Hypertrophic cardiomyopathy in cats: the silent diagnosis

If you are reading this, a vet has probably just told you your cat has HCM, or a heart murmur, or has used the phrase "thickened heart muscle". Or you may be somewhere much harder: your cat was well yesterday, and today they are in an emergency clinic, suddenly breathless or dragging a back leg, and someone has said the word "cardiomyopathy" for the first time. Both of these are frightening, and neither is your fault. The aim of this article is to give you a calm, accurate picture of what is happening, why it so often stays hidden until a crisis, and how to think about the decisions ahead.

Hypertrophic cardiomyopathy (HCM) is the most common heart disease in cats. It is a condition in which the muscular wall of the heart's main pumping chamber, the left ventricle, becomes abnormally thick. That thickening leaves less room inside the chamber for blood to fill, and makes the muscle stiffer and less able to relax between beats. Over time this can lead to a backlog of pressure and fluid, to abnormal heart rhythms, and to blood clots forming inside the heart. Many cats with HCM live for years with no outward sign at all, which is exactly what makes it so difficult.

What is HCM in cats?

HCM is a disease of the heart muscle, not the heart valves. In plain terms, the wall of the main pumping chamber grows too thick, so the chamber holds less blood and cannot relax properly to fill.

The heart still contracts, often very strongly, but a stiff, thick-walled chamber is a problem in two ways. It fills poorly, so less blood is pushed out with each beat, and the raised pressure inside backs up into the chamber behind it, the left atrium, which stretches and enlarges. That enlarged, sluggish left atrium is where much of the danger sits. It is where clots can form as a result of turbulent blood flow, and it is the pressure point from which fluid eventually floods into or around the lungs.

HCM has a strong genetic component in some breeds. Maine Coons and Ragdolls are known to carry specific gene mutations, and Bengals, British Shorthairs, Sphynx and Persians are also over-represented. It is common in ordinary domestic shorthairs and longhairs too, where no single cause is identified. It usually appears in middle age, though it can show up young in the predisposed breeds. International Cat Care has a clear overview at icatcare.org if you want to read further.

Why is it so often silent?

Cats are very good at hiding illness, and HCM hides better than most. A large number of affected cats show no symptoms whatsoever until the day something goes seriously wrong.

There are a few reasons for this. Cats are naturally low-activity animals who sleep most of the day, so a heart that is quietly struggling rarely gets pushed hard enough to give itself away, the way a dog's might on a walk. Cats also do not typically develop the persistent cough that alerts dog families to heart trouble. And because early HCM often produces no murmur, or only a faint intermittent one, many cats pass a routine health check looking entirely well.

The unfortunate truth is that for a significant number of cats, the first sign of HCM is the crisis itself: sudden breathlessness, a suddenly paralysed leg, or, in the worst cases, sudden death. This is not a reflection of anything missed at home. It is the nature of a disease that does its damage on the inside, quietly, and then announces itself all at once.

Where HCM is picked up before a crisis, it is usually by chance: a murmur, or a particular heart rhythm called a gallop rhythm, heard during a vaccination check or a pre-anaesthetic examination, which then leads to a scan.

What are the signs of HCM in cats?

For many cats there are no signs until a crisis. When signs do appear, they usually come from one of three things: fluid backing up into the lungs (heart failure), a blood clot lodging in an artery, or an abnormal rhythm.

The signs worth knowing, roughly in the order families tend to meet them, are these.

Faster or harder breathing. An increase in the resting or sleeping breathing rate is one of the earliest and most useful warnings that fluid is starting to build. A cat at rest who is breathing quickly, with visible effort, or with an open mouth, is a genuine emergency and needs to be seen straight away. Open-mouth breathing in a cat almost always means serious trouble.

Reduced energy and hiding. A cat who is quieter, sleeping more, hiding away, or no longer jumping up to their usual spots may be telling you their heart is struggling, even before the breathing changes.

