HCM: The Silent Heart Disease That Affects One in Seven Cats

Most feline illnesses announce themselves eventually — a limp, a rash, a change in the litter tray, weight sliding off month by month. Hypertrophic cardiomyopathy, almost always shortened to HCM, is different. It is the most common heart disease in cats, thought to affect roughly one in seven of the general cat population, and yet the majority of cats living with it look completely, convincingly normal. They eat, they play, they sit in the window. Then one evening the same cat is breathing hard in the corner of the room, or cannot use its back legs, and the owner is at an emergency clinic hearing about a disease they have never heard of.

That gap — between how well affected cats look and how serious the condition is — is the reason this article exists. HCM cannot be prevented and, at present, cannot be cured, but it can very often be detected before the crisis, and cats found early and monitored well can live comfortable years. Here is what the disease does, which cats are most at risk, the signs worth watching for, and the simple home check cardiologists consider the most useful thing an owner can do.

What HCM Actually Is

The left ventricle does the heart's heavy work, squeezing oxygenated blood out to the body. In HCM, the muscular wall of that chamber thickens abnormally — hypertrophy — but not in the useful way an athlete's muscles thicken. The thickened wall intrudes into the chamber, so it holds less blood, and crucially the muscle becomes stiff and struggles to relax between beats.

A stiff chamber that will not fill properly causes pressure to back up behind it, into the left atrium and then into the blood vessels of the lungs. Three consequences follow, and they are the three things that actually harm cats:

• Congestive heart failure. Pressure forces fluid out of the lung vessels into the lung tissue (pulmonary oedema) or into the space around the lungs (pleural effusion). The cat effectively cannot get enough air.
• Blood clots. Blood swirling slowly in an enlarged left atrium can clot. If that clot is pumped out and lodges where the main artery divides towards the hind legs, it causes feline aortic thromboembolism — the so-called saddle thrombus. It is sudden, agonisingly painful and a genuine emergency.
• Arrhythmias. Diseased heart muscle can generate dangerously abnormal rhythms, which in a minority of cats cause collapse or sudden death.

Importantly, many cats with mild HCM never progress to any of these. A cat can carry a modestly thickened heart for its entire life and die of something else entirely at fifteen. The disease is a spectrum, not a sentence.

A veterinary cardiology overview of what HCM is, why it hides so well, and what a diagnosis really means.

Which Cats Get It

Any cat of any age can develop HCM, including young ones — it has been diagnosed in cats under a year old. Most affected cats are middle-aged males of no particular breed, simply because most cats are of no particular breed. That said, several pedigrees carry a clearly increased risk, and two have known genetic mutations that can be tested for:

• Maine Coon — the MYBPC3 A31P mutation is a recognised risk factor, and DNA testing is standard in responsible breeding programmes.
• Ragdoll — a separate MYBPC3 mutation (R820W) is associated with earlier, often more severe disease.
• Sphynx, British Shorthair, Persian, Bengal, Norwegian Forest Cat and Devon Rex are all over-represented, though no single reliable gene test exists for them yet.

Two caveats matter. A negative gene test does not mean a cat cannot develop HCM — the known mutations explain only a fraction of cases — and a positive test does not guarantee illness, since some carriers never develop a thickened heart. Genetic testing is a breeding tool, not a diagnosis; only imaging the heart shows what a particular cat's heart is doing now. It is also worth ruling out the imposters: an overactive thyroid and high blood pressure can both thicken heart muscle in ways that mimic HCM but are treatable at the source, which is why a good workup includes a thyroid test and a blood pressure reading.

The Warning Signs — And Why There Often Aren't Any

Cats hide illness superbly, and they compensate for heart disease by doing less. A cat that quietly stops jumping onto the wardrobe does not look sick; it looks like a cat that has grown out of jumping. So the honest headline is that many cats with HCM show no signs at all until they crash. What a vet may pick up on a routine exam is a heart murmur, a gallop rhythm (an extra sound in the beat) or an irregular rhythm — useful flags, but imperfect ones: plenty of healthy cats have innocent murmurs, and plenty of cats with significant HCM have none.

The signs owners can watch for, in rough order of how early they appear:

• An increased resting or sleeping breathing rate — the earliest reliable home signal, and the subject of the next section.
• Reduced activity and stamina: sleeping more, playing less, no longer jumping to favourite high spots, hiding.
• Rapid or laboured breathing with visible effort in the belly, especially at rest.
• Open-mouth breathing or panting that is not explained by heat or a car journey. In cats this is a red flag — go to a vet immediately.
• Loss of appetite, weight loss, poor coat.
• Sudden hind-limb weakness or paralysis, crying in pain, cold back paws, pale or bluish paw pads — the classic saddle thrombus. This is a genuine emergency; go straight to a vet.
• Fainting or collapse.

One myth worth killing: coughing is not a typical sign of heart disease in cats. Dogs in heart failure cough; cats in heart failure breathe fast. A coughing cat is far more likely to have asthma or another airway problem. Do not use coughing as your heart-trouble alarm, and do not dismiss fast breathing because there is no cough.

