Hypertrophic cardiomyopathy (HCM) is the most common heart disease in cats. Most cats with it look completely healthy, and many never develop symptoms. A smaller group goes on to heart failure, a clot that suddenly paralyzes the back legs, or sudden death. You can’t tell from the outside which group your cat is in, and a stethoscope check often misses the disease.
What HCM is
In HCM, the muscle wall of the left ventricle (the chamber that pumps blood out to the body) grows too thick. Thick muscle is also stiff, so it can’t relax between beats and blood has trouble flowing in. Over time the left atrium (the chamber that feeds blood into the ventricle) has to push harder and gets stretched larger. Most of the trouble in HCM starts with that enlarged atrium.
When the atrium is enlarged, blood backs up into the lungs and fluid collects there. That is congestive heart failure (CHF). Blood also moves slowly inside the big atrium and can form a clot. If the clot breaks loose, it travels down the aorta and often lodges where the aorta splits to the back legs (a saddle thrombus), and the back legs suddenly go limp.
HCM is a problem of the heart muscle itself, often inherited. The muscle can also thicken because of high blood pressure or an overactive thyroid. That isn’t HCM, and the thickening can go back down once the cause is controlled.
How common is it?
About 15% of cats have HCM, and it gets more common with age. That figure comes from heart ultrasound screening of cats that looked healthy. Most affected cats have no symptoms, and routine checkups don’t include a heart ultrasound, so it usually goes unnoticed.
Most cats with HCM stay at the stage where changes show on ultrasound but the cat feels fine. Among cats diagnosed while they still had no symptoms, about 23% died of a heart-related cause within five years. That is an average over a whole group and can’t predict what happens to one cat. Some stay stable for years; others slowly get worse.
Breeds at higher risk
Several breeds are more likely to develop HCM: Maine Coon, Ragdoll, British Shorthair, Persian, Bengal, Sphynx, Norwegian Forest Cat, and Birman. Mixed-breed cats get it too, and so can cats of other breeds.
Maine Coons and Ragdolls each have a known HCM mutation, and a DNA test exists for it, used mainly to screen cats before breeding. A positive result is a reason to ask your vet about an earlier heart ultrasound. A negative result doesn’t mean the cat won’t get HCM.
Signs to watch for
Most cats show nothing until something serious happens. It’s hard to notice a cat slowing down when it already sleeps 16 hours a day, and cats with heart disease don’t cough the way dogs do. The signs that do show up almost always come from a complication.
Faster or harder breathing. This is the most useful early warning of heart failure. If a cat that used to breathe quietly in its sleep is now working at it, with the chest and belly heaving, breathing through an open mouth, or hiding to breathe alone, go to the emergency vet now. Open-mouth breathing in a resting cat is never normal and is usually serious.
Back legs that suddenly go limp, with severe pain. This is a clot lodged in the aorta (aortic thromboembolism, ATE, also called a saddle thrombus). The legs feel cold, the paw pads turn pale or blue, and the cat cries out in pain. Go to the emergency vet immediately. It can happen in a cat nobody knew had heart disease.
Fainting or collapse. A brief collapse, especially during stress or activity, can mean a dangerous heart rhythm.
Slowly eating less, losing weight, low energy. Many illnesses look like this, not only heart disease, but in an older cat or a high-risk breed it’s worth a checkup.
Go to the emergency vet right away for: open-mouth breathing at rest, fast and labored breathing, back legs that suddenly won’t move with crying in pain, or collapse.
Why a stethoscope misses it
A heart check at every annual exam doesn’t mean the heart is fine. About 30-45% of healthy cats have a heart murmur, and nearly 7 in 10 cats with HCM have none.
A gallop (an extra heart sound) or an irregular rhythm is more likely to mean real heart disease, but many cats with HCM sound normal. A normal stethoscope exam is somewhat reassuring, but it can’t rule out HCM. Only a heart ultrasound can confirm it.
How HCM is diagnosed
A heart ultrasound (echocardiogram) is the standard test for HCM. The vet measures how thick the left ventricle wall is, watches how the heart moves, and checks the size of the left atrium. Atrium size says the most about risk, because heart failure and clots both start with an enlarged atrium.
NT-proBNP is a blood test that rises when the heart muscle is stretched. It’s mostly used in a cat that is breathing hard, to help tell whether the problem is the heart or the lungs. It isn’t accurate enough to screen healthy-looking cats and doesn’t replace an ultrasound. Some clinics have a quick in-house version that can help when an ultrasound isn’t available.
An ECG is used when an irregular heartbeat is suspected, not to screen for HCM.
Chest X-rays show whether fluid has built up in or around the lungs, and are used once a cat has symptoms and heart failure is suspected.
Blood pressure and thyroid tests are standard too, to rule out thickening caused by high blood pressure or an overactive thyroid.
Stages of HCM
Vets sort HCM into five stages:
| Stage | What it means |
|---|---|
| A | A high-risk breed with no sign of heart disease yet |
| B1 | HCM without symptoms; the left atrium usually isn’t clearly enlarged, and the short-term risk of heart failure or a clot is low |
| B2 | HCM without symptoms, but the left atrium is clearly enlarged, and the short-term risk of heart failure or a clot is higher |
| C | The cat has had heart failure or a clot |
| D | Heart failure that no longer responds well to treatment |
How each stage is managed
There’s no cure for HCM, and the thickened muscle doesn’t go back to normal. Treatment aims to delay or reduce heart failure and clots.
Stages A and B1. No medicine has been shown to slow the disease in cats with no symptoms and low risk. The plan is a recheck ultrasound on the schedule your cardiologist sets, and counting resting breaths at home.
Stage B2. With a clearly enlarged left atrium, the risk of a clot goes up, and the vet usually prescribes a clot-prevention medicine.
Stage C. A cat that has had heart failure needs a diuretic to clear fluid from the lungs, usually long-term, with the dose adjusted by the vet based on breathing rate. Whether other heart medicines help depends on how the heart is working, and that’s the cardiologist’s call. After a clot, early care focuses on pain relief and supportive treatment, followed by long-term clot prevention.
What a clot (ATE) looks like
Within minutes of the clot lodging, the cat can’t walk. The back legs feel cold and stiff, the paw pads turn pale or blue, and the cat is in severe pain, crying out and panting with its mouth open. It can strike a cat that seemed perfectly healthy, with no warning. Don’t wait to see if it passes. Go straight to the emergency vet.
Many cats are euthanized on arrival because the attack is so severe. Of the cats that are treated, about 7 in 10 survive the first day, and some recover a lot of function and live a year or more. Getting to the emergency vet fast is the part an owner can actually help with.
Counting resting breaths at home (for diagnosed cats)
Once a cat is diagnosed, the most useful thing to do at home is count its breaths while it sleeps every day. This is the resting respiratory rate. A healthy cat asleep usually breathes fewer than 30 times a minute, and the number stays about the same from day to day. A number that keeps climbing, or stays above 30, is often the earliest sign of fluid in the lungs, hours or even days before the cat looks unwell. Catch it early, and your vet can adjust the diuretic before it turns into a middle-of-the-night emergency.
How to count: wait until your cat is asleep or fully relaxed (not while purring, which pushes the count up). Watch the chest or belly rise and fall. Each rise and fall is one breath. Count for 30 seconds and double it, or count a full minute. Try to do it around the same time each day, write the numbers down, and bring them to your vet at rechecks.
A healthy cat that counts high once after playing hard is nothing to worry about, and this method can’t tell whether a healthy cat has HCM.
For what else to watch at home as your cat gets older, see our senior cat care guide.
Frequently Asked Questions
How long can a cat live with HCM? It depends on the stage. Many cats with HCM never get sick from it and live a normal lifespan. Of cats diagnosed while they still had no symptoms, about 23% died of a heart-related cause within five years. Cats that have already had heart failure or a clot usually have less time, and care shifts toward keeping them comfortable.
Can HCM be cured? No. The thickened muscle doesn’t go back to normal, and treatment aims to delay or reduce heart failure and clots. If the thickening comes from high blood pressure or an overactive thyroid, it isn’t HCM, and it can improve once that condition is under control.
Should I get my cat genetically tested? The test is mainly for Maine Coons and Ragdolls used for breeding, so the mutation isn’t passed on. Pet Maine Coons and Ragdolls can be tested too: a positive result is a reason to talk to your vet about an earlier heart ultrasound, and a negative one doesn’t mean the cat won’t get HCM. For other breeds, no genetic test is recommended.
Why didn’t my vet hear anything wrong at the annual exam? Because HCM often makes no sound. Many cats with HCM have no murmur, while about 30-45% of healthy cats do. A normal stethoscope exam can’t rule HCM out. Only a heart ultrasound can confirm it.
If your cat is a high-risk breed such as a Maine Coon or Ragdoll, or something sounded off at a checkup, ask your vet about one heart ultrasound to use as a baseline. If your cat is already diagnosed, counting its breathing at home every day helps you spot changes early.
References
- Luis Fuentes, V., Abbott, J., Chetboul, V., et al. (2020). ACVIM consensus statement guidelines for the classification, diagnosis, and management of cardiomyopathies in cats. Journal of Veterinary Internal Medicine, 34(3), 1062-1077. PMC
- Paige, C. F., Abbott, J. A., Elvinger, F., & Pyle, R. L. (2009). Prevalence of cardiomyopathy in apparently healthy cats. Journal of the American Veterinary Medical Association, 234(11), 1398-1403. PubMed
- Meurs, K. M., Sanchez, X., David, R. M., et al. (2005). A cardiac myosin binding protein C mutation in the Maine Coon cat with familial hypertrophic cardiomyopathy. Human Molecular Genetics, 14(23), 3587-3593. PubMed
- Meurs, K. M., Norgard, M. M., Ederer, M. M., Hendrix, K. P., & Kittleson, M. D. (2007). A substitution mutation in the myosin binding protein C gene in ragdoll hypertrophic cardiomyopathy. Genomics, 90(2), 261-264. PubMed
- Borgeat, K., Wright, J., Garrod, O., Payne, J. R., & Luis Fuentes, V. (2014). Arterial thromboembolism in 250 cats in general practice: 2004-2012. Journal of Veterinary Internal Medicine, 28(1), 102-108. PMC
- Ljungvall, I., Rishniw, M., Porciello, F., Häggström, J., & Ohad, D. (2014). Sleeping and resting respiratory rates in healthy adult cats and cats with subclinical heart disease. Journal of Feline Medicine and Surgery, 16(4), 281-290. PubMed
- Porciello, F., Rishniw, M., Ljungvall, I., et al. (2016). Sleeping and resting respiratory rates in dogs and cats with medically-controlled left-sided congestive heart failure. The Veterinary Journal, 207, 164-168. PubMed