Sudden hind-leg weakness or paralysis. A cat who suddenly cannot use one or both back legs, often crying out in pain, with cold paws and pads that look pale or bluish, is very likely suffering an arterial blood clot. This is a medical emergency and one of the most painful things that can happen to a cat.

Collapse or fainting. An abnormal heart rhythm can cause a cat to collapse briefly, or in the worst cases to die suddenly with no warning.

A murmur on its own is not a symptom and does not confirm HCM, and plenty of cats with murmurs have structurally normal hearts. It is the pattern, and a scan, that gives the diagnosis.

Saddle thrombus: the clot that changes everything

One of the most feared complications of HCM is arterial thromboembolism (ATE), often called a saddle thrombus. A clot forms in the enlarged, sluggish left atrium, breaks free, and lodges where the main artery divides to supply the back legs.

The result is sudden, severe, and unmistakable. The cat loses the use of one or both hind legs, usually with intense pain, cold limbs, and pads that have lost their colour. Many families understandably think their cat has been hit by a car or has broken its back, because the onset is so abrupt. In reality the blood supply to the legs has been cut off.

ATE is one of the hardest situations in feline medicine, for two reasons. The pain is severe and needs urgent, strong relief. And the outlook is genuinely guarded: even with intensive hospital treatment, a proportion of cats do not survive the initial event or the days that follow, and clots can recur even when a cat pulls through. Because of this, a saddle thrombus is one of the points at which many families and vets have an honest conversation about whether continuing is the kindest path, or whether a peaceful goodbye is. There is no wrong answer here. A cat in this much pain, with an uncertain outlook, is a situation where choosing to let go is an act of mercy, not of giving up.

If a clot is caught and treated and your cat recovers, your vet will usually start blood-thinning medication, most often clopidogrel, to reduce the chance of it happening again. But the risk never fully goes away, and knowing that in advance helps families prepare rather than be blindsided.

Congestive heart failure in cats

The other way HCM becomes evident is via the symptoms of congestive heart failure (CHF), where the rising pressure inside the heart forces fluid out into or around the lungs.

In cats this fluid tends to collect either within the lung tissue (pulmonary oedema) or in the space around the lungs (pleural effusion), and both make breathing difficult. The cat breathes faster and harder, may sit hunched with elbows out, and in a crisis may breathe with an open mouth. This is an emergency. A cat in this state is frightened and short of air, and needs oxygen and treatment urgently.

The good news is that heart failure, once diagnosed, can often be controlled for a meaningful stretch of time. Emergency treatment usually means oxygen, a diuretic such as furosemide to draw the fluid off, and sometimes draining fluid from around the lungs with a needle, which can bring dramatic relief. Once stable, cats are typically managed at home on a diuretic, often alongside other medication tailored to their scan findings. The way heart failure develops and is treated has a lot in common with the same process in dogs, and our guide to heart failure in dogs walks through that shared picture in more detail.

Treatment does not cure HCM or stop it progressing. What it does, often well, is buy comfortable time and keep the breathing under control. As with any progressive heart disease, doses tend to need adjusting as things advance.

How is HCM diagnosed?

The single test that confirms HCM is an echocardiogram, an ultrasound scan of the heart. It is the only way to see the thickened muscle directly and measure the enlarged left atrium.

Your vet may first suspect a problem from a murmur, a gallop rhythm, or an irregular heartbeat heard during an examination, but none of these confirms the diagnosis on its own. A blood test called NT-proBNP can help flag a heart under strain and is sometimes used to decide whether a scan is worthwhile, particularly in a breathless cat where the cause is not yet clear. Chest X-rays show whether fluid has collected in or around the lungs. An ECG is used if the rhythm sounds abnormal, and blood pressure and a thyroid blood test are usually checked too, because both high blood pressure and an overactive thyroid can thicken the heart muscle in ways that mimic HCM and are treatable in their own right.

