Aug 28, 2026
Cat
Symptoms
What Is HCM in Cats, and Why Does It Matter So Much?
HCM affects roughly 1 in 7 cats and often causes no symptoms until it's advanced. Learn the warning signs, high-risk breeds, and how it's diagnosed.
Hypertrophic cardiomyopathy (HCM) is the most common heart disease in cats, affecting roughly 1 in 7 — and most cats show no symptoms at all until the disease is already advanced. It causes the heart's main pumping chamber to thicken abnormally, making it harder for the heart to pump blood effectively. Left undetected, HCM can progress to heart failure, dangerous blood clots, or sudden death, which is exactly why screening matters more here than with most conditions.
What Actually Happens in HCM?
HCM causes the wall of the heart's left ventricle to thicken abnormally. That thickened muscle makes it harder for the heart to relax between beats, disrupts normal heart rhythm, and creates conditions where blood can pool and clot more easily. Left untreated, this progression can lead to congestive heart failure, arterial thromboembolism (a blood clot that blocks circulation, often to the hind legs), or sudden cardiac death — sometimes as the very first visible sign of the disease.
Which Cats Are Most at Risk?
HCM can occur in cats of any breed, including mixed-breed cats, but certain breeds carry a well-documented elevated risk:
Maine Coon
Ragdoll
British Shorthair
Persian
Bengal
Sphynx
Norwegian Forest Cat
Birman
Chartreux
Maine Coons and Ragdolls are the only breeds with an identified genetic mutation (in the MYBPC3 gene) linked to HCM, and genetic blood tests are available for both. That said, this known mutation accounts for only a small share of all feline HCM cases — most cats who develop the disease, including many purebreds in the at-risk list above, don't carry a currently identified genetic marker. HCM is typically an adult-onset disease, though it's been diagnosed in cats as young as 4 months and as old as 16 years, and prevalence increases with age. Male cats are affected more often than females.
What Are the Warning Signs?
This is the hardest part of HCM: a meaningful share of affected cats — some estimates put it around a quarter of cats over 9 — show no symptoms at all. When signs do appear, they range from subtle to severe:
Early or non-specific signs:
Lethargy
Reduced appetite
More advanced signs:
Rapid or labored breathing
Open-mouth breathing
Weakness or collapse
Fainting (syncope)
Sudden hind-leg weakness or paralysis, along with pain and cold limbs — a sign of a blood clot (arterial thromboembolism) and a true emergency
In some cases, sudden death is the first and only sign — which is why breed-based screening exists in the first place, rather than waiting for symptoms to prompt a vet visit.
How Is HCM Diagnosed?
Echocardiogram (heart ultrasound) is the gold-standard test and the most reliable way to directly visualize wall thickness and heart function.
Genetic testing is available for Maine Coons and Ragdolls specifically, though a negative result doesn't rule out HCM entirely, since other genetic factors can still be involved.
NT-proBNP blood testing can help flag cats with moderate to severe HCM who aren't showing outward symptoms.
Additional testing — bloodwork, blood pressure screening, and thyroid testing — is often used to rule out other conditions that can cause similar heart thickening, like hyperthyroidism or high blood pressure, since these "look-alikes" need to be excluded for an accurate diagnosis.
Is There a Cure?
No — there's currently no cure for HCM in cats. Some cats with mild disease show no symptoms and live a normal lifespan without major intervention. Others progress to heart failure or clot-related complications and need ongoing management, which may include medications to help the heart relax, control heart rate, or reduce clotting risk, along with regular monitoring.
Frequently Asked Questions
How common is HCM in cats?
It's the most common heart disease in cats, affecting an estimated 1 in 7 (around 14-17%) across the general cat population.
Which cat breeds are at the highest risk for HCM?
Maine Coons, Ragdolls, British Shorthairs, Persians, Bengals, Sphynx, Norwegian Forest Cats, Birmans, and Chartreux are all documented as higher-risk breeds, though HCM can occur in any cat, including mixed breeds.
Can a cat have HCM with no symptoms?
Yes, and it's common — many cats, especially in earlier stages, show no outward signs at all. This is why breed-based screening and periodic echocardiograms matter more for HCM than for conditions that reliably announce themselves with symptoms.
Is there a genetic test for HCM?
Yes, but only for Maine Coons and Ragdolls, where a specific mutation in the MYBPC3 gene has been identified. A normal genetic test result lowers risk but doesn't rule out HCM completely, since other cats develop the disease through mutations that haven't been identified yet.
What's the first sign of a blood clot from HCM?
Sudden hind-leg weakness or paralysis, often with visible pain and cold limbs, can indicate a clot has blocked blood flow (arterial thromboembolism). This is a medical emergency and needs immediate veterinary care.
Have a High-Risk Breed? Get Guidance Built Around Your Cat
Because HCM often shows no symptoms until it's advanced, knowing your cat's breed-specific risk is one of the most useful things you can bring to a vet visit. PetFirst Assist uses your cat's actual profile — including breed — to help you understand symptoms and recognize when something is worth a closer look.
PetFirst Assist is an informational tool and does not replace professional veterinary diagnosis or treatment. Screening and diagnosis for HCM should come from your veterinarian, and sudden hind-leg weakness or breathing difficulty should be treated as an emergency.
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