By Dial A Vet Veterinary Team
Hypertrophic cardiomyopathy (HCM) is a disease where the heart muscle, especially the left ventricle, becomes abnormally thick. The thickened wall can stiffen, reduce filling, and create turbulent flow. Some cats live for years with mild disease found only on a scan. Others develop heart failure, dangerous blood clots, or sudden decline. Calm monitoring and early recognition of emergencies matter more than internet prognosis charts.
Key takeaways
- HCM is the most common cardiomyopathy in cats.
- Many cats show no signs until a murmur, gallop sound, breathing trouble, or clot event appears.
- Sudden hind-leg pain and paralysis can signal a saddle thrombus emergency.
- Diagnosis relies on echocardiography (heart ultrasound), not sound alone.
- Treatment aims to manage heart failure signs, reduce clot risk when indicated, and support quality of life. No cure guarantees.
Signs that may appear
- Faster or laboured breathing at rest
- Open-mouth breathing (always urgent in cats)
- Lethargy or hiding
- Reduced appetite
- Fainting or collapse (less common)
- Sudden pain, cold hind legs, and inability to walk (possible arterial thromboembolism)
- Sometimes no outward signs until a vet hears a murmur or gallop
Related reading: breathing problems in cats, heart failure medicines discussion (general education only), and anaemia in cats for other causes of weakness.
Breeds and risk
Maine Coons, Ragdolls, British Shorthairs, Sphynx, and some other breeds have known predispositions, and genetic tests exist for certain mutations. Mixed-breed cats develop HCM too. Breeding cats in at-risk lines should follow veterinary genetic and screening advice.
When it is an emergency
Seek emergency care immediately if your cat has:
- Open-mouth breathing, blue gums, or extreme breathing effort
- Sudden inability to use one or both back legs with pain or crying
- Collapse or unresponsiveness
Do not force a struggling cat into a carrier without careful wrapping; minimise stress and travel promptly.
How vets diagnose HCM
A murmur or gallop raises suspicion but does not confirm HCM. Blood pressure measurement, thyroid testing in older cats (to rule out hyperthyroidism contributions), chest X-rays, NT-proBNP blood markers in some cases, and echocardiography are used. Heart ultrasound measures wall thickness and assesses obstruction, atrial size, and clot risk features.
Management categories (no doses)
- Monitoring preclinical cats with scheduled rechecks
- Medicines to manage congestive heart failure when fluid builds up in or around the lungs
- Anti-clotting strategies when atrial enlargement or prior clot risk is significant
- Treating concurrent hyperthyroidism or hypertension when present
- Stress reduction, weight optimisation, and activity tailored to comfort
Your vet or a veterinary cardiologist individualises the plan. We do not prescribe drug doses here, and we do not promise survival times.
Living with an HCM diagnosis
Learn your cat's normal resting breathing rate at home (count breaths while sleeping). Rising rates can flag early fluid build-up. Keep a go-to emergency clinic address. Ask before starting fluids under the skin for other diseases, because extra fluid can worsen heart failure.
Home breathing-rate practice
While your cat sleeps, count how many breaths occur in 15 seconds and multiply by four, or count for a full minute. Many healthy cats rest under about 30 breaths per minute, but your vet will give a personalised threshold. Rising rates, belly effort, or open-mouth breathing mean you should seek care without waiting for business hours when signs are severe.
Clot risk education for families
Arterial thromboembolism is extremely painful. Cats may cry, drag the back legs, and have firm, cold muscles. This is a true emergency. Some cats are treated supportively; outcomes vary and decisions are individual. Knowing the signs means you will not wait overnight. Ask your cardiology team whether clot-prevention medicine is indicated for your cat's atrial size and history.
FAQ
Does a heart murmur always mean HCM?
No. Murmurs have many causes, and some HCM cats have no murmur.
Can diet reverse HCM?
No specific diet cures HCM. Balanced nutrition and healthy weight still help overall care.
Should related cats be screened?
Discuss echocardiographic screening and genetics with your vet for breeding or related household cats.
Is anaesthesia safe?
Cats with HCM need careful anaesthetic planning. Always tell the clinic about the diagnosis before procedures.
Talk with a vet
If your cat has been diagnosed with HCM or you have noticed breathing changes or sudden leg weakness, a Dial A Vet veterinarian can help you triage and prepare questions. Book a Dial A Vet consult.
References
- Cornell Feline Health Center: Hypertrophic cardiomyopathy
- ACVIM: Consensus statements on classification of cardiomyopathies in cats
- Merck Veterinary Manual: Cardiomyopathy in cats
Disclaimer: Educational only. This page does not replace an examination or advice from your own veterinarian. If you are worried about your pet right now, contact your local vet or nearest emergency animal hospital.