For the predisposed breeds, particularly Maine Coons and Ragdolls, genetic testing and screening echocardiograms are widely used, and responsible breeders screen their cats before breeding.

How long can a cat live with HCM?

There is no single answer, because HCM behaves so differently from one cat to the next. A cat diagnosed by chance with mild thickening and no symptoms may live a normal lifespan and die of something else entirely. A cat who presents in heart failure or with a clot has a more guarded outlook, often measured in months rather than years.

What the outlook is mostly affected by is not the diagnosis itself but whether, and how, the disease has started to cause problems. A cat with mild HCM and a normal-sized left atrium is in a very different position from a cat whose atrium is badly enlarged and who has already had fluid on the lungs or a clot. This is why the scan findings matter so much, and why your vet will talk in terms of your particular cat rather than averages. Rather than fix on a number, the more useful questions are how your cat is in themselves today, whether their breathing is controlled, and what their scan showed about the risk ahead.

When is it time?

This is the question that brings many families to an article like this, and it deserves a straight answer. There is no blood value or scan measurement that tells you it is time. The decision is led by your cat's quality of life, not by a number.

HCM tends to force the question in one of two ways. Sometimes it is a slow decline: a cat on heart-failure medication whose breathing becomes harder to control, who needs ever-higher doses, who is tiring, hiding, off their food, and no longer doing the small things they enjoy. When good days become rare and the difficult ones are not improving despite treatment, the conversation has shifted from managing the disease to keeping your cat comfortable.

Other times HCM forces the question in a single moment: a saddle thrombus, or a sudden flooding of the lungs, where your cat is in real distress and the outlook is poor. In that situation, choosing a peaceful, immediate goodbye rather than a long, painful, uncertain hospital stay is a loving decision, and one that many vets will gently support. It can feel brutally fast, because it is, but sparing a frightened, painful cat is not something to reproach yourself for.

Our quality of life assessment is a short, structured way to think this through when the decline is gradual. It will not give you a number that decides for you. It is there to give shape to what you are already noticing across breathing, comfort, appetite, energy, and the small pleasures your cat still has. If you are weighing it up, our piece on when is it time walks through the decision itself in more depth.

Whatever path HCM takes with your cat, the decision almost always feels too early until, in hindsight, it feels too late. Both of those feelings are normal, and neither means you got it wrong.

How we can help

We are a vet-led at-home euthanasia service. When the time comes, our visiting vets come to you, take time to explain everything, and use sedation first so your cat is calm and settled before the final injection. For a cat, being able to stay in their own home, in a familiar warm spot, with no stressful carrier and no car journey to a clinic, often matters a great deal. Our home euthanasia for cats is built for exactly that. Most visits last around an hour, and there is no rush.

If you would simply like to talk through where your cat is, with no obligation either way, you are welcome to get in touch. The line is open seven days a week, including evenings and weekends. There is no script and no pressure, only time to listen. We work across the UK.

A final note

Living with a cat who has HCM often means carrying a quiet worry alongside the ordinary days, the knowledge that a well-looking cat may not be as well as they seem. That is a hard thing to hold. Whether you are newly diagnosed and watching a breathing rate each evening, or standing in an emergency clinic making the hardest call of all, you are not alone in it. Whatever stage you are in, our team is on the phone any day, evenings and weekends included.

Sources

• International Cat Care, hypertrophic cardiomyopathy (HCM) owner guidance: https://icatcare.org/advice/hypertrophic-cardiomyopathy-hcm/
• American College of Veterinary Internal Medicine (ACVIM) consensus guidelines on the classification, diagnosis and management of cardiomyopathies in cats.
• British Small Animal Veterinary Association (BSAVA), Manual of Canine and Feline Cardiorespiratory Medicine, current edition.
• Royal College of Veterinary Surgeons (RCVS), Code of Professional Conduct, guidance on euthanasia and end-of-life care.