The Sixty-Second Check Every Owner Should Know

If you take one practical thing from this article, take this. The sleeping respiratory rate (SRR) is the number of breaths a cat takes per minute while genuinely asleep — the most sensitive early warning of fluid building up in the lungs that you can measure at home, for free.

Wait until your cat is properly asleep — not purring, not dozing with one eye on you, since purring and movement ruin the count. Watch the chest and count one breath for each full rise and fall, for a full minute or for 30 seconds doubled. Write the number down.

• Normal: a healthy sleeping cat breathes roughly 15 to 30 times per minute, and most sit comfortably under 30.
• Concerning: a consistent SRR above 30, or a clear upward drift from your cat's own established baseline, warrants a call to your vet.
• Urgent: above 40, or any laboured or open-mouth breathing, means immediate veterinary attention.

The power of the measurement is the trend, not any single reading, so build a baseline while your cat is well: a couple of counts a week, jotted in a notes app or on the calendar. For a cat already diagnosed with heart disease, cardiologists commonly ask for a daily count — a rising SRR frequently precedes visible distress by a day or more, and that head start is exactly what turns an emergency into an appointment.

A veterinary cardiology team demonstrates the counting technique — it takes under a minute and needs no equipment.

How HCM Is Diagnosed

Suspicion usually starts with the stethoscope, but no vet can diagnose HCM by listening alone. The steps that follow typically include:

• Echocardiogram (heart ultrasound) — the gold standard, and the only test that directly measures wall thickness, chamber size and how the heart is filling. Non-invasive, no anaesthetic needed in most cats.
• NT-proBNP blood test — a marker released by stressed heart muscle. A useful triage tool for deciding whether a murmur justifies imaging, or whether a breathless cat's problem is heart or lung. It supports a decision; it does not replace the scan.
• Chest X-rays — poor at judging wall thickness but excellent for confirming fluid in or around the lungs when a cat is in trouble.
• Blood pressure, thyroid level and general bloods — to catch the treatable mimics described above.
• ECG — if an irregular rhythm is heard or suspected.

What a murmur does and does not tell you, and when it is worth investigating further.

Treatment and Living With It

There is no drug that reverses the thickening, and — this surprises people — a cat with mild, symptom-free HCM is often best left on no medication at all, monitored with repeat scans. Treatment is aimed at the complications, and looks broadly like this:

• Clot prevention. Cats with an enlarged left atrium are usually started on clopidogrel, which outperforms aspirin at preventing repeat thromboembolism. This is one of the highest-value interventions available.
• Congestive heart failure. Diuretics such as furosemide remove the accumulated fluid; a vet may also drain fluid from around the lungs, which brings immediate relief. ACE inhibitors, and pimobendan in selected cases, may be layered in.
• Obstruction and arrhythmias. Beta-blockers such as atenolol are sometimes used where outflow is obstructed or the rhythm is a problem, though evidence that they extend life is limited.
• Pain relief and emergency care for thromboembolism, which is intensely painful and needs immediate attention rather than a wait-and-see night at home.

Day to day, life with an HCM cat is mostly unremarkable: keep up the SRR counts, give medication on time, keep the cat lean, avoid unnecessary stress, and attend the recheck scans. Two practical points are easy to forget. Tell every vet who treats your cat about the heart diagnosis before any anaesthetic or intravenous fluids — a heart that cannot fill properly tolerates fluid overload badly. And resist the internet's supplements: no diet, taurine top-up or heart tonic prevents or reverses HCM. (Taurine deficiency causes a different disease, dilated cardiomyopathy, largely designed out of commercial cat food since the 1980s.)

Prognosis genuinely varies. Cats diagnosed incidentally with mild disease and a normal-sized left atrium often survive many years, sometimes a normal lifespan. Cats who have already had congestive heart failure or a thromboembolism face a far more guarded outlook, frequently measured in months to a couple of years, though some do better than the averages. Research into drugs that might slow the thickening itself is promising, but nothing has yet displaced early detection and careful monitoring as what matters most.

The Bottom Line

HCM is common, quiet and unpreventable, which sounds bleak until you notice what that combination actually implies: the outcome depends far more on when it is found than on anything you could have done to avoid it. So book the annual exam — and twice-yearly once your cat is seven or older — so a stethoscope regularly meets that chest. Ask about screening if you own one of the higher-risk breeds, and buy from breeders who scan and test their cats. Learn your own cat's sleeping breathing rate while it is healthy, so an abnormal number is obvious rather than debatable. And treat fast, laboured or open-mouth breathing, or sudden hind-leg weakness, as an emergency every single time. A cat whose HCM is caught on a routine visit has options. A cat whose HCM announces itself at 2am has far fewer.

This article is general information and not a substitute for veterinary advice. HCM can only be diagnosed by a vet, and the drugs mentioned here are prescription medicines that must never be given without veterinary direction — some human heart medicines, and aspirin in particular, are dangerous to cats at the wrong dose. If your cat is breathing rapidly or with effort, breathing through an open mouth, or has suddenly lost the use of its back legs, seek emergency veterinary care immediately.