Here is the short version: the most common heart disease in cats thickens the heart muscle from the inside, often causes no symptoms you could possibly notice, and in too many cats the first sign is a sudden collapse or a clot that paralyzes the back legs. Screening studies suggest it hides in roughly one of every seven seemingly healthy cats. The disease cannot be prevented or cured, but it can be found, and finding it early is the entire game. This guide explains what it is, which cats carry the risk, why a stethoscope is not enough, and what you can actually do.
The most common feline heart disease, and a quiet one
Hypertrophic cardiomyopathy, usually shortened to HCM, means the muscle of the heart's main pumping chamber grows abnormally thick. A thick heart is a stiff heart, and a stiff heart cannot relax and fill properly between beats. Among cats with any heart disease, more than 85 percent have this one, which makes it the dominant cardiac problem in the species.1 Cornell's feline center states it plainly: HCM is the most common heart disease of cats.2
What makes it dangerous is not rarity. It is silence. The same trait that runs through the most serious cat diseases, including the chronic kidney disease we cover separately, runs through this one: it advances without obvious symptoms, so the body absorbs the damage long before an owner sees a reason to worry.
How common is it, really
More common than most owners would ever guess. A screening study often called CatScan examined 780 apparently healthy cats and found that about 15 percent had HCM.3 These were not sick cats brought in for heart trouble. They were ordinary cats that looked fine. The 2020 consensus statement from the American College of Veterinary Internal Medicine puts general-population prevalence near that same 15 percent, and notes it climbs to roughly 29 percent in older cats.4
Payne et al., CatScan study, J Vet Cardiol 2015; ACVIM consensus, 2020
Sit with that for a moment. In a room of fourteen healthy-looking cats, the statistics say two of them have a heart condition nobody knows about. That is the reason this article exists.
The breeds and the genes
HCM can appear in any cat, including ordinary domestic shorthairs, but some breeds carry clearly higher risk. The consensus statement lists Maine Coon, Ragdoll, British Shorthair, Persian, Bengal, Sphynx, Norwegian Forest Cat, and Birman among the predisposed breeds.4
Two of those have a known genetic cause. In 2005, researchers identified a specific mutation in a gene called MYBPC3 in Maine Coons with familial HCM, the first time a spontaneous HCM-causing mutation had been pinned down in any non-human species.5 Two years later the same group found a different mutation in the same gene in Ragdolls.6 Genetic tests now exist for both. They are useful for breeding decisions, but they come with a sharp limit worth understanding: the mutations are nearly specific to those two breeds, and a cat can test negative and still develop HCM. A clear test is reassuring, not a guarantee.4
Why your vet's stethoscope is not enough
With many diseases, a careful physical exam gets you most of the way. HCM is not one of them. A heart murmur, the sound vets listen for, is genuinely unreliable in cats. A large share of perfectly healthy cats have one, and plenty of cats with real HCM have none at all.4 So a murmur cannot confirm the disease and the absence of a murmur cannot rule it out. Listening helps, but it misses too many cats to be the test you rely on.
This is the single most important practical point in this article. If your cat is a high-risk breed, or has any hint of heart trouble, normal listening results are not the same as a clean bill of cardiac health.
The clot that announces the disease
The cruelest way HCM reveals itself is a blood clot. When the heart's upstream chamber enlarges and blood pools, a clot can form, break loose, and travel down the aorta until it jams where the artery splits toward the hind legs. Veterinarians call it arterial thromboembolism, or a saddle thrombus. It causes sudden, severe pain and paralysis of the back legs, and it is frequently the very first sign that a cat had heart disease at all.
The outcomes are sobering and worth stating honestly. In a study of 127 affected cats, among those treated, about 45 percent survived to leave the hospital, and survival afterward was shorter for cats that also had heart failure.7 In everyday general practice the picture is harder still, because many cats are in such pain and distress at presentation that euthanasia is chosen. None of this is meant to frighten. It is meant to explain why catching the disease in the quiet years, before a clot is ever possible, matters so much.
Smith et al., J Vet Intern Med, 2003
How it is actually diagnosed
The test that settles the question is an echocardiogram, an ultrasound of the heart, ideally read by a cardiologist. It measures the thickness of the heart walls and the size of the chambers, which is what confirms HCM and grades how far it has progressed.4 Chest X-rays are far less sensitive and can look normal in mild or even moderate disease, so they are not a screening tool for this.4
There is also a blood test, NT-proBNP, which rises when heart muscle is stressed. A point-of-care version can flag cats that deserve a full scan: in one multi-center study it detected moderate-to-severe hidden heart disease with about 89 percent sensitivity.8 It is a smart triage step, especially for screening at-risk cats or sorting out a cat in breathing distress, but it does not replace the echocardiogram and it can miss milder disease.4
What the stages mean
The 2020 consensus statement sorts cats into stages, the same shorthand a cardiologist will use with you.4 Stage A is an at-risk cat, such as a predisposed breed, with no detectable disease yet. Stage B is HCM with no symptoms, split into B1 for lower immediate risk and B2 for higher risk, usually because the upstream chamber has enlarged. Stage C is a cat that has had heart failure or a clot. Stage D is heart failure that no longer responds to treatment. Most cats with HCM are found, or could be found, in the symptom-free B stages, which is exactly where there is the most time to work with.
What owners can actually do
Start with honesty about the limits: there is no proven way to prevent HCM and no cure.2 That can feel discouraging, so here is the part you do control. You can get at-risk cats screened, you can keep affected cats monitored so heart failure is caught early and treated, breeders can test for the known mutations and avoid doubling them up, and you can track the one home number that flags trouble sooner.
That number is the same one cardiologists use for dogs, and it works for cats too. A resting or sleeping cat almost always breathes fewer than 30 breaths per minute.9 A sustained rise above that can be one of the earliest signals that fluid is backing up into the lungs, and it is a reason to call your vet rather than wait and see.
What to do this week
- If your cat is a Maine Coon, Ragdoll, Sphynx, Persian, or another at-risk breed, ask your vet about heart screening, which may mean an NT-proBNP blood test, a referral for an echocardiogram, or both.
- Learn your cat's normal sleeping breathing rate this week, while it is healthy, so you have a baseline. Count chest rises for a full minute.
- Do not take a clean stethoscope exam as proof the heart is fine, especially in a high-risk breed. Ask whether imaging is warranted.
- If you are getting a pedigree kitten, ask the breeder whether the parents were screened or genetically tested, and treat that as a real selection factor.
HCM is common, quiet, and serious, which is an unsettling combination. The encouraging part is that quiet diseases reward attention more than almost anything else. A blood test, a scan when it is warranted, and a minute spent watching a sleeping cat breathe are small things. For a disease that so often arrives without warning, they are how you give yourself a warning.