal of all HCM treatments is to improve the ability of the ventricle to fill with blood, to prevent or at least delay the onset of congestive heart failure, and to prevent devastating complications like blood clots.

There are many possible medications and treatments. The ones your vet chooses will be based on his personal experience and preferences, and also, of course, the condition of your particular cat and the particular symptoms that need to be addressed.

In mild cases of HCM, the benefit of treatment has not been well studied. You and your vet will have to take into consideration what is found on ultrasound, and any other health conditions your cat may be suffering from, plus, the temperament of your cat, and the cost of treatment, before deciding what to do. Close monitoring of the cat without treatment may be an option, depending upon these many factors.

In more advanced cases of HCM, your vet may recommend drugs that:

These are only a few of the many potential treatments that your veterinarian may recommend.

How long will my cat live if she has HCM? What is her prognosis?

There are no simple answers when it comes to this disease. In some cats the disease progresses rapidly, over a period of months. Other cats remain the same for years, surviving with only mildly compromised heart function. A slow, gradual progression of disease is most common, however.

If your cat has just been diagnosed, your vet might recommend a second echocardiogram within three to six months to assess the progression of the disease. If no significant progression is noted then, cats with mild HCM are usually just rechecked every year.

Cats with advanced disease will naturally require more monitoring.

But you can do your part at home, too. Look for changes in your cat: lameness, lethargy, loss of appetite, difficulty breathing.

Keep tabs on your cat’s respiratory rate. You can use the free Cardalis app to help you keep track. Try to check his breathing rate daily by counting the number of breaths in 30 seconds and multiplying by two. (You might want to try this on a sleeping cat, or at least one that is not purring.) A cat’s normal respiratory rate is 36 breaths per minute or less. If your cat’s rate is consistently higher than that, check in with your vet.

Two large studies looked at survival times for cats with HCM, and both came to the same conclusions:

• Cats who get a blood clot but survive the first 24 hours, have a median survival time of two to six months.
• Cats with congestive heart failure will survive for three to 18 months.
• Cats with no symptoms will live for three to five years.[27]

A final thought on HCM

Wherever you are in this journey with your cat, know that there are never going to be enough days together, no matter how many there are. It’s easy to feel overwhelmed with treatment options and worry. But there are so many beautiful moments with a cat at every stage of life. Try, if you can, to focus on them.

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Dawn LaFontaine is a lifelong animal lover who always seems to have a little pet hair in her keyboard. Her blog, Kitty Contemplations, helps cat guardians better understand and care for the special beings they share their lives and homes with. Her cat-products business, Cat in the Box, sells beautiful, well-made, and award-winning products that she designed to meet the biological needs of cats.

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FOOTNOTES 

[1] “What Is Cardiomyopathy in Adults?” Www.heart.org, https://www.heart.org/en/health-topics/cardiomyopathy/what-is-cardiomyopathy-in-adults.

[2] “Hypertrophic Cardiomyopathy.” Cornell University College of Veterinary Medicine, 23 July 2018, https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/hypertrophic-cardiomyopathy.

[3] “Landmark Heart-Health Study Sheds New Light on Feline Hypertrophic Cardiomyopathy.” AAHA, https://www.aaha.org/publications/newstat/articles/2018-06/landmark-heart-health-study-sheds-new-light-on-feline-hypertrophic-cardiomyopathy/.

[4] “Hypertrophic Cardiomyopathy.” Cornell University College of Veterinary Medicine.

[5] Plotnick, Arnold. “Hypertrophic Cardiomyopathy.” Manhattan Cat Specialists, 28 Oct. 2019.

[6] “Feline Hypertrophic Cardiomyopathy (HCM).” MedVet, 8 Dec. 2016, https://www.medvetforpets.com/feline-hypertrophic-cardiomyopathy-hcm/.

[7] Plotnick, Arnold. “Hypertrophic Cardiomyopathy.”

[8] Cunningham, Suzanne M., and Kursten V. Roderick. “Introduction to Heart and Blood Vessel Disorders in Cats - Cat Owners.” MSD Veterinary Manual, MSD Veterinary Manual, 8 Oct. 2021, https://www.msdvetmanual.com/cat-owners/heart-and-blood-vessel-disorders-of-cats/introduction-to-heart-and-blood-vessel-disorders-in-cats.

[9] “Signs & Symptoms of Hypertrophic Cardiomyopathy (HCM) Heart Disease in Cats, Treatment Options in Upstate NY |.” Upstate Veterinary Specialties, 27 Apr. 2017, https://www.uvsonline.com/hypertrophic-cardiomyopathy-hcm/.

[10] “Hypertrophic Cardiomyopathy.” Cornell University College of Veterinary Medicine.

[11] “Hypertrophic Cardiomyopathy in Cats (HCM).” Default, https://www.vetspecialists.com/vet-blog-landing/animal-health-articles/2020/04/29/hypertrophic-cardiomyopathy-in-cats.

[12] “Signs & Symptoms of Hypertrophic Cardiomyopathy (HCM) Heart Disease in Cats, Treatment Options in Upstate NY |.” Upstate Veterinary Specialties.

[13] ibid

[14] “Hypertrophic Cardiomyopathy.” Cornell University College of Veterinary Medicine.

[15] “Hypertrophic Cardiomyopathy in Cats (HCM).”

[16] Kittleson, Mark D. “Acquired Heart and Blood Vessel Disorders in Cats - Cat Owners.” MSD Veterinary Manual, MSD Veterinary Manual, 8 Oct. 2021, https://www.msdvetmanual.com/cat-owners/heart-and-blood-vessel-disorders-of-cats/acquired-heart-and-blood-vessel-disorders-in-cats.

[17] Plotnick, Arnold. “Hypertrophic Cardiomyopathy.”

[18] “Hypertrophic Cardiomyopathy.” Cornell University College of Veterinary Medicine.

[19] “IDEXX SNAP Feline ProBNP Test— Use NT-ProBNP at Point of Care to Assess Stretch and Stress on the Heart.” https://www.idexx.com/files/snap-felinebnp-update.pdf. Accessed 19 Oct. 2021.

[20] “Hypertrophic Cardiomyopathy in Cats (HCM).”

[21] “Feline Hypertrophic Cardiomyopathy (HCM).” MedVet.

[22] “A Better Way to Screen Cats for Heart Disease.” Tufts Now, 17 Oct. 2019, https://now.tufts.edu/articles/better-way-screen-cats-heart-disease.

[23] Plotnick, Arnold. “Hypertrophic Cardiomyopathy.”

[24] ibid

[25] “Feline Hypertrophic Cardiomyopathy (HCM).” MedVet.

[26] “Signs & Symptoms of Hypertrophic Cardiomyopathy (HCM) Heart Disease in Cats, Treatment Options in Upstate NY |.” Upstate Veterinary Specialties.

[27] Plotnick, Arnold. “Hypertrophic Cardiomyopathy.”